18 Thomas J. Rosol1 and Donald J. Meuten2 1 The Ohio State University, USA 2 North Carolina State University, USA Endocrine glands are collections of specialized cells that synthesize, store, and release their secretions directly into the bloodstream. Hormones affect the rates of specific chemical reactions in target cells and other body tissues. Endocrine glands act in concert with the nervous system and are involved in integrating and coordinating a wide variety of activities to maintain internal homeostasis of the body. There is a close anatomical relationship between endocrine cells and the capillary network. The capillary endothelial cells have fenestrae covered by a single membrane that facilitate rapid transport of raw materials and secretory products between the bloodstream and endocrine cells. Hormones secreted by mammals are divided chemically into three major groups: polypeptides (about 80%), steroids (about 15%), and tyrosine derivatives (about 5%). By knowing the chemical nature of a hormone, it is possible to predict its mechanism of action, receptors, solubility, half‐life in blood, and its plasma protein‐binding characteristics (Table 18.1). Table 18.1 Comparison of major classes of hormones Polypeptide hormones are produced by organs such as the adenohypophysis, pancreatic islets, parathyroid glands, thyroid C cells, and gastrointestinal tract and typically bind and activate G protein‐coupled seven transmembrane domain receptors (GPCR). Cyclic adenosine monophosphate (cAMP) and calcium ion (Ca2+) are common cytoplasmic mediators of polypeptide hormone action. For example, activated hormone receptors can stimulate the enzyme adenylyl cyclase in the plasma membrane of target cells, followed by the intracellular formation of cAMP from adenosine triphosphate (ATP), and subsequent activation of cAMP‐dependent protein kinases. In addition, activated hormone receptors can increase cytoplasmic Ca2+ from the extracellular fluid or intracytoplasmic vesicular stores by stimulation of phospholipase C, which hydrolyzes membrane phospholipid to release the second messengers, inositol triphosphate (IP3) and diacylglycerol. IP3 binds to membrane Ca2+ channels and permits the entry of increased Ca2+ into the cytoplasm. Steroid hormones produced by organs such as the adrenal cortex and gonads account for approximately 15% of mammalian hormones. After steroid hormones are within target cells they bind initially to cytoplasmic homodimeric or heterodimeric steroid receptors, then the hormone–receptor complex is translocated into the nucleus, where it binds to specific regulatory sites on the DNA in association with accessory proteins to effect up‐ or downregulation of gene expression. The third chemical group of hormones is composed of the tyrosine derivatives. They account for approximately 5% of mammalian hormones and include the catecholamines (epinephrine and norepinephrine) secreted by the adrenal medulla and the iodothyronines (thyroxine (T4) and triiodothyronine (T3)) produced by follicular cells of the thyroid gland. Catecholamines share a similar mechanism of action with polypeptide hormones, whereas iodothyronines more closely resemble steroid hormones (Table 18.1). There are certain morphological differences among endocrine cells that secrete polypeptide and steroid hormones, which are also found in neoplastic cells derived from the different endocrine glands. Normal and neoplastic cells concerned with the synthesis of polypeptide hormone have a well‐developed endoplasmic reticulum with many attached ribosomes for assembly of hormone. They also have a prominent Golgi apparatus for packaging hormone into small granules for intracellular storage and transport. Secretory granules are unique for cells that secrete polypeptide hormones (and catecholamines) and provide a mechanism for intracellular storage of preformed hormone. The membrane‐limited granules represent macromolecular aggregations of active hormone and specific‐binding proteins, such as chromogranin A. Chromogranin A is a useful, but nonspecific, immunohistochemical (IHC) marker for peptide hormone‐secreting cells. Neoplasms derived from endocrine cells that secrete polypeptide hormone or catecholamine may release recently synthesized hormone on a continuous or episodic basis and often have only a few characteristic secretory granules in their cytoplasm. Therefore, IHC staining for cytoplasmic hormone or chromogranin A may not accurately reflect the amount of hormone produced and secreted. In situ hybridization for mRNA or serum concentrations of hormone often more accurately indicate the amount of hormone produced by neoplastic cells. Neoplasms derived from polypeptide hormone‐secreting endocrine cells usually consist of one predominant cell type and are associated with the secretion of one major polypeptide hormone. However, there is evidence from immunocytochemical and electron microscopic investigations that some endocrine tumors may be composed of more than one type of neoplastic cell and be capable of synthesizing multiple hormones (e.g., pancreatic islet cell tumors stain positive for insulin, glucagon, somatostatin, and pancreatic polypeptide); however, an overproduction of one hormone (e.g., insulin) usually predominates and is responsible for the clinical disease syndrome. Neoplasms derived from steroid hormone‐secreting endocrine cells are characterized by having large lipid bodies in their cytoplasm that contain cholesterol, cholesterol esters, and other precursor molecules for hormone synthesis. The lipid bodies are in close proximity to an extensive tubular network of smooth endoplasmic reticulum and large mitochondria that contains the hydroxylase and dehydrogenase enzyme (cytochrome P450) systems necessary for the attachment of various side chains and radicals to the basic steroid nucleus. Steroid hormone‐producing cells lack secretory granules and do not store significant amounts of preformed hormone. They are dependent on continued synthesis to maintain the normal secretory rate for a particular steroid hormone. The histopathological separation between nodular hyperplasia, adenoma, and carcinoma is often more difficult in endocrine glands than in most other organs of the body. However, criteria for the separation should be established and applied in a uniform manner. There appears to be a continuous spectrum of proliferative lesions between focal (nodular) hyperplasia and adenomas derived from a specific population of endocrine cells. Sometimes the distinction of hyperplasia, adenoma, and carcinoma is straightforward. Metastases are clear evidence of carcinoma. However, there are ambivalent cases in which the distinction is not clear. For example, pituitary adenomas may appear to invade adjacent neuropil, when this usually represents expansile growth along paths of least resistance. Diagnoses may be facilitated by clinical data, serum hormone concentrations, provocative testing, histopathology, and sometimes arbitrary size distinctions to classify the lesion. A common feature of endocrine glands is that prolonged stimulation of a population of secretory cells predisposes to the development of tumors. Long‐continued stimulation may lead to the development of clones of cells within the hyperplastic endocrine glands that grow more rapidly than the rest and are more susceptible to genetic alterations that lead to neoplastic transformation when exposed to the right combination of promoting agents. Focal (nodular) hyperplasia usually appears as multiple small areas that are well demarcated, but not encapsulated from normal cells. There is minimal compression of adjacent normal cells. Cells of focal hyperplasia closely resemble the cells of origin; however, the cytoplasmic area may be slightly enlarged and the nucleus more hyperchromatic than in the normal endocrine cell. These lesions are often incidental, do not produce clinical disease(s) and are not due to stimulation from a trophic hormone or absence of negative feedback. As a generalization, nodular hyperplasia is nonfunctional and diffuse hyperplasia is functional. Diffuse hyperplasia is usually due to a trophic hormone or lack of negative feedback. For example, diffuse parathyroid chief cell hyperplasia occurs in secondary nutritional or renal hyperparathyroidism. Excessive focal growth of endocrine cells is the consequence of aberrant secretion of growth stimulating and/or function stimulating hormone(s). Nodules arising in hyperplastic endocrine glands may be of polyclonal as well as of clonal origin. Hyperfunction and cellular hypertrophy associated with endocrine hyperplasia are completely reversible when the overstimulation ceases; however, chronic and severe hyperplasia of endocrine tissues may not be fully reversible. Pathogenic mechanisms that can result in non‐neoplastic endocrine hyperplasia include (1) pathologic overproduction of trophic hormones (e.g., ACTH by a corticotroph adenoma), (2) disruption of negative feedback control system as in iodine‐deficient goiter or parathyroid chief cell hyperplasia associated with chronic renal disease or nutritional imbalances, and (3) exogenous administration of trophic hormones. There are several characteristics of non‐neoplastic endocrine hyperplasia that are not explained solely on the basis of simple systemic overstimulation and that emphasize the overlap between non‐neoplastic and neoplastic growth in endocrine glands. These unique characteristics of non‐neoplastic endocrine hyperplasia include the following: (1) hyperplasia is not a fully reversible process, (2) hyperplasia focal or nodular and not uniformly diffuse, (3) hyperplastic nodules may grow autonomously, (4) hyperplastic nodules may secondarily acquire the features of autonomous growth, (5) hyperplastic endocrine nodules may be clonal as well as polyclonal or be of both types, and (6) both hypo‐ and hyperfunction may develop in hyperplastic nodules of endocrine organs. As a rule, most focal hyperplastic lesions in endocrine tissues are nonfunctional and incidental lesions. Adenomas are solitary nodules in one endocrine gland (or occasionally in both for paired endocrine glands) that usually are larger than the multiple areas of focal hyperplasia. They are sharply demarcated from the adjacent normal glandular parenchyma by a thin, partial to complete, fibrous capsule. The adjacent parenchyma is compressed to varying degrees depending on the size of the adenoma. Cells composing an adenoma are phenotypically uniform and may closely resemble the cells of origin morphologically and in their architectural pattern of arrangement. However, adenomas often have histological differences from normal glands such as multiple layers of cells or solid clusters of secretory cells subdivided into packets by a fine fibrovascular stroma. The term adenoma should be used to designate true neoplasms arising in an endocrine organ that grow autonomously in the absence of known systemic or locally acting growth stimulating agents. The separation of adenoma from focal hyperplasia in endocrine organs solely on the basis of histological criteria is unreliable and arbitrary using existing morphological methods. Many adenomas are clearly identifiable based on size, histology, and compression of adjacent normal tissue, but there is considerable overlap between focal hyperplasia and small adenomas. Carcinomas that are easy to classify are large, invasive, and metastatic. However, in the absence of these classical criteria there can be considerable subjectivity in the separation of a carcinoma and adenoma. Carcinomas are often larger than adenomas and have regions of necrosis and/or hemorrhage. Histopathological features of malignancy include intraglandular invasion, invasion into and through the capsule of the gland, formation of tumor cell thrombi within vessels, and metastases at distant sites. Malignant endocrine cells may be more pleomorphic (including oval or spindle shaped) than normal, but nuclear pleomorphism and other cytological characteristics are not consistent criteria to distinguish