NORMAL OR HYPERPLASTIC TISSUE

Normal and hyperplastic tissues are both composed primarily of mature cell types. Normal cells display uniformity in cellular, nuclear, and nucleolar size and shape. Cytoplasmic volume is usually high relative to the nucleus (Figs. 2-1 and 2-2). Hyperplasia is a nonneoplastic enlargement of tissue that can occur in response to hormonal disturbances or tissue injury. Hyperplastic tissue has a tendency to enlarge symmetrically in comparison to neoplasia. Cytologically, hyperplastic cells have a higher nuclear-to-cytoplasmic ratio than normal cells. Examples of hyperplastic responses include nodular proliferations within the parenchyma of the prostate (Fig. 2-3), liver, and pancreas (Fig. 2-4).

FIGURE 2-1 Normal skeletal muscle. Tissue aspirate. Dog. Numerous threadlike myofibrils compose each cell whose nucleus is small, condensed, and oval. Cross-striations, characteristic of skeletal muscle, are barely visible against the dark blue cytoplasm but seen in the magnified view (inset). (Wright-Giemsa; IP.)

FIGURE 2-2 Normal salivary gland. Tissue aspirate. Dog. The gland has uniform features of nuclear size, nuclearto-cytoplasmic ratio, and cytoplasmic content. (Wright-Giemsa; HP oil.)

FIGURE 2-3 Canine prostatic hyperplasia. Tissue aspirate. Dog. The presenting clinical sign in this case involves blood dripping from the prepuce. Cytologically, the nuclear size is uniform; however, the nuclear-to-cytoplasmic ratio is increased as indicated by the close proximity of nuclei to each other. (Wright-Giemsa; HP oil.)

FIGURE 2-4 Nodular hyperplasia of the pancreas. Tissue aspirate. Dog. Ultrasound examination reveals a hypoechoic mass in the area of the pancreas. Cytologically, hyperplastic parenchymal organs commonly display binucleation. (Wright-Giemsa; HP oil.)

CYSTIC MASS

Cystic lesions contain liquid or semisolid material. The low-protein liquid usually contains a small number of cells. These benign lesions may result from proliferation of lining cells or tissue injury. Examples include seroma (Fig. 2-5), salivary mucocele, apocrine sweat gland cyst, epidermal/follicular cyst, and cysts associated with noncutaneous glands such as the mammary gland or prostate (Fig. 2-6).

FIGURE 2-5 Seroma. Tissue aspirate. Dog. Blood-tinged fluid is removed from a swelling on the neck. Cytologically, low cellularity and low protein content is visible in the background. Large mononuclear cells with fine cytoplasmic granularity predominate along with low numbers of erythrocytes. (Wright-Giemsa; HP oil.)

FIGURE 2-6 Prostatic cyst. Histopathology. Dog. Cuboidal epithelial cells line large cystic spaces that represent dilated ducts. (H&E; LP.)

INFLAMMATION OR CELLULAR INFILTRATE

Inflammatory conditions are classified cytologically by the predominance of the cell type involved. Recognition of the inflammatory cell type often suggests an etiologic condition.

Purulent or suppurative lesions contain greater than 85% neutrophils; they are then classified by the presence or absence of degeneration affecting the neutrophil. Nondegenerate neutrophils are morphologically normal and predominate in relatively nontoxic environments such as immune-mediated conditions, neoplastic lesions (Fig. 2-7), and sterile lesions caused by irritants such as urine and bile. Degenerate neutrophils display nuclear swelling and decreased stain intensity termed karyolysis (Fig. 2-8), indicating rapid cell death in a toxic environment (Perman et al., 1979). Increased nuclear staining with coalescence of the nucleus into a single round mass and an intact cellular membrane characterizes pyknosis (Fig. 2-9), a slow, progressive change often within a relatively nontoxic environment. An end stage of cell death, termed karyorrhexis, may be seen cytologically as the result of pyknosis of hypersegmented nuclei (Fig. 2-10). Degenerate neutrophils predominate in bacterial infections, particularly gram-negative types. Under septic conditions, bacteria may be found intracellularly (Fig. 2-11).

FIGURE 2-7 Nondegenerate neutrophils. Synovial fluid. Dog. Nonseptic inflammation with well-segmented neutrophils appears secondary to adjacent neoplasia of the bone. (Wright-Giemsa; HP oil.)

FIGURE 2-8 Karyolysis, karyorrhexis. Tissue aspirate. Dog. Mild to moderate karyolysis of neutrophils is evident by the decreased nuclear stain intensity and swollen nuclear lobes. Pyknosis of multiple nuclear segments appear as dark, dense, round structures, termed karyorrhexis (arrow), in this case of bacterial dermatitis. (Wright-Giemsa; HP oil.)

