7.1 Introduction

The endocrine system is a collection of discrete organs and cells distributed throughout the body whose role is to maintain homeostasis among different organs of the body. They act by secreting their cellular products (hormones) directly into the bloodstream where they act on cells at distant sites (endocrine effects); at nearby sites (paracrine effects); or on the secreting cell itself (autocrine effects). The function of the endocrine glands is usually controlled by negative feedback loops, where the release of the hormone is controlled by circulating levels of the factors released by the target organs, or plasma levels of the secreted hormone itself. The controlling mechanisms involve interactions between the central nervous system, the endocrine glands and their target organs, so disruption of one component in the homeostatic axis can have knock-on effects on the endocrine gland, sometimes leading to proliferative changes in cellular components of the endocrine gland. A breakdown in the normal homeostatic mechanisms of the endocrine system, as a result of ageing, can lead to the hyperplastic or neoplastic changes commonly seen in senile rodents.

The anatomy and histology of the pituitary, adrenal, thyroids and parathyroids are presented below, but there are several sources of information (some of which are available online) that provide details of the basic anatomy of the mouse (Cook 1965, 1983; Hummel et al. 1966) and provide guidance on general necropsy and histology practices, which include descriptions of procedures to follow for the dissection and trimming of these tissues (Ruehl-Fehlert et al. 2003; Kittel et al. 2004; Knoblaugh et al. 2012). The endocrine portion of the pancreas is discussed in Chapter 3 with the exocrine pancreas.

7.2 Adrenals

The adrenals in the mouse are small, cream/pale tan in colour and can be hard to identify, so some authors recommend leaving them within the perirenal fat pad attached to the kidney during fixation and processing (Chapter 1). However, weighing of the adrenals requires them to be removed from the kidneys, and the surrounding fat trimmed away. Care needs to be taken not to damage the adrenal capsule during this process. The small size of the adrenals also makes them difficult to cut so that adequate sections of medulla are present. In preparation for processing, mouse adrenals should be retained in toto in cassettes with biopsy sponges to ensure that they are not lost during processing. To improve sectioning, mouse adrenals are best embedded longitudinally in toto without other tissues and sectioned to reach the optimal level, which presents the largest cut surface (Kittel et al. 2004) (Figure 7.1). It is possible to embed the adrenals attached to the cranial pole of the kidney but this makes it hard to produce adequate sections of both tissues demonstrating all relevant areas (cortex, medulla and papilla of the kidney, and cortex and medulla of the adrenal).

Figure 7.1 Normal cross section of mouse adrenal showing cortex and medulla.

7.2.3 Artefacts

Care is needed in sectioning the adrenal to ensure that the medulla is presented on the section. Tangential sections can give the appearance of variations in the proportion of cortex to medulla. Extension of medullary cells along the hilus through the cortex towards the outer cortex is a normal feature and does not represent a proliferative change (Capen et al. 2001, Frith et al. 2007) (Figure 7.2).

Figure 7.2 Section of adrenal illustrating extension of medullary cells along hilus towards outer cortex (arrow).

7.2.4 Anatomy and histology of the adrenals

The paired adrenal glands are ovoid in shape and located adjacent to the anterior pole of the kidneys. The left gland is typically heavier than the right; they are slightly larger in females than in males and the female adrenal is paler because of the presence of more lipids. The outer zona glomerulosa is composed of small basophilic cells below the cortical capsule, and the more eosinophilic cells of the zona fasciculata form columns extending towards the medulla (Nyska and Maronpot 1999; Bielohuby et al. 2007). The X-zone is a unique feature of the mouse adrenal; a layer of basophilic cells at the corticomedullary region that forms postnatally and is fully formed at weaning (Figure 7.3). In males the X-zone degenerates rapidly at puberty without vacuolation; in females the zone undergoes slow vacuolar degeneration and can be present for several weeks (Figure 7.4). The thickness and persistence of the X-zone can vary with strain of mouse. The zone disappears rapidly during pregnancy (Rosol et al. 2001).

Figure 7.3 X zone of a female CD-1 mouse appearing as a distinctive layer of basophilic cells between the zona fasciculata and the medulla.

Figure 7.4 Vacuolation of the x-zone in a female cd-1 mouse.

The medulla is composed of a homogeneous population of polyhedral chromaffin cells arranged in variably sized packets, ganglion cells, venules and capillaries, with conspicuous sinusoids. The cytoplasm of the chromaffin cells is finely granular and slightly more basophilic than the cells of the cortex. Chromaffin cells stain when labeled with antibodies for tyrosine hydroxylase, chromogranin and synaptophysin. The ganglion cells are scattered randomly within the medulla (Nyska and Maronpot 1999; La Perle and Jordan 2012).