an adenoma from a carcinoma. Mitotic figures may be frequent in malignant endocrine cells, but the significance of this criterion can vary considerably since mitotic figures can also occur in adenomas. In addition, predicting biological behavior of endocrine carcinomas is difficult based only upon histopathologic evaluation, and historical incidences based on species and organ can be helpful for prognoses. For example, parathyroid carcinomas in dogs have a very low incidence of metastasis, and well‐differentiated adrenal cortical carcinomas in dogs have a moderate to high chance of metastasis. Many neoplasms derived from endocrine glands are functionally (endocrinologically) active, secrete an excessive amount of hormone either continuously or episodically, and result in dramatic clinical syndromes of hormone excess. Functional versus nonfunctional tumors cannot be always identified using histopathology alone. Integration of clinical signs, clinical pathology data, serum hormone concentrations, and determination of atrophy of uninvolved cells of the same cell type can provide additional clarity on the functionality of a tumor. Examples of functional tumors in animals that are described in this chapter include, among others, the hypoglycemia of β‐cell neoplasms of the pancreatic islets in dogs and ferrets; hyperthyroidism associated with thyroid follicular tumors in cats and dogs; hypercalcemia with primary hyperparathyroidism; growth hormone‐secreting pituitary tumors; hypercalcitoninism with thyroid C‐cell tumors; hyperadrenocorticism associated either with adrenocorticotropin (ACTH)‐secreting pituitary corticotroph adenomas or neoplasms derived from the adrenal cortex in dogs; and hypertension resulting from overproduction of catecholamines by tumors of the adrenal medulla. Quantitation of hormone levels in serum or plasma and/or the measurement of hormonal metabolites in the urine over a 24‐hour period of excretion are often essential to confirm that an endocrine tumor is functional. Morphologically, an endocrine tumor often can be interpreted as endocrinologically active if the rim of normal tissue around the tumor, the opposite of paired endocrine glands, or the nontumorous endocrine glands undergo trophic atrophy due to negative feedback inhibition by the elevated hormone levels or by an altered blood constituent (e.g., elevated blood calcium). The non‐neoplastic secretory cells (especially the cytoplasm) become smaller than normal, and eventually the number of cells is decreased. Functional pituitary neoplasms secreting an excess of a particular trophic hormone (e.g., ACTH) will be associated with diffuse and sometimes marked hypertrophy and hyperplasia of target endocrine cells (e.g., ACTH stimulation of the zona fasiculata and reticularis in the adrenal cortex). There are few identified causes of pituitary tumors. Pituitary tumors are a common finding in 20% of the human population and up to one‐third of the tumors induce clinical disease.1 Genetic mutations are an infrequent cause of pituitary tumors in humans and it appears that tumor formation is promoted by normal hormones that regulate pituitary cells and local growth factors involved in fetal pituitary growth. Estrogens and gonadotropin‐releasing hormone (GnRH) analogs/agonists induce pituitary tumors in rats. Pars intermedia adenomas in horses are associated with a reduction in hypothalamic production of dopamine, which is a growth inhibitor for melanotrophs. Mutations in growth regulatory or tumor suppressor genes may occur after hyperplasia is induced by stimulation from endogenous hormones or growth factors or suppression of growth inhibitors.1,2 Administration of salmon calcitonin to Sprague–Dawley and Fischer 344 rats for 1 year increased the incidence of nonfunctional pituitary adenomas.3 The nonfunctional pituitary tumors expressed and secreted the pituitary glycoprotein hormone α‐subunit. Long‐term therapy for osteoporosis with salmon calcitonin is no longer recommended in humans because meta‐analysis of 21 clinical trials revealed a mild increase in the overall rate of malignancies in calcitonin‐treated patients.4 The ACTH‐producing cell in the hypophysis is a large chromophobic cell. Following adrenalectomy the ACTH‐producing chromophobic cells have the highest rate of DNA synthesis, hormone synthesis, and hormone secretion of all the hypophyseal cell types. The chromophobe adrenalectomy cell is morphologically distinct from gonadectomy or thyroidectomy cells and from other cell types in the normal hypophysis. The cytoplasm is often compressed between or indented by the neighboring pituitary cells. Functional tumors arising in the pituitary gland in domestic animals are most commonly derived from corticotroph (ACTH‐secreting) cells in the pars distalis or pars intermedia (in dogs) and associated with a clinical syndrome of cortisol excess (Cushing’s disease) (see Figure 18.1A).5 Cushing’s disease was initially described in association with basophil adenomas of the hypophysis in human patients. Functional chromophobe adenomas are encountered most frequently in dogs, occasionally in cats, and infrequently in other animal species. They develop in adult to aged dogs and occur in a number of breeds. Boxers and Boston terriers have a higher incidence of functional (ACTH‐producing) pituitary tumors. The clinical manifestations and lesions that develop are primarily the result of a long‐term overproduction of cortisol by hyperplastic adrenal cortices. These clinical changes are the result of the combined gluconeogenic, lipolytic, protein catabolic, and anti‐inflammatory actions of glucocorticoid hormones on many organ systems of the body. Unless the pituitary tumor is large there are few if any clinical signs directly referable to the tumor. There are other causes of cortisol excess in dogs, including functional (zona fasciculata) adrenal cortical neoplasms. Figure 18.1 Functional chromophobe adenoma, dog. (A) Corticotroph adenoma (arrow) in the hypophysis with bilateral adrenal cortical hyperplasia in a dog. The hypothalamus is compressed by the dorsally expanding pituitary adenoma. (B) Corticotroph adenoma, diffuse and sinusoidal type. In the diffuse regions of the adenoma there are sheets of chromophobic tumor cells that lack a characteristic pattern of arrangement. Capillaries are indistinct and few in number. In the sinusoidal regions the neoplastic cells are separated into compartments by endothelial‐lined vascular sinusoids some of which have coalesced to form regions of hemorrhage. (C) Corticotroph adenoma, diffuse type. There are sheets and cords of chromophobic tumors cells with thin fibrous trabeculae containing capillaries. The adenoma is compressing the neuropil and has a thin fibrous capsule. (D) Corticotroph adenoma, POMC IHC. Most tumor cells are darkly stained and contain a high level of POMC precursor protein. (E) Corticotroph adenoma, ACTH IHC. Most tumor cells have minimal cytoplasmic staining for ACTH due to active secretion of the peptide or heterogeneity of ACTH production by the cells. Note the hypertrophy of the corticotrophs with abundant cytoplasm for production of ACTH. Some of the corticotrophs have abundant cytoplasmic accumulation of ACTH and are darkly stained. A number of clinical and functional alterations develop in dogs with corticotroph (ACTH‐secreting) adenomas. Dogs with pituitary‐dependent hyperadrenocorticism have adenomas derived from cells either from the pars distalis (majority) or the pars intermedia. Immunocytochemical staining of the tumor cells are usually positive for pro‐opiomelanocortin (POMC, the precursor protein of the secreted peptides from corticotrophs and melanotrophs), ACTH, β‐lipotrophin, and β‐endorphin (Figure 18.1D,E).6 Nearly all of the clinicopathologic abnormalities are due to the high circulating concentrations of cortisol, not ACTH. If blood cortisol levels are decreased by specific therapy (mitotane or trilostane) of the hyperplastic adrenal cortices, the physical and laboratory abnormalities will return to normal even though the pituitary tumor continues to produce an excess of ACTH. It is unknown if the pituitary tumor in dogs treated in this manner grow at an increased rate due to the lack of negative feedback from the decreased cortisol. Redistribution of adipose tissue leads to prominent fat pads on the dorsal midline of the neck. The muscles of the extremities and abdomen are weakened and atrophied. The loss of tone of abdominal muscles and muscles of the abaxial skeleton results in gradual abdominal enlargement (pot belly) and lordosis. Atrophy of the temporal muscles may result in concave indentations and palpable prominences of underlying skull bones. A cortisol‐associated myopathy contributes to the pendulous abdomen and muscle weakness. Hepatomegaly due to increased glycogen deposition, fat accumulation, and vacuolation of smooth endoplasmic reticulum in hepatocytes due to enzyme induction by cortisol contributes to the development of the distended abdomen. Cytoplasmic vacuolation of hepatocytes due to glycogen accumulation is a nearly constant lesion in these dogs, and with exogenous steroids it can occur in 48 hours. Skin lesions often occur in dogs with hyperadrenocorticism, especially over points of wear. The hair coat becomes thin, rough, and dry. Hair shafts can be easily broken and dislodged from their follicles. The disease progresses in a bilaterally symmetrical pattern. The skin is coarsely wrinkled and often “paper thin” due to a loss of collagen and elastin fibers in the dermis and subcutis, often with severe atrophy of the epidermis and pilosebaceous apparatus. The majority of hair follicles are inactive and are in the telogen phase of the growth cycle. The prominent comedomes observed in the skin, particularly on the ventral abdomen, represent hair follicles distended with keratin and debris. The outer stratum corneum is thickened considerably, giving the skin surface a dry and scaly appearance. Other skin changes include hyperpigmentation and mineralization. Cutaneous mineralization is a characteristic lesion in up to 30% of dogs with hyperadrenocorticism and is still considered pathognomonic. Numerous mineral crystals deposited along collagen and elastin fibers in the dermis may protrude through the atrophic and thinned epidermis. A mild to moderate granulomatous inflammatory reaction often accompanies the deposition of mineral in soft tissues. Another common site for mineralization is the interalveolar septae of the lung. Mineralization of these tissues is underappreciated in H&E‐stained sections; however, if histochemical stains for Ca/P are used (e.g., Von Kossa), mineralization will be present. The pathogenesis of the mineralization is related to the increased protein catabolism and formation of an organic matrix that attracts and binds calcium and phosphorus. Dermal vessels are often prominent and readily visible through the thin skin. In an occasional dog with marked abdominal distension and loss of supporting dermal collagen and elastin fibers, the superficial vessels become severely stretched and dilated, forming striae similar to those described in humans with Cushing’s syndrome. The syndrome of long‐term cortisol excess is often complicated by an increased susceptibility to infection with the development of bacterial or fungal infections in the skin, urinary tract, conjunctiva, and lung. Multifocal areas of suppurative folliculitis and dermatitis develop near the lip folds and footpads and elsewhere in the skin. A frequent serious complication in dogs with hyperadrenocorticism is a suppurative bronchopneumonia that can be fatal if not detected early and treated appropriately. Clinical laboratory abnormalities due to cortisol excess in dogs are characteristic and include leukocytosis, mature neutrophilia, lymphopenia, eosinopenia, and monocytosis. These changes are common; however, erythrocytosis with nucleated red blood cells is seen much less frequently. Serum chemistry abnormalities include a marked increase in alkaline phosphatase (ALP) (steroid