FIGURE 2-9 Pyknosis. Chylous effusion. Dog. Chronic inflammation of this fluid produces neutrophil nuclei that have condensed into a large, often single, dark, round structure related to the slow progression of cellular change in a nonseptic environment. The pyknotic cell in this case also contains a second, smaller round nuclear fragment. (Wright; HP oil.)

FIGURE 2-10 Karyorrhexis. Tissue aspirate. Inflammatory response with evidence of multiple pyknotic nuclear segments in the center cell. (Wright-Giemsa; HP oil.)

FIGURE 2-11 Bacterial sepsis. Tissue aspirate. Dog. Markedly karyolytic neutrophils are present with intracellular and extracellular coccoid bacteria. Karyolysis is so severe that the cells are barely recognizable as neutrophils. (Wright; HP oil.)

Histiocytic or macrophagic lesions contain a predominance of macrophages, suggesting chronic inflammation (Fig. 2-12). In granulomas, activated macrophages that morphologically resemble epithelial cells are termed epithelioid macrophages. These cells may merge to form giant multinucleated forms (Fig. 2-13). The granulomatous lesions are often associated with foreign body reactions and mycobacterial infection.

FIGURE 2-12 Macrophagic inflammation. Tissue imprint. Dog. Nodular lung disease with numerous large mononuclear cells having abundant gray cytoplasm and many cells with multiple cytoplasmic vacuoles. (Wright-Giemsa; HP oil.)

FIGURE 2-13 Multinucleate giant cell. Tissue aspirate. Cat. Skin lesion with pyogranulomatous inflammation, including many giant cells related to the presence of fungal hyphae. Pictured is a cell with seven distinct nuclei and abundant granular blue-gray cytoplasm. (Wright-Giemsa; HP oil.)

Pyogranulomatous or mixed cell inflammatory lesions contain a mixture of neutrophils and macrophages (Fig. 2-14) that may include increased numbers of lymphocytes or plasma cells. This type of inflammation is often associated with foreign body reactions, fungal infections, mycobacterial infections, panniculitis, lick granulomas, and other chronic tissue injuries.

FIGURE 2-14 Pyogranulomatous or mixed cell inflammation. Chylous effusion. Dog. Chronic chylous effusion contains a variety of cell types, including nondegenerate neutrophils, vacuolated macrophages, small to medium lymphocytes, and two mature plasma cells. (Wright; HP oil.)

Eosinophilic lesions contain greater than 10% eosinophils in addition to other inflammatory cell types (Fig. 2-15). They are seen with or without mast cell involvement. This inflammatory response is associated with eosinophilic granuloma, hypersensitivity or allergic conditions, parasitic migrations, fungal infections, mast cell tumors, and other neoplastic conditions that induce eosinophilopoiesis.

FIGURE 2-15 Eosinophilic inflammation. Transtracheal wash. Cat. Clinical presentation of a chronic cough in this cat with suspected pulmonary allergy. Fluid contains 95% eosinophils. Pictured are several eosinophils that stain pink to blue-green and adhere to pink mucous material. (Wright-Giemsa; HP oil.)

Lymphocytic or plasmacytic infiltration is often associated with allergic or immune reactions, early viral infections, and chronic inflammation. The lymphoid population is heterogeneous, with small or intermediate-sized lymphocytes and plasma cells mixed with other inflammatory cells (see Fig. 2-14). A monomorphic population of lymphoid cells without other inflammatory cells present suggests lymphoid neoplasia.

RESPONSE TO TISSUE INJURY

Cytologic samples often contain evidence of tissue injury in addition to cyst formation, inflammation, or neoplasia. These changes include hemorrhage, proteinaceous debris, cholesterol crystals, necrosis, and fibrosis.

Hemorrhage that is pathologic can be distinguished from blood contamination encountered during the cytologic collection. Blood contamination is associated with the presence of numerous erythrocytes and platelets. Acute hemorrhage is associated with engulfment of erythrocytes by macrophages termed erythrophagocytosis (Fig. 2-16). Chronic hemorrhage is associated with active macrophages containing degraded blood pigment within their cytoplasm—for example, blue-green to black hemosiderin granules (Figs. 2-17 and 2-18) or yellow rhomboid hematoidin crystals (Fig. 2-18). Hemosiderin represents an excess aggregation of ferritin molecules or micelles. This form of iron storage becomes visible by light microscopy and stains blue with the Prussian blue reaction. Hematoidin crystals do not contain iron although they are formed during anaerobic breakdown of hemoglobin such as may occur within tissues or cavities. Hematomas often contain phagocytized erythrocytes if the lesion is acute, or hemosiderin-laden macrophages if the lesion is chronic.

FIGURE 2-16 Erythrophagocytosis. Cerebrospinal fluid. Cat. Many erythrocytes are in the background along with one large macrophage that engulfed numerous intact red cells. The cat had confirmed infection with feline coronavirus (feline infectious peritonitis). (Wright; HP oil.)