The blood supply to the adrenal glands is formed by three groups of arteries, constituting a plexus within the capsule. An anastomizing network of capillary sinusoids form the vascular system of the cortex, which descends between the cortical cords, forming a second plexus in the lower cortex, which drains into small venules in the medulla, which converge into the central vein of the medulla. The medulla is supplied by small arterioles descending from the capsular plexus through the cortex into the medulla, which ramifies into a network of capillaries surrounding the medullary cells before draining into the central vein of the medulla. The cells of the medulla are therefore exposed to fresh arterial blood as well as blood rich in corticosteroids, which has an important influence on the synthesis of hormones by the adrenal medulla (Rosol et al. 2001).

Accessory adrenocortical tissue is frequently seen in close association with the adrenal capsule (Figure 7.5). This occurs more commonly in females than males. They usually exhibit two distinct zones of the adrenal cortex (zona glomerulosa and fasciculata) but medullary tissue is absent. The accessory tissue can undergo the same aging changes as the main adrenal gland (Taylor 2011). Accumulation of lipogenic pigment, or ceroid, at the corticomedullary junction is a common age-related change in many strains and stocks of mice, occurring more commonly in females than males (Figure 7.6 and Figure 7.7). Pigment is deposited in cortical cells and macrophages, and stains positively with the Periodic Acid Schiff (PAS) stain (Taylor 2011). The adrenal glands are described as a common site for deposition of amyloid in different strains of mice, particularly the CD-1 mouse (Brayton 2007; Frith et al. 2007). However, the occurrence of amyloidosis in CD-1 mice has shown time-related changes and can vary considerably between animal suppliers (Maita et al. 1988; Engelhardt et al. 1993; Taylor 2011).

Figure 7.5 Accessory adrenocortical tissue. These tissues have a normal cortical structure, and zona glomerulosa and fasciculata can be evident. Medullary chromaffin cells are never seen.

Figure 7.6 Corticomedullary pigment (arrows) in a control two year old female mouse.

Figure 7.7 PAS positive staining of corticomedullary pigment shown in Figure 7.6.

Other than hyperplasia of cortical subcapsular cells, proliferative changes including neoplasms are a relatively rare occurrence in most strains of mice (Nyska and Maronpot 1999; Brayton 2007). Subcapsular cell (spindle cell) hyperplasia is a common age-related change in the cortex in a number of strains of mice, more often in females than males. It rarely occurs before 3 months of age, but the number of subcapsular cells increases with age, forming focal, wedge-shaped accumulations, or proliferating diffusely along the outer cortex. Two types of subcapsular cell have been identified: type-A cells, fusiform in shape with scant basophilic cytoplasm, and type-B cells, larger, round cells with eosinophilic or vacuolated cytoplasm (Figures 7.8 and 7.9). There are strain differences in the occurrence of these changes, and also in the association of mast cells with subcapsular cell hyperplasia (Kim et al. 2000; Taylor 2011). Sex hormones also play a part in the development of subscapular cell hyperplasia, as gonadectomy leads to an increased incidence in males (Bernichtein et al. 2009).

Figure 7.8 Subcapsular cell hyperplasia—type A cells predominate.

Figure 7.9 subcapsular cell hyperplasia—type B cells predominate.

Focal hypertrophy/hyperplasia of the cells of the zona fasciculata is also seen occasionally (Nyska and Maronpot 1999; Capen et al. 2001) (Figure 7.10). Adrenal medullary hyperplasia occurs at a lower incidence in mice compared to rats but occurs as a focal or diffuse lesion. Focal hyperplasia is seen with an increased size of the affected cells with increased cytoplasmic basophilia (Figure 7.11). Diffuse changes involve an increased number and volume of all medullary cells (Nyska and Maronpot 1999; Capen et al. 2001).

Figure 7.10 Focal cortical hypertrophy. Discrete focus of enlarged cells with eosinophilic cytoplasm.

Figure 7.11 Focal medullary hyperplasia (arrow).

7.3 Pituitary

Seymour et al. (2004) recommend retention of the pituitary in the skull at necropsy, and sectioning of the decalcified skull transversely to include a cross section of the pituitary. (Figure 7.12) While the process of decalcification may not affect morphology with standard H&E staining it may limit the ability to use IHC on sections. An alternative is to fix the pituitary in situ at necropsy, which allows dissection with minimal damage prior to embedding and avoids decalcification (Mahler and Elwell 1999; Kittel et al. 2004). If the gland is to be weighed, however, removal at necropsy is necessary and care should be taken to avoid damage.

Figure 7.12 Pituitary gland in situ (arrow), located within the sella turcica.

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