and hepatic isoenzymes), a mild increase in alanine transaminase (ALT) (associated with the steroid‐induced hepatopathy), and moderate hyperglycemia. Increases in ALP will be present in over 90% of dogs; however, the magnitude varies considerably. If ALP and the urine cortisol/creatinine ratio are not increased, then it is highly unlikely the dog has Cushing’s disease. Differentiation of the types of hyperadrenocorticism can be accomplished clinically with a variety of stimulatory and suppressive tests.7 The pituitary gland usually is enlarged in dogs with corticotroph adenomas (up to 4 cm in diameter). However, the occurrence or severity of functional disturbance has no consistent relationship to the size of the neoplasm. Small chromophobe adenomas (microadenomas) are as likely to be endocrinologically active as larger neoplasms. The magnitude of expansion of corticotroph adenomas is dependent upon the degree of insensitivity to negative feedback by glucocorticoids.8 Microadenomas may only be found on histopathology and these are difficult to distinguish from hyperplasia. Fortunately, the differentiation of microadenoma and hyperplasia does not change treatments or prognosis. The larger adenomas are often firmly attached to the base of the sella turcica, but without evidence of erosion of the sphenoid bone. In the animal species most likely to develop pituitary neoplasms (dog and horse), the diaphragma sella is incomplete. Therefore, the line of least resistance in the dog and horse favors dorsal expansion of the gradually enlarging mass with resulting invagination into the infundibular cavity, dilatation of the infundibular recess and third ventricle, and eventual compression and replacement of the hypothalamus and thalamus. This differs from the situation in humans or ruminants where the complete diaphragma sella (dura mater separating the hypophysis from the cranial cavity) favors ventrolateral growth of the neoplasm and erosion of the sphenoid bone. Dorsal expansion of the larger pituitary neoplasms results in either compression of the overlying hypothalamus or extension into and replacement of the parenchyma of the hypothalamus and occasionally the thalamus. The dorsal extension of the tumor is not interpreted to be a criterion of malignancy for pituitary tumors (Figures 18.2A,B, 18.6A, and 18.7A). Focal areas of hemorrhage, necrosis, mineralization, and liquefaction are frequently encountered in larger pituitary neoplasms. Pituitary macrotumors in dogs can be readily identified by magnetic resonance imaging and many produce CNS signs or impair vision. Figure 18.2 Nonfunctional chromophobe adenoma of the pituitary gland. (A) Ventral view of the brain of a 4‐year‐old male Siamese cat illustrating a large chromophobe adenoma. The neoplasm has completely incorporated the pituitary, extended into the brain, and destroyed the optic nerves. (B) Frontal section of the brain and cross‐section of the adrenal glands. Note compression of the brain by the adenoma and the atrophy of the adrenal cortices (zona fasiculata and reticularis) due to the lack of ACTH. The zona glomerulosa is present as a thin rim of tan‐colored tissue. It persists because it is only partially influenced by ACTH. Dogs with functional corticotroph adenomas have bilateral enlargement of the adrenal glands (Figure 18.1A) due to chronic excessive stimulation by ACTH. The hypertrophy and hyperplasia is often striking and is due to an increased amount of cortical parenchyma, primarily in the zona fasciculata and to a lesser extent in the zona reticularis. Nodules of yellow‐orange cortical tissue are often identified outside the capsule in the periadrenal fat and extending into the medulla. The corticomedullary junction is irregular, and the medulla frequently is compressed. The normal ratio of cortex:medulla:cortex is approximately 1:1:1 in dogs. Pituitary adenomas are composed of well‐differentiated secretory cells supported by fine connective tissue septa. Chromophobe adenomas are subclassified into sinusoidal and diffuse types on the basis of the predominant pattern of neoplastic cells. The tumor cells in the sinusoidal type are separated into compartments of varied sizes and shapes by delicate, often incomplete, connective tissue septa containing capillaries or small venules. The sinusoidal type of pituitary tumor is more vascular than the diffuse type, and in some areas the blood sinusoids attain considerable size and appear to be lined by neoplastic cells. When the tumor cells palisade along the connective tissue septa or blood sinusoids, they are more elongated and have oval or spindle‐shaped nuclei. The tumor cells in the diffuse type of adenoma lack a characteristic architectural arrangement and appear as sheets or masses of large chromophobic cells (Figure 18.1B,C). Blood vessels are small and the connective tissue stroma is sparse. Corticotroph adenomas are composed of either large or small chromophobic cells. Large cell chromophobes make up the majority of adenomas of this type. They are polyhedral and have large vesicular nuclei with one or two prominent nucleoli and an abundant eosinophilic cytoplasm with distinct cell boundaries. Small cell chromophobes constitute the remaining pituitary adenomas of this type. They are roughly half the size of large cell chromophobes and have small dark nuclei with indistinct nucleoli and a small amount of cytoplasm. Mitotic figures are infrequent in both types of chromophobic cells. Remnants of the pars distalis may be identified near the periphery of the pituitary adenomas. Demarcation between the neoplasm and the pars distalis is not distinct and there rarely is a capsule. Acidophils and occasionally basophils are incorporated within the neoplasm near the margin. The pars distalis is either partly replaced by the neoplasm or severely compressed and composed principally of heavily granulated acidophils. The posterior lobe and infundibular stalk are either infiltrated and disrupted by tumor cells or completely incorporated within the larger neoplasms. The hypothalamus is severely compressed or replaced by the large, dorsally expanding, corticotroph adenomas (Figure 18.1B). There are increased numbers of fibrous astrocytes and hemosiderin‐laden macrophages, perivascular hemorrhages, a loss of neurons, and axonal degeneration within the hypothalamus and occasionally in the thalamus. Focal areas of hemorrhage, coagulation and liquefactive necrosis, mineralization, and cholesterol clefts often occur within the larger corticotroph adenomas. Cells constituting functional corticotroph adenomas in dogs have evidence of secretory activity.9 Endoplasmic reticulum and the Golgi apparatus are well developed in neoplastic corticotrophs. The predominating neoplastic cells are large, relatively electron dense, and polyhedral or cuboidal. The outline of the neoplastic cells is irregular, and cytoplasmic projections extend between neighboring cells or encompass them completely. The nucleus usually is centrally located and irregular in shape with deep indentations and contains one or two dense nucleoli. The neoplastic cells are supported by a reticular framework of follicular cells.10,11 Follicular cells are stellate and have long cytoplasmic processes that extend between the neoplastic cells and terminate on the extracellular accumulations of colloid or on perivascular spaces. Cells in functional corticotroph adenomas contain mature secretory granules as demonstrated by electron microscopy.12 Secretory granules vary in number from cell to cell but usually are numerous. Secretory granules can be observed in tumors that are poorly fixed, fixed in formalin or had a prolonged post‐mortem delay in processing. This feature is useful in poorly differentiated tumors to establish if a tumor is of endocrine origin and produces polypeptide secretory granules. Pituitary adenomas arising in both the pars distalis and intermedia, associated with cortisol excess in dogs, are composed of cells that immunocytochemically stain selectively for POMC, ACTH and MSH, β‐endorphin, and β‐lipotrophin (Figure 18.1D,E).6 POMC staining in the cytoplasm is usually intense and staining for ACTH and MSH is variable between cells depending on the rate of hormone synthesis and number of secretory granules. Focal areas of hyperplasia and microadenomas, composed of similar ACTH/MSH cells, also occur in both lobes of the adenohypophysis. In spite of hypercortisolemia and neoplastic transformation of corticotrophs in dogs with pituitary‐dependent hyperadrenocorticism, corticotrophs usually remain responsive to corticotrophin‐releasing hormone and other factors.13 Nonfunctional (endocrinologically inactive) pituitary tumors are most common in dogs, cats, and parakeets and are rare in other species. In contrast to the functional adenomas, there is no indication of breed or sex predisposition. Although these chromophobe adenomas appear to be endocrinologically inactive, they may result in significant functional disturbances by virtue of compression atrophy of the pars nervosa and pars distalis or extension into the overlying brain. Animals with nonfunctional pituitary adenomas usually are presented with clinical disturbances related to dysfunction of the central nervous and neurohypophyseal systems or lack of secretion of pituitary trophic hormones with diminished end‐organ function (e.g., thyroid follicular cells, adrenal cortex, and gonads). The clinical history often includes depression, incoordination and other disturbances of balance, weakness, collapse with exercise, and a marked change in personality. In longstanding cases there may be blindness with dilated and fixed pupils. The body condition varies from a progressive loss of weight to obesity. The animals often appear to be dehydrated and the owner may have noticed increased water consumption and frequent urination. Parakeets with chromophobe adenomas often develop exophthalmos due to extension of neoplastic cells along the optic nerve, disturbances of balance and falling down from a perch, and diarrhea (associated with disturbances in water balance). Most of the clinical signs are due to the relatively large size of these tumors which compress and or infiltrate adjacent tissues. A consistent finding with both functional and nonfunctional pituitary tumors is the excretion of large volumes of dilute urine with a low specific gravity (approximately 1.007). Water intake is increased correspondingly. Disturbances of water balance (diabetes insipidus) is the result of either a direct diuretic effect exerted on the kidney (peripheral) by the elevated cortisol level or an interference with the synthesis and release of antidiuretic hormone (ADH; central). The posterior lobe, infundibular stalk, and hypothalamus are often compressed or disrupted by the infiltration of neoplastic cells in dogs with pituitary tumors. This interrupts the nonmyelinated axons that transport ADH from the site of production in the hypothalamus (primarily in the supraoptic nucleus) to the site of release in the capillary plexus of the posterior lobe. Compression of neurosecretory neurons in the hypothalamus by the tumor may result in decreased ADH synthesis. Nonfunctional pituitary adenomas usually reach considerable size before they cause obvious clinical signs (Figures 18.2 and 18.6). These features are likely related. Nonfunctional tumors continue to grow until their physical size produces clinical signs. Tumors that are endocrinologically active produce clinical diseases that result in detection when some of the pituitary lesions are microscopic. The proliferating tumor cells in nonfunctional tumors incorporate the remaining structures of the adenohypophysis and infundibular stalk. The neoplasms are firmly attached to the base of the sella turcica, but there usually is no evidence of erosion of the sphenoid bone. In dogs and cats the diaphragma sella is incomplete, so the line of least resistance favors dorsal expansion of the progressively enlarging adenoma, resulting either in a broad‐based indentation or extension into the overlying brain (Figure 18.2A,B). The entire hypothalamus may be compressed and replaced by the tumor, which extends through the thalamus and protrudes into the lateral ventricles. The optic nerves are compressed and incorporated within the large neoplastic mass on the ventral aspect of the brain, accounting for the blindness observed clinically. The adrenal glands of animals with large nonfunctional pituitary adenomas may be small and consist primarily of medullary tissue surrounded by a narrow zone of atrophic cortex (Figure 18.2B). The adrenal cortex appears as a thin yellow‐brown rim composed of a moderately thickened capsule and secretory cells of the outer layer, zona glomerulosa. Approximately 10% of the zona glomerulosa is dependent on ACTH signaling and the majority is dependent on serum concentrations of Na+ and K+. The zonae fasciculata and reticularis are severely atrophied and secrete subnormal amounts of glucocorticoid hormones. These dogs may have secondary Addison’s disease in which clinical signs are due to decreased serum glucocorticoids, but with adequate mineralocorticoids and serum Na+ and K+ concentrations. Thyroid glands in animals with large pituitary adenomas may be either near normal or reduced in size, although to a much lesser degree than the adrenal cortex (Figure 18.2B). In severe or longstanding cases the majority of follicles are large, lined by flattened (atrophic) follicular cells, and distended with a densely stained colloid with little evidence of endocytotic activity because of a lack of thyroid‐stimulating hormone (TSH). Seminiferous tubules in the testis are small and have little evidence of active spermatogenesis. The tumor cells are cuboidal to polyhedral and either are arranged in diffuse sheets or subdivided into small packets by fine connective tissue septae. Numerous small capillaries are present throughout the neoplasm. Special histochemical techniques for pituitary cytology fail to demonstrate specific secretory granules within the cytoplasm of tumor cells. The histogenesis of these tumors is uncertain, but they appear to be derived from pituitary cells that have not differentiated sufficiently to synthesize and secrete a specific trophic hormone. Adenoma derived from cells of the pars intermedia is the most common type of pituitary tumor in older horses, the second most common type in dogs, infrequent in certain strains of laboratory rats and nonhuman primates, and rare in other species. Nonbrachycephalic breeds of dogs develop adenomas in the pars intermedia more often than brachycephalic breeds.5 Adenomas of the pars intermedia in dogs either are endocrinologically inactive and associated with varying degrees of hypopituitarism and diabetes insipidus or are endocrinologically active and secrete excessive ACTH, leading to bilateral adrenal cortical hyperplasia and the syndrome of cortisol excess. The clinical signs in these dogs are similar to those described previously for corticotroph adenomas of the pituitary gland. Malignancy is rare. Two cell populations have been identified in the pars intermedia of normal dogs by immunocytochemistry.14 The predominant cell type (A cell) stains strongly for α‐MSH as in the pars intermedia of other species. A second cell type (B cell) in the canine pars intermedia stains intensely for ACTH but not for α‐MSH. This second cell population accounts for the high bioactive ACTH concentration found in the pars intermedia of dogs and most likely gives rise to corticotroph adenomas of the pars intermedia in dogs with the syndrome of cortisol excess. The clinical syndrome associated with tumors or hyperplasia of the pars intermedia in horses (PPID; pituitary pars intermedia dysfunction) is characterized by polyuria, polydipsia, increased appetite, muscle weakness, somnolence, intermittent hyperpyrexia, and generalized hyperhidrosis (Figure 18.3A,B).15 The affected horses often develop a striking hypertrichosis (hirsutism) because of failure of the cyclic seasonal shedding of hair. The hair over most of the trunk and extremities is long (as much as 10–13 cm), abnormally thick, wavy, and often matted together (Figure 18.3A). Horses with larger tumors may have hyperglycemia (insulin‐resistant) and glycosuria, probably the result of a downregulation of insulin receptors on target cells induced by the chronic excessive intake of food and hyperinsulinemia. Figure 18.3 Adenoma and hyperplasia of the pars intermedia (PI), horse. (A) Hirsutism resulting from a failure of cyclic shedding in a horse with a PI adenoma. (B) Longitudinal section of a pituitary gland from a horse with PI hyperplasia (H) and an early adenoma (A) compressing the pars nervosa (scale = 1 cm). (C) Pituitary gland of a horse with a multinodular PI adenoma compressing the pars distalis and nervosa and overlying hypothalamus (scale = 1 cm). (D) Multinodular PI adenoma with multifocal hemorrhage, compression of the pars distalis (PD) and compression and infiltration of the pars nervosa (N). The PD has prominent capillaries filled with blood. (E) PI adenoma with compression and hemosiderin‐laden macrophages in the pars nervosa (PN) due to chronic hemorrhage. The adenoma cells are small and round or polyhedral to spindle shaped with abundant eosinophilic cytoplasm. (F) PI adenoma. Note the elongated, chromophobic, spindle‐shaped tumor cells. (G) PI adenoma. IHC for α‐MSH. Most tumor cells have moderate to intense positive cytoplasmic staining. The tumor cells with moderate staining are likely actively secreting hormone. (H) PI adenoma. IHC for ACTH. There is minimal to no staining for ACTH in the tumor cells. It is thought that most PI adenomas in horses secrete a low level of ACTH. (I) Compressed pars distalis adjacent to a PI adenoma. IHC for ACTH. Note the scattered cells with intense cytoplasmic staining for ACTH consistent with cytoplasmic storage of secretory granules. The disturbances in carbohydrate metabolism and increased appetite, hirsutism, and hyperhidrosis are considered to be primarily a reflection of deranged hypothalamic function caused by the large pituitary tumors. Adenomas of the pars intermedia in affected horses often extend out of the sella turcica, expand dorsally because of the incomplete diaphragma sella, and severely compress the overlying hypothalamus (Figure 18.3C). The hypothalamus is known to be the primary center for homeostatic regulation of body temperature, appetite, and cyclic shedding of hair. In addition to the space‐occupying effects, adenomas of the pars intermedia in horses are usually endocrinologically active but, the biochemical events in the pars intermedia lesions stimulate a unique pathogenesis with a different profile of pituitary hormones. Tumor tissue and plasma from horses with pars intermedia adenoma or hyperplasia contain high concentrations of immunoreactive peptides, such as corticotropin‐like intermediate lobe peptide (CLIP), α‐ and β‐melanocyte stimulating hormones (α‐ and β‐MSH), and β‐endorphin, which are derived from POMC and processed in the pars intermedia. POMC is a biosynthetic precursor of ACTH and other pituitary peptides and a high molecular weight (31,000–37,000 daltons) glycoprotein that undergoes different posttranslational processing in the pars distalis and intermedia (Figure 18.5). In the normal pars distalis, POMC is processed to ACTH (4500 Da), β‐lipotrophin, and γ‐lipotrophin, whereas in the normal pars intermedia POMC is cleaved into α‐MSH, CLIP (that contains amino acids 18–39 of ACTH), β‐MSH, and β‐endorphin. Plasma cortisol strongly inhibits ACTH secretion by the pars distalis, but has a much smaller effect on peptides secreted by the pars intermedia, which are under tonic dopaminergic inhibitory control. Plasma cortisol and ACTH concentrations may be modestly elevated in horses with pars intermedia adenomas. The cortisol levels often lack the normal diurnal rhythm. The modest elevations of plasma ACTH appear to be due to the different processing of POMC in tumors derived from cells of the pars intermedia. This may explain the normal or slightly elevated blood cortisol levels and normal or mildly hyperplastic adrenal cortices observed in some horses with adenomas of the pars intermedia.16 The plasma and tumor levels of pars intermedia–derived peptides (CLIP, α‐MSH, β‐MSH, and β‐endorphin) are disproportionately elevated (40 times or more) compared to those of ACTH, apparently as the result of selective posttranslational processing of POMC similar to the normal pars intermedia. Immunoreactive peptides have been found in pars intermedia tumor extracts that have a larger molecular weight than those present in normal pituitary tissue.17 The larger peptides may be derived from improper intranuclear processing of POMC mRNA. Horses have a marked seasonal variation in secretion of α‐MSH and ACTH from the pars intermedia. Serum concentrations of α‐MSH and ACTH are significantly greater in the autumn months. Therefore, control ranges of the hormone concentrations must be adjusted depending on the season. The sensitivity of serum α‐MSH and ACTH concentrations for diagnosis of early PPID is therefore greater in the autumn.16 Other tests that have value are endogenous ACTH, α‐MSH, thyrotropin‐releasing hormone (TRH) response test, combined hormone response tests, and measurement of ACTH after oral administration of domperidone.7 These tests may prove helpful in recognizing PPID in its early stages. The dexamethasone suppression test is useful for more advanced cases and it is easy to perform. Basal cortisol is not diagnostic and loss of diurnal secretion of cortisol is controversial. ACTH stimulation is not a useful test as it recognizes <20% of the cases likely because adrenocortical hyperplasia is not a prominent feature of PPID. Endogenous ACTH is also used to diagnose PPID. Using a 10–50 pg/mL reference range and a cut‐off of >55 pg/mL indicates PPID with a disease range of 104–1000 pg/mL. Variables include the cut‐off value (critical), time of day, and different reference ranges for ponies versus horses. It is imperative to use values set by the reference lab that analyzes the samples. Although this is a simple one‐collection procedure it does not recognize horses in the early stages of PPID and misses some horses in the late stages of the disease. Therefore basal ACTH is not ideal and provocative testing is recommended. Assays for ACTH should be validated for horses. Measurement of α‐MSH may be better than ACTH in horses because MSH is produced primarily in the pars intermedia and ACTH is primarily secreted from the pars distalis. Plasma α‐MSH hormone concentration >91 pmol/L is considered diagnostic of PPID. However, there is seasonal variation in mean concentrations and ranges. Therefore additional guidelines considered diagnostic for PPID are if plasma α‐MSH is >19 pmol/L in spring, summer, or winter or is >148 pmol/L in the fall. Plasma α‐MSH and ACTH concentrations increase as daylight decreases, from maximum daylight hours to 12 hours of daylight, but serum insulin does not fluctuate. This occurs in normal horses and ponies and those with PPID, hence the season of the year should be considered when interpreting results. Ambiguous results can be repeated at a later time, even in a different season. Horses and ponies receiving pergolide (dopamine receptor agonist) have a reduced increase in α‐MSH and lower plasma ACTH. Another diagnostic approach is to measure α‐MSH after a dexamethasone suppression test (DST). An α‐MSH >90 pmol/L after a DST is the cut‐off value between normal horses and horses with PPID. The pars intermedia is partially regulated by dopaminergic input from neurons in the hypothalamus and loss of dopaminergic inhibition is hypothesized to stimulate the pars intermedia, causing the hyperplastic lesions that lead to neoplasia and PPID syndrome. Domperidone is a synthetic benzimidazole used to treat fescue endophyte agalactia in mares and it blocks dopamine receptors. Therefore, the correct dose of domperidone should block dopamine receptors and permit melanotrophs to release the pars intermedia peptides α‐MSH, β‐endorphin, CLIP, and ACTH and the concentrations of these substances should be greater in horses with PPID than in normal horses. Basal ACTH is not consistently increased in horses with PPID; however, domperidone administration increased ACTH in horses with pituitary lesions characteristic of PPID.18 Adenomas of the pars intermedia in dogs produce only a moderate enlargement of the pituitary gland. The pars distalis is readily identifiable and sharply demarcated from the anterior margin of the neoplasm. The tumor may extend across the residual hypophyseal lumen and result in compression atrophy, but usually the posterior lobe is incorporated within the tumor and the infundibular stalk is intact. Degenerative changes within the neoplasm are minimal. Adenomas of the pars intermedia in horses result in symmetrical enlargement of the hypophysis. Small adenomas often occur concurrently with nodular or diffuse hyperplasia of the pars intermedia (Figure 18.3B). Large adenomas extend out of the sella turcica and may severely compress the overlying hypothalamus (Figure 18.3C). The optic nerves are often displaced and compressed by the tumor; however, visual deficits are noted infrequently. The adenomas are yellow to white, multinodular, and incorporate the pars nervosa. On sectioning of the pituitary mass, multiple areas of hemorrhage are often present, and the pars distalis can be identified as a compressed subcapsular rim of tissue on the anterior margin (Figure 18.3D,E). Adenomas of the pars intermedia in horses are partly encapsulated and sharply delineated from the compressed parenchyma of the pars distalis. The tumors are subdivided into nodules or compartments by fine septa of connective tissue that contain numerous capillaries. Areas of hemorrhage and necrosis are infrequent, although hemosiderin‐laden macrophages may be present within the connective tissue septa and pars nervosa (Figure 18.3E). The tumor cells are arranged in cords and nests along the capillaries and connective tissue septa. Tumor cells are large, cylindrical, spindle shaped or polyhedral, and have an oval hyperchromatic nucleus (Figure 18.3F). The histological pattern is often reminiscent of the prominent pars intermedia of normal horses. Cuboidal tumor cells often form follicular structures that contain dense eosinophilic colloid. In other areas the spindle‐shaped cells palisade around vessels. The cytoplasm is lightly eosinophilic and granular. Mitotic figures are uncommon. The compressed remnant of pars distalis is atrophic but contains granulated acidophils and basophils. The neurohypophysis is often infiltrated by an extension of neoplastic cells, compressed, and replaced by fibrous astrocytes and hemosiderin‐laden macrophages (Figure 18.3D,E). The hypothalamus is also compressed to varying degrees, depending upon the size of the adenoma, and has increased glial cells and a loss of nerve cell bodies. Lesions in the pars intermedia (PI) have been graded (normal, focal hyperplasia, diffuse adenomatous hyperplasia, microadenoma, and adenoma), which correlated with the circulating concentration of pituitary hormones.18 Adenomas of the pars intermedia in dogs appear to arise from the lining epithelium of the residual hypophyseal lumen covering the infundibular process. They are relatively small and more strictly localized than corticotroph (chromophobe) adenomas in dogs arising in the pars distalis. Adenomas of the pars intermedia extend across the residual hypophyseal lumen to compress the pars distalis and are sharply demarcated from the pars distalis, but they are not encapsulated. The histological appearance is strikingly different from adenomas arising in the pars distalis in that there are numerous large colloid‐filled follicles interspersed between nests of chromophobic cells of varying size (Figure 18.4A). The follicles are lined by simple columnar epithelium, which is partly ciliated and contains interspersed mucin‐secreting goblet cells. The follicular colloid is densely eosinophilic and periodic acid–Schiff (PAS) positive. The nests of cells between the follicles are primarily chromophobic, but an occasional acidophilic or basophilic cell may be present. Figure 18.4 Functional pars intermedia (PI) adenoma from a dog with Cushing’s disease. PI adenomas grow centrally in the pituitary gland and compress the pars distalis as they grow. (A) The chromophobic tumor cells are small to large, round to polyhedral, with oval nuclei, and frequently form colloid‐filled follicles lined by cuboidal to low columnar epithelium that may be partially ciliated. (B) ACTH IHC. Tumor cells are lightly to intensely stained for ACTH. Lightly stained tumor cells have abundant cytoplasm and likely are actively secreting ACTH. (C) α‐MSH IHC. The tumor cells have diffuse, moderate to intense cytoplasmic staining for α‐MSH. Endocrinologically active (ACTH‐secreting) adenomas of the pars intermedia in dogs have prominent large corticotrophs with abundant eosinophilic cytoplasm and more widely scattered follicles. Dense bands of fibrous connective tissue are occasionally interspersed between the follicles and nests of chromophobic cells, particularly in the endocrinologically inactive adenomas of the pars intermedia. Mitotic figures are observed infrequently. The neoplastic cells compress and frequently invade the pars nervosa and infundibular stalk. Electron microscopy of adenomas of the pars intermedia in horses reveals numerous secretory granules in the cytoplasm. Their mean diameter is approximately 300 nm, and they have a closely applied limiting membrane. The rough endoplasmic reticulum and Golgi apparatus are particularly well developed. Immunocytochemical staining of adenomas of the pars intermedia are similar to that of the non‐neoplastic equine pars intermedia.19 There is a strong diffuse cytoplasmic staining for pro‐opiomelanocortin (POMC), a moderately strong staining for α‐MSH and β‐endorphin, a weak staining for ACTH, and negative immunostaining for prolactin, glial fibrillary acidic protein, and neuron‐specific enolase (Figure 18.3G,H). The immunocytochemical findings support the biochemical studies that indicate horses with PPID develop a unique clinical syndrome that is the result of hypothalamic and neurohypophyseal derangement as well as an autonomous production of excess amounts of POMC‐derived peptides (Figure 18.5). The clinical syndrome in horses with pituitary tumors is distinctly different from that in Cushing’s disease, which occurs in dogs, cats, and human patients. Corticotroph adenomas in dogs and humans associated with Cushing’s disease are characterized by variable immunostaining for ACTH and weak to moderate immunostaining for α‐MSH (Figure 18.4B,C). Some corticotrophs that contain numerous cytoplasmic secretory granules will stain intensely for ACTH, but many will have modest staining due to little storage of preformed hormone and rapid secretion. Figure 18.5 The precursor of ACTH and related peptides, pro‐opiomelanocortin (POMC) is processed differently in the pars distalis and pars intermedia. Plasma cortisol exerts primary negative feedback control on the pars distalis, whereas the pars intermedia is predominantly under dopaminergic control. ACTH, adrenocorticotropic hormone. LPH, lipotrophic hormone. MSH, melanocyte‐stimulating hormone. CLIP, corticotropin‐like intermediate peptide. Neoplasms derived from granulated acidophils are uncommon in all domestic animal species, but are common in many strains of adult rats. Acidophil adenomas and rarely adenocarcinoma have been reported in dogs, sheep, and cats.20–22 Acidophil adenomas in sheep are rare, may attain considerable size, and remain confined to the sella turcica. Sheep have a complete diaphragma sella separating the pituitary from the brain (Figure 18.6A). The remaining adenohypophysis and neurohypophysis are compressed severely, and the sella turcica is enlarged and deepened due to pressure‐induced osteolysis (Figure 18.6B). Increased development of mammary tissue and galactorrhea has been observed in sheep with acidophil adenomas, suggesting an overproduction of prolactin by the tumor cells. Figure 18.6 Acidophil adenoma, ewe. (A) Note the severe compression of the pars distalis and overlying brain. Size is not a useful criterion for malignancy in pituitary tumors. Large adenomas can lead to loss of brain tissue due to expansile growth and compression. (B) Erosion of sella turcica (hypophyseal fossa of the sphenoid bone). The adenoma remained confined to the sella turcica due to the complete diaphragma sellae. Tumor growth enlarged and deepened the sella turcica. Scales = 1 cm. A spectrum of clinical problems have been associated with acidophil adenomas in cats, including diabetes mellitus, diabetes insipidus, cranial nerve deficits, and muscular atrophy. Clinical laboratory evaluation often reveals acidosis, hyperglycemia and glycosuria, and resistance to insulin therapy. Acidophil adenomas in cats with clinical signs of diabetes mellitus have degranulation and vacuolation of the beta cells of the pancreatic islets. This suggests that the tumors were secreting excess growth hormone, which resulted in a downregulation of insulin receptors and resistance to the action of insulin. Feline acidophil adenomas have been associated with insulin‐resistant diabetes mellitus and acromegaly and localization of growth hormone in the cytoplasm of tumor cells.21,23 Cats with growth hormone‐secreting pituitary adenomas also develop degenerative arthritis with joint cartilage proliferation and chronic renal disease associated with periglomerular fibrosis and mesangial proliferation in the glomerulus. Acidophil adenomas enlarge the pituitary gland and indent the overlying hypothalamus to varying degrees (Figure 18.7A). The enlarged hypophysis is composed of irregular columns of acidophils interspersed between numerous blood‐filled sinusoids. The fibrous stroma is sparse. Although the degree of cytoplasmic granulation of acidophils varies from cell to cell, the predominating type of neoplastic acidophil usually contains many secretory granules. The nuclei of the densely granulated acidophils are small, oval, and hyperchromatic. Sparsely granulated (chromophobic) cells are often interspersed between the densely granulated acidophils (Figure 18.7B). Their cytoplasm is more abundant and lightly acidophilic. The nucleus is large, round, and vesicular. Mitotic figures are observed infrequently. Secretory granules of the acidophils are evident on H&E‐stained sections but are more readily visualized as bright red granules when stained either with acid fuchsin–aniline blue or Crossman’s modification of Mallory’s trichrome stain. Orange‐G stains the granules an intense yellow‐orange, but they are PAS negative (Figure 18.7B). Figure 18.7 Acidophil adenoma associated with acromegaly, cat. (A) The adenoma has caused severe atrophy of the normal hypophysis and has protruded dorsally into the brain. Note the characteristic white color of the tumor. The dark regions dorsally are secondary hemorrhage and necrosis. This tumor was considered benign because the dorsal extension of the tumor was due to expansile growth and not invasion of the brain tissue. In addition, there is a meningioma in the dilated ventricle dorsal to the acidophil adenoma. (B) Slidders OFG (orange, fuchsin, green) stain. The neoplastic acidophils stain orange. The intensely stained cells have abundant cytoplasmic secretory granules. The moderately stained cells are likely in the active phase of growth hormone secretion. Colloid‐containing follicles lined by follicular cells are found occasionally within acidophil adenomas in dogs. The colloid is intensely PAS positive. Numerous sinusoids are distended with erythrocytes, detached neoplastic cells, and large masses of fibrin. The pars nervosa and infundibular stalk are compressed to varying degrees, partly replaced by fibrous astrocytes, and infiltrated at the periphery by neoplastic cells; however, this limited extension of neoplastic cells into adjacent parts of the pituitary gland is not interpreted as a criterion of malignancy. Two types of acidophils have been found within pituitary acidophil tumors. The predominating type of acidophil is smaller and contains many secretory granules. The plasma membranes of adjacent cells are relatively straight with uncomplicated interdigitations and are connected by an occasional desmosome. The Golgi apparatus is comparatively small and associated with few pro‐secretory granules. The rough endoplasmic reticulum is composed of small, flattened membranous sacs with attached ribosomes. A few mitochondria are distributed randomly throughout the cytoplasm. The less common type of neoplastic acidophil has a greater cytoplasmic and nuclear area, and the cytoplasm contains numerous organelles but few mature secretory granules. The rough endoplasmic reticulum is extensive and consists of aggregates of lamellar arrays of granular membranes. The Golgi apparatuses are prominent and are composed of agranular membranes associated with numerous small pro‐secretory granules. Mitochondria are more numerous in this type of acidophil. These hypertrophied acidophils are considered to be actively synthesizing and secreting cells. Immunoreactive prolactin cells occur in small groups of large polygonal cells with prominent granules in the ventrocentral and cranial parts of the normal canine pars distalis. A diffuse increase in this population of cells occurs in pregnant animals near parturition. Growth hormone‐secreting cells are present singly along capillaries in the dorsal region of the pars distalis near the pars intermedia. They are small, round to oval, and have fine cytoplasmic granules. Somatotrophs frequently undergo diffuse hyperplasia and hypertrophy in old dogs, especially females. Pituitary carcinomas are rare compared with pituitary adenomas, but they have been seen in dogs and cows.24,25 These carcinomas are usually endocrinologically inactive, but may cause significant functional disturbances by destruction of the pars distalis and neurohypophysis, leading to panhypopituitarism and diabetes insipidus. The designation of carcinoma is based primarily on definitive invasion of the tumor into the CNS or adjacent structures combined with hemorrhage, necrosis, and anaplasia. Metastases are reported, but are exceedingly rare. Pituitary carcinomas are large, extensively invade the overlying brain, and aggressively infiltrate into the sphenoid bone of the sella turcica (Figure 18.8A,B). Metastasis is rare but occurs in regional lymph nodes or to distant sites, such as the spleen, kidney or liver (Figure 18.8C). Figure 18.8 Pituitary chromophobe carcinoma, dog. (A) Extensive dorsal invasion into the brain. (B) Invasion into the sphenoid bone of the sella turcica. (C) Metastasis to the liver (arrowheads). Metastasis is rare even with pituitary carcinomas that are invasive to the brain or sphenoid bone. Malignant tumors of pituitary chromophobes are highly cellular and often have large areas of hemorrhage and necrosis. Giant cells, nuclear pleomorphism, and mitotic figures are encountered more frequently than in chromophobe adenomas; however, pituitary cytology is not a dependable criterion of malignancy. Invasion of neoplastic cells into the adjacent sphenoid bone, vascular invasion with formation of tumor cell thrombi, extensive aggressive invasion (not just extension along lines of least resistance) into the overlying brain, and establishment of metastases at distant sites are criteria for the diagnosis of pituitary carcinoma. Limited extension of neoplastic cells into the adjacent pars nervosa and infundibular stalk are observed frequently with larger pituitary adenomas. Craniopharyngioma is a benign tumor that is derived from epithelial remnants of the oropharyngeal ectoderm of dorsal extensions of the craniopharyngeal duct (Rathke’s pouch). They occur in animals younger than those with other types of pituitary neoplasms and are present either in a suprasellar or infrasellar location. Craniopharyngiomas are one cause of panhypopituitarism and dwarfism in young dogs resulting from a subnormal secretion of somatotropin and other trophic hormones beginning at an early age, prior to closure of the growth plates; however, most pituitary neoplasms of this type develop in young adult (2‐ to 4‐year‐old) dogs. Malignant craniopharyngiomas have been reported in two middle‐aged cats.26 It has been proposed that pleomorphic neoplasms in the suprasellar region of younger dogs be classified as germ cell tumors rather than craniopharyngiomas.27 The diagnosis of germ cell tumors was based upon three criteria: (1) midline suprasellar location, (2) presence within the tumor of several distinct cell types (one population resembling a seminoma or dysgerminoma and others suggesting teratomatous differentiation into secretory glandular and squamous elements), and (3) positive staining for α‐fetoprotein. The source of the germ cells is unknown but various germ cell tumors are reported in the brain (such as dysgerminomas and teratomas). The clinical signs are due to the large size of this type of pituitary tumor and are usually a combination of several factors, including (1) lack of secretion of pituitary trophic hormones resulting in trophic atrophy and subnormal function of the adrenal cortex and thyroid gland (Figure 18.9A), gonadal atrophy and, occasionally, a failure to attain somatic maturation due to a lack of growth hormone secretion, (2) diabetes insipidus, (3) deficits in cranial nerve function, and (4) CNS dysfunction due to extension into the overlying brain. Figure 18.9 Craniopharyngioma in a young (3‐year‐old) female dog with severe nervous system clinical signs. (A) Dorsal extension and compression of the hypothalamus and thalamus. The large neoplasm has incorporated the adenohypophysis and neurohypophysis, resulting in severe secondary trophic atrophy of the adrenal cortex. The adrenal glands consist predominantly of medulla surrounded by a thin rim of cortex composed of capsule and zona glomerulosa. The zonae fasiculata and reticularis are severely atrophic. Although the thyroid follicular cells were flattened and atrophic, the overall gland size was within normal limits due to distension of the follicles with colloid. The disparity in sizes of the thyroids is not significant. Scale = 1 cm. (B) The craniopharyngioma consists of cords and solid areas of squamous epithelial cells with eosinophilic cytoplasmic keratinization and fragments of nuclear chromatin. The neoplastic epithelial cells are large and pleomorphic with vesicular nuclei and prominent nucleoli and the cells frequently have discrete vacuoles in their cytoplasm. There is an admixture with smaller cuboidal to polyhedral cells. H&E. (C) The cords of keratinizing epithelial cells stain intensely for keratin. Note that the keratin IHC accentuates visualization of the intracytoplasmic vacuoles. This tumor did not metastasize, which is typical of craniopharyngioma. Craniopharyngiomas are often large and grow along the ventral aspect of the brain where they can incorporate several cranial nerves and destroy much of the pars distalis and pars nervosa. In addition, they may extend dorsally into the hypothalamus and thalamus (Figure 18.9A); however, dorsal growth of the tumor is not considered to be evidence of malignancy but rather extension along lines of least resistance. Craniopharyngiomas have alternating solid and cystic areas. The histological characteristics of craniopharyngiomas are distinctive and unique for an intracranial tumor on the ventral aspect of the brain. The solid areas are composed of nests of epithelial cells (cuboidal, columnar, or squamous cells) with prominent focal areas of keratinization (Figure 18.9B,C) and occasional mineralization that compress the overlying hypothalamus. The areas of keratin accumulation are densely eosinophilic and are frequently associated with fragments of nuclear chromatin. The neoplastic epithelial cells are large and pleomorphic with vesicular nuclei and prominent nucleoli, and they frequently have discrete vacuoles in their cytoplasm (Figure 18.9B,C). Often there is an admixture with smaller cuboidal to polyhedral cells that have acidophilic secretory granules that stain positive for either prolactin or growth hormone by immunocytochemistry. The cystic spaces are lined by either columnar or squamous cells and contain keratin debris and colloid. Colloid‐containing follicles may be formed that are lined either by cuboidal or by columnar cells and contain variable amounts of eosinophilic colloid. An alternative interpretation is that these suprasellar pleomorphic neoplasms in dogs are of germ cell origin and that the secretory (glandular) and squamous elements represent teratoma differentiation.27 Additional cases need to be studied utilizing a spectrum of immunocytochemical stains for the protein and glycoprotein pituitary hormones, α‐subunit, chorionic gonadotropin, α‐fetoprotein, and placental ALP to determine which of the pleomorphic neoplasms in the suprasellar region are derived from remnants of the oropharyngeal epithelium of the craniopharyngeal duct and which have their origin from multipotential germ cells. Despite their relatively large size, squamous differentiation and heterogeneous cytology these tumors are invariably benign in dogs. In the two cats reported with craniopharyngiomas, they were classified as malignant due to local bone invasion and anaplasia. Tumors composed of granulated basophils are one of the rarest pituitary tumors in animals. Cushing’s disease in humans was initially attributed to hypersecretion of ACTH by small basophilic adenomas in the pars distalis. Current evidence suggests they are a possible cause for a small percentage of patients with Cushing’s disease in humans. Early reports on corticotropin‐secreting pituitary tumors in dogs with hyperadrenocorticism reflected this concept and considered them to be basophil adenomas. Corticotroph (chromophobe) adenomas of the pars distalis and pars intermedia are responsible for the great majority of cases of Cushing’s‐like disease in dogs. Basophil adenomas in humans may secrete thyrotropin (TSH), resulting in bilateral enlargement of both thyroid lobes (goiter). Serum thyroxine, triiodothyronine, and TSH are elevated and responsive to TRH. The neoplastic cells contain small secretory granules (diameter <150 nm) with prominent rough endoplasmic reticulum and Golgi apparatuses, characteristic of pituitary thyrotrophs. The pituitary gland is occasionally either partially or completely destroyed by metastatic tumors from distant sites. Examples include lymphoma of cattle and dogs, malignant melanoma of horses and dogs (Figure 18.10), transmissible venereal tumor, and adenocarcinoma in the mammary gland of dogs. The easiest way to differentiate metastasis to the pituitary from a primary pituitary neoplasm is to identify the primary site distant to the pituitary gland and compare the histology of the neoplasms. IHC for pituitary‐specific hormones and nonpituitary tumor markers can also be used. Figure 18.10 Equine brain (fixed). Metastatic melanoma to the pituitary gland. Most tumors in the pituitary are primary; metastases usually are incidental findings. In addition, the pituitary gland may be destroyed by local infiltration or compression from adjacent neoplasms, such as an osteosarcoma of the sphenoid bone, ependymoma arising in the infundibular recess of the third ventricle, malignant meningioma (Figure 18.11), and a glioma (infundibuloma) of the infundibular stalk. Figure 18.11 Malignant meningioma (arrowheads), dog, arising on the ventral aspect of the brain that invaded into the brain and pituitary gland. Adenomas of the adrenal cortex occur most frequently in neutered domestic ferrets and old dogs (8 years and older) and sporadically in cats, horses, cattle, goats, and sheep.1–3 Adrenal cortical carcinomas occur less frequently than adenomas, and they occur most often in ferrets and dogs, and rarely in other species. Carcinomas develop in adult to older animals. Humans have a high incidence (5%) as adults of nonfunctional adenomas (incidentalomas) and carcinomas are very rare. In dogs, adenomas and carcinomas are often functional. The stem and proliferative cells of the adrenal cortex occur in the subcapsular region and junction of the zona glomerulosa and fasiculata.4 Differentiation of the cortical cells occurs in a linear manner in the cords of cells from the zona glomerulosa to the cortical medullary junction. ACTH from the pituitary gland induces expression of the melanocortin‐2 receptor (MC2R, also known as ACTH receptor) on the cortical cells that differentiate to produce glucocorticoids. The stem cells of the adrenal cortex are thought to be the origin of adrenocortical tumors that result from genetic and epigenetic alterations of the cortical cells.1 Gonadectomy increases pituitary secretion of luteinizing hormone (LH) due to reduced negative feedback from sex steroids. High levels of LH induce proliferation of adrenocortical stem cells and differentiation into sex steroid‐producing cells (particularly in ferrets and mice), which may represent the cells of origin of adrenocortical tumors in these animals.5 Neutered domestic ferrets have a high incidence of nodular adrenocortical hyperplasia, adenomas, and carcinoma.6 Castrated male goats are reported to have a higher incidence of cortical adenomas than intact males.7 Adrenocortical tumors that occur in gonadectomized animals often have expression of LH receptor and GATA4, which is a transcription factor specific for sex steroid‐producing cells. Adenomas and carcinomas of the adrenal cortex in dogs may be functional (endocrinologically active) and secrete excessive amounts of cortisol. There are multiple pathogenic mechanisms that result in the syndrome of cortisol excess (Figure 18.12G).8 The clinical signs of functional adrenal cortical tumors in dogs usually are the result of cortisol excess and are similar to those described previously for corticotroph (ACTH‐secreting) adenomas of the pituitary. The clinical picture of adrenal cortical carcinoma may be complicated by compression of adjacent organs by a large tumor, invasion into the aorta or posterior vena cava, and metastasis to distant sites. Ultrasonography and/or radiographic detection of adrenal enlargement with or without calcification has proven to aid in the diagnosis of adrenal neoplasms.9 In horses, adrenal tumors have been reported to be associated with endocrine disturbances. Figure 18.12 Adrenal cortical tumors and hyperplasia, dog. (A) Well‐demarcated functional adenoma from a dog (right). Note that the adenoma has the same texture and color of the normal non‐neoplastic cortex. Their yellow tan color is due to the lipid content in cortisol‐producing cells and can be used to help differentiate cortical and medullary adrenal tumors. The cortex of the contralateral adrenal (left) and the ipsilateral normal cortex have moderate atrophy due to cortisol production from the adenoma, which causes negative feedback to the pars distalis and decreased production of ACTH. The approximate normal ratio of cortex:medulla:cortex is 1:1:1. Bar = 5 mm. (B) Carcinoma (right) from a dog with Cushing’s syndrome. The carcinoma was functional and secreted an excess of cortisol that resulted in prominent cortical atrophy of the contralateral adrenal gland (lower middle). The pale nodule in the medulla of the atrophic adrenal was a nodule of hyperplasia and not a metastasis. The carcinoma had large regions of necrosis (collapsed red tissue). A longitudinal section of kidney is at the left. There were no metastases to the kidney (left). These tumors often metastasize to the liver and lungs. Bar = 1 cm. (C) Adenoma (arrowheads) from a dog with compression of the cortex and medulla and central fibrosis (F). This tumor was nonfunctional and there was no atrophy of the unaffected adrenal cortices. There is an incidental focus of mineralization above the letter F. (D) Central region of the adenoma in (C). Most of the neoplastic cells are similar to cells in the normal zona fasiculata, they have fine cytoplasmic fat vacuoles and are very well differentiated. Some of the neoplastic cells are spindle shaped with fewer cytoplasmic vacuoles and mild atypia. There are also bands of fibrous connective tissue (upper right). (E) Adenoma, dog. The adenoma occupies the majority of the adrenal gland. There are large regions of hemorrhage and dilated vascular spaces. These can mimic a hemangioma. (F) It can be challenging to identify the margin of an adrenocortical adenoma since the neoplastic cells often blend into the normal cortex and there may not be a dividing capsule. In this example, the adenoma (A) is on the left and the tumor cells have fewer cytoplasmic fat vacuoles and more eosinophilic cytoplasm. The normal zona fasciculata (ZF) is on the right. Some tumors will contain foci of extramedullary hematopoiesis with megakaryocytes. In this sample, some of the smaller dark cells are nucleated red blood cells. (G) Multiple pathogenic mechanisms of cortisol excess in dogs. (H) Nodular cortical hyperplasia in a dog with multiple extracapsular (arrowheads) extensions. These lesions are common, incidental and nonfunctional. There is no atrophy of the adjacent cortex. Inset: Higher magnification of extracortical hyperplasia. Adrenal tumors are recognized with high frequency (~20%) in ferrets that are neutered as adolescents (in the United States and Japan) and kept as household pets and living to a more advanced age (4–5 years of more).3 The adrenal enlargements are either bilateral (~15%) or unilateral (~85%) due to nodular hyperplasia (56%), adenoma (16%), or carcinoma (26%).10 However, there may be considerable variability between pathologists in the diagnosis of adrenal tumors in ferrets due to their pleomorphic growth patterns. In contrast to adrenal tumors in dogs that produce excess glucocorticoids, adrenal tumors in ferrets produce sex steroids. Clinical signs in ferrets include vulvar enlargement; bilaterally symmetrical alopecia, especially on the ventral abdomen and medial aspects of the rear legs; polyuria; and polydipsia. Adrenal tumors develop in adult ferrets (mean age 5 years), with females more frequently affected than males (sex ratio of 2:1 or greater) and the left side more commonly than the right. Other functional disturbances including anemia, thrombocytopenia, pyometra, endometrial hyperplasia, and squamous metaplasia of the prostate gland are consistent with an overproduction of estrogenic steroids. About one‐third of ferrets with adrenal cortical tumors also have neoplasms derived from the insulin‐producing beta cells of the pancreatic islets, which can be associated with hypoglycemia and elevated levels of serum insulin, resulting in seizures, lethargy, and weakness. The most consistent endocrinologic change in ferrets with adrenal tumors is an elevation in plasma levels of 17β‐estradiol. It is presumed that the 17β‐estradiol is produced by the tumor directly, but an alternative possibility would be that the adrenal tumors secrete androgenic steroids that are aromatized peripherally to estrogenic steroids. There is no increase in circulating 17β‐estradiol in response to exogenous ACTH, but plasma levels decrease following adrenalectomy. Plasma cortisol and corticosterone levels in ferrets with adrenal tumors are normal or below normal and the contralateral adrenal cortex is not atrophic, as would be expected if the adrenal tumor was secreting excess cortisol. However, the urinary cortisol:creatinine ratio has been reported to be elevated in ferrets with adrenal cortical tumors.11 This is probably due to the stress of the disease rather than secretion of excess cortisol by the neoplasm. The clinical signs in ferrets with adrenal cortical tumors can be effectively reversed by adrenalectomy. Primary hyperaldosteronism occurs infrequently in cats and rarely in dogs with adrenocortical adenomas or carcinomas that secrete excessive amounts of aldosterone.12 Clinical signs include hypertension, blindness, and weakness due to hypokalemic polymyopathy. Cortical adenomas usually are well‐demarcated single nodules in one adrenal gland, but they may be bilateral in approximately 10% of the dogs. Larger cortical adenomas are yellow to red, distort the external contour of the affected gland, and are partially or completely encapsulated. Adjacent cortical parenchyma is compressed, and the tumor may extend into the medulla (Figure 18.12A). Smaller cortical adenomas are more yellow or similar in color to the normal adrenal cortex because of the high lipid content. They are surrounded by mildly compressed cortex and may be difficult to distinguish from areas of nodular cortical hyperplasia observed frequently in old dogs. Nodular hyperplasia usually consists of multiple foci of various sizes in both adrenals with no evidence of encapsulation and is often associated with extracapsular nodules of hyperplastic cortical tissue extending into the periadrenal connective tissues and into the adrenal medulla. Nodular hyperplasia does not cause clinical disease or atrophy of the contralateral or ipsilateral adrenal cortex. Adrenal cortical carcinomas are larger than adenomas and may be more likely to develop in both glands (Figure 18.12B). In dogs they are composed of a variegated, yellow to brownish red, friable tissue that incorporates most or all of the affected adrenal gland. They are often fixed in location because of extensive invasion of surrounding tissues and the posterior vena cava, forming a large tumor cell thrombus. Carcinomas may attain considerable size in cattle (as much as 10 cm or more in diameter) and have multiple areas of mineralization or ossification. Functional (cortisol‐secreting) adrenal cortical adenomas and carcinomas are associated with profound cortical atrophy of the contralateral gland because of negative feedback inhibition of the pituitary ACTH secretion by the elevated blood cortisol levels (Figure 18.12B). The atrophic cortex consists primarily of the adrenal capsule and zona glomerulosa with few secretory cells remaining in the zonae fasciculata and reticularis. A similar parenchymal atrophy is present in the uncompressed cortex around functional adenomas. The severity of the atrophy can be mild to marked, most likely due to chronicity. The adrenal medulla appears expanded and relatively more conspicuous because of the lack of surrounding cortical parenchyma. Cortical adenomas are composed of well‐differentiated steroid hormone‐producing cells that resemble secretory cells of the normal zona fasciculata or reticularis (Figure 18.12C–F). Tumor cells are arranged in broad trabeculae or nests separated by small vascular spaces. The abundant cytoplasmic area of tumor cells is lightly eosinophilic, often vacuolated, and filled with many lipid droplets. Some tumor cells can be spindle shaped, often with fewer cytoplasmic vacuoles. Adenomas are partially or completely surrounded by a thin fibrous connective tissue capsule and a rim of compressed cortical parenchyma. The junction of an adenoma with normal cortex can be subtle in areas without a fibrous capsule (Figure 18.12F). Focal areas of mineralization, extramedullary hematopoiesis, and accumulations of fat cells may be found in cortical adenomas. Extramedullary hematopoiesis with megakaryocytes and erythroid and granulocytic colonies is a characteristic finding in canine adrenal cortical adenomas. Larger adenomas have areas of necrosis and hemorrhage with vascular proliferation near the center (Figure 18.12E). Nodular cortical hyperplasia and myelolipoma must be differentiated histologically from adrenal cortical adenomas. The presence of a partial or complete fibrous capsule surrounding one or two progressively expanding areas of proliferating cortical cells suggests an adenoma rather than nodular hyperplasia. Nodular hyperplasia appears as multiple small nodules of cortical tissue, non‐encapsulated and located most commonly outside the capsule (Figure 18.12H), next in the cortex, and sometimes in the medulla. Myelolipoma is a benign lesion encountered in the adrenal glands of cattle and nonhuman primates and infrequently in other animals. It is composed of well‐differentiated adipose cells and hematopoietic tissue, including both myeloid and erythroid elements. Areas of well‐differentiated lamellar bone formation may occur in myelolipomas. Although the origin of myelolipomas is uncertain, they appear to develop by metaplastic transformation of cells in the adrenal cortex or cells lining adrenal sinusoids. In addition, teratomas have been reported in the ferret adrenal gland, and are rare in other species.13 Adrenal cortical carcinomas are composed of more highly pleomorphic cells than adenomas, which are subdivided into groups by a fibrovascular stroma of varying thickness. The architecture of the affected adrenal is completely obliterated by the carcinoma. The pattern of growth varies between individual tumors and within the same carcinoma, resulting in the formation of trabeculae, lobules, or nests of tumor cells. Tumor cells usually are large and polyhedral with a vesicular nucleus, prominent nucleoli, and densely eosinophilic or vacuolated cytoplasm. Anaplastic carcinomas may have spindle‐shaped cells with a smaller and more lightly eosinophilic cytoplasm. Areas of hemorrhage within the tumors are common because of rupture of thin‐walled vessels. Invasion of tumor cells through the adrenal capsule into adjacent tissues and into vessels and lymphatics, forming emboli, is frequently detected in carcinomas of the adrenal cortex and when present makes the diagnosis of carcinoma straightforward. Adrenal adenomas and carcinomas in ferrets are typically pleomorphic, with small undifferentiated cells, larger well‐differentiated cortical cells with cytoplasmic lipid vacuoles, and variable degrees of spindle cell differentiation (Figure 18.13A,B).14 Some carcinomas develop myxoid differentiation with lumen‐like spaces containing alcian blue‐positive acid mucopolysaccharides (Figure 18.13B).15 IHC markers for the tumor cells include vimentin, α‐inhibin, synaptophysin, estrogen receptor, and GATA4 (Figure 18.13C,D). The neoplastic spindle cells may also differentiate to smooth muscle‐like cells and stain immunohistochemically positive for α‐smooth muscle actin and desmin. Figure 18.13 Adrenal cortical carcinoma, ferret. (A) Multinodular cortical carcinoma occupying the majority of the adrenal gland with regions of hemorrhage and dilated vascular spaces. Adrenal cortical tumors are common in ferrets and usually are associated with clinical signs of hyperestrogenism (e.g., alopecia, vulvar swelling, prostatic squamous metaplasia). (B) Adrenal cortical carcinoma with cords of small round cells and polygonal cells containing cytoplasmic fat vacuoles and follicle‐like structures containing basophilic mucin. (C) Adrenal cortical carcinoma immunohistochemically stained for the transcription factor GATA4. Most tumor cells are mildly to markedly positive for GATA4. (D) GATA4 IHC. The adrenal cortical carcinoma cells have marked nuclear positive staining. IHC for specific cytochrome P450 enzymes in adrenocortical tumors has the potential to characterize the type of differentiation of the tumor cells, especially in functional tumors.16 For example, tumors that secrete aldosterone would be expected to express aldosterone synthase (CYP11B2). Additional IHC markers for adrenocortical tumors include steroidogenic factor‐1 (SF‐1), steroid receptor co‐activator‐1 (SRC‐1), StAR protein (steroidogenic acute regulator protein), α‐inhibin, calreticulin, GATA6 (GATA4 in ferrets), Melan‐A, and synaptophysin. Tumor cells are typically negative for keratin and chromogranin A. Adrenal cortical adenomas usually are slow growing, well demarcated, relatively small tumors that may be associated with a hypersecretion of cortisol or infrequently other adrenal steroid hormones (e.g., aldosterone, androgens, or estrogens). Carcinomas of the adrenal cortex are larger, locally invasive, and metastasize to distant sites. They often invade through the thin wall of the posterior vena cava, forming a large tumor cell thrombus, and into the adventitial layer of the abdominal aorta in dogs and cattle. Metastases are found primarily in the liver, kidney, lungs, and mesenteric lymph nodes. It can be challenging to differentiate adrenal cortical adenomas from carcinoma in dogs, especially in biopsy specimens. Histologic features alone are often not sufficient, but sometimes the pathologist is restricted by the sample provided. Many cortical carcinomas are well differentiated, but can still have metastatic potential. Definitive criteria of malignancy are local invasion through the adrenal capsule into surrounding tissues, vascular invasion, and metastasis. Indicators of malignancy for adrenocortical carcinomas in dogs included size (greater than 2 cm), fibrosis, capsular invasion, trabecular growth pattern, decreased cytoplasmic vacuolation, hemorrhage, necrosis, and increased proliferation index (Ki67 positivity was 9% in carcinomas and 1% in adenomas).17 In addition, metastases, such as in the lungs or liver, can be difficult to identify as originating from the adrenal gland. IHC markers to help identify adrenal cortical neoplasms, such as metastases, include steroidogenic factor‐1 (SF‐1), steroid receptor co‐activator‐1 (SRC‐1), α‐inhibin, calreticulin, GATA4, Melan‐A, and synaptophysin. SF‐1 and SRC‐1 have a high degree of specificity and sensitivity for adrenocortical tumors. Differentiation of adrenocortical adenomas from carcinomas in ferrets is also challenging, since the neoplasms are often multinodular with different cell types and degrees of differentiation. Capsular invasion and metastasis (especially to the liver) are definitive criteria of invasion. Complete surgical excision of early invasive carcinomas often has a good prognosis with a low incidence of metastasis.18 Tumor‐like nodules of ectopic adrenal cortical tissue can occur in the region of the adrenal glands and in or on the surface of the gonads, kidney, liver or other abdominal organs. Ectopic adrenal tissue occurs most commonly on or near the equine gonads and infrequently in dogs and other species.19,20 If the ectopic adrenal tissue occurs on the testicle it can be located within the scrotum after the testicle normally descends. The nodules range from small microscopic foci to grossly visible single to multiple yellow tumor‐like nodules. These are typically incidental findings and histologically are composed of normal adrenal cortex in the usual layered arrangement as the normal gland with a fibrous capsule. They do not contain adrenal medullary tissue. The nodules may be submitted as surgically biopsies in order to identify their origin and confirm that they do not represent a neoplasm. It is possible for ectopic adrenal tissue to undergo neoplastic transformation to functional or nonfunctional cortical neoplasms in rare instances. Pheochromocytomas are the most common tumors in the adrenal medulla of animals, although other tumors may develop from the neuroectodermal cells, which differentiate into either secretory cells or sympathetic ganglion cells (Figure 18.14).1–4 Neuroblastomas are uncommon; they arise from primitive neuroectodermal cells, often in younger animals, and form a large intra‐abdominal neoplasm that may metastasize to peritoneal surfaces. Ganglioneuromas are rare; they usually are well‐differentiated small tumors that have sympathetic ganglion cells and neurofibrils. Figure 18.14 Histogenesis of tumors of the adrenal medulla. Pheochromocytomas develop most often in dogs, cattle, and horses and infrequently in other domestic animals.1–5 They are common in some strains of rats.1,6 Pheochromocytomas in dogs usually develop in middle‐aged to older dogs with no apparent gender or breed predisposition. Occasionally, pheochromocytomas may develop concurrently with calcitonin‐secreting C‐cell (ultimobranchial) tumors of the thyroid gland.7,8 This appears to represent a multicentric neoplastic transformation of multiple types of endocrine cells of neuroectodermal origin in the same individual and resembles the syndrome of multiple endocrine neoplasia (MEN) in human patients. Functional pheochromocytomas are associated with tachycardia, hypertension, edema, and cardiac hypertrophy, which are attributed to excessive catecholamine secretion. Arteriolar sclerosis and widespread medial hyperplasia of arterioles occur in dogs with pheochromocytomas that were associated with paroxysmal hypertension. Infrequently, hyperadrenocorticism may occur concurrently in dogs with pheochromocytoma.9 Hypercalcemia may occur in sporadic cases of pheochromocytoma.10 Norepinephrine is the principal catecholamine extracted from pheochromocytomas in dogs. This is similar to normal pups, where norepinephrine is the predominant catecholamine, but in adult dogs epinephrine predominates. The catecholamine content in pheochromocytomas from bulls has been found to be higher than in the normal adrenal medulla.11 Plasma and urinary metabolites of epinephrine or norepinephrine (metanephrine and normetanephrine, respectively) are increased in animals with functional pheochromocytomas.12,13 Many adrenal medullary tumors in animals are found as incidental findings at necropsy or surgery. However, malignant pheochromocytomas are often large and invade into the posterior vena cava, forming a intravascular tumor cell thrombus. The vena cava is distended greatly and partially occluded by the thrombus, leading to impaired venous return and potentially severe regional hemorrhage. In a study of 50 dogs with pheochromocytomas, local tumor invasion was present in 52%, regional lymph node metastasis in 12%, and distant metastases in 24%.2 Pheochromocytomas are tumors of chromaffin cells and are almost always located in the adrenal gland, although a few extra‐adrenal tumors have been found along the posterior aorta and vena cava in sites analogous with the organ of Zuckerkandl in humans. They usually are unilateral, but may be bilateral. Although size varies considerably, pheochromocytomas can be large (10 cm or more in diameter) and incorporate the majority of the affected adrenal. A small remnant of the adrenal gland usually can be found at one pole. Smaller tumors are completely surrounded by a thin compressed rim of adrenal cortex (Figure 18.15B,C).
Tumors of the Endocrine Glands
Hormone class
Primary site of action
Receptors
Solubility
Half‐life in blood
Plasma‐binding protein
Polypeptides and catecholamines
Plasma membrane
Transmembrane proteins
Aqueous
Minutes
None
Steroids and thyroid hormones
Nucleus
Nuclear proteins
Lipophilic
Hours to days
Specific binding proteins
TUMORS OF THE PITUITARY GLAND
Functional corticotroph (chromophobe) adenoma in pars distalis
Incidence
Clinical characteristics
Macroscopic pathology
Histopathology
Ultrastructural and immunocytochemical characteristics
Nonfunctional chromophobe adenoma in pars distalis
Incidence
Clinical characteristics
Macroscopic pathology
Histopathology
Adenoma of the pars intermedia
Incidence
Clinical characteristics
Macroscopic pathology
Histopathology
Ultrastructural and immunohistochemical characteristics
Acidophil adenoma of pars distalis
Incidence
Clinical characteristics
Histopathology
Ultrastructural and immunocytochemical characteristics
Pituitary chromophobe carcinoma
Incidence
Macroscopic pathology and histopathology
Craniopharyngioma (intracranial germ cell tumor)
Incidence
Clinical features
Macroscopic pathology
Histological characteristics
Basophil adenoma of pars distalis
Metastatic tumors to the pituitary gland
References
TUMORS OF THE ADRENAL GLAND
Tumors of the adrenal cortex: adenoma, carcinoma, myelolipoma
Incidence and pathogenesis
Clinical characteristics
Macroscopic pathology
Histopathology
Growth and metastasis
Ectopic adrenal cortex
References
Tumors of the adrenal medulla: pheochromocytoma, neuroblastoma, ganglioneuroma
Incidence and classification
Clinical characteristics
Macroscopic pathology
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