Commonly sampled internal organs include the liver, spleen, and thyroid gland often by ultrasound-guided biopsy. Artifacts commonly encountered involve ultrasound gel that appears as particulate eosinophilic material similar to stain precipitate.

Liver

Cytodiagnostic Groups for Liver Cytology

1.  Normal epithelial cells

2.  Cellular degeneration or injury

3.  Hyperplasia or adenoma

4.  Pigment abnormalities

5.  Inflammation

6.  Malignant neoplasia

7.  Hematopoietic tissue

Normal Hepatic Tissue

 Hepatocytes consist of clumps or sheets of large uniform cells. Cells are characterized by low nuclear to cytoplasmic ratios, lightly basophilic and granular cytoplasm, centrally placed round nucleus that has stippled chromatin and a prominent nucleolus. Occasionally cells are binucleated and nucleoli are multiple.

 Biliary epithelium consists of sheets of small uniform cells with high nuclear to cytoplasmic ratios. Large biliary ducts are lined by a simple columnar epithelium. Nucleoli are often indistinct.

 Mast cells and macrophages may be occasionally found in low numbers.

Hepatocellular Degeneration or Injury

 Hydropic change (vacuolar degeneration) produces a foamy appearance within the cytoplasm of hepatocytes that is the result of swelling of endoplasmic reticulum related to increased intracellular water. This may be seen in tissue anoxia and toxic hepatopathies.

 Fatty change appears as discrete clear vacuoles within the hepatocyte cytoplasm that is the result of accumulation of lipids that can freely coalesce. This appearance is often diagnostic for lipidosis.

 Glycogen accumulation occurs with altered glucose metabolism such as seen in steroid hepatopathy. Cytoplasm appears foamy, similar to hydropic change. Steroid induced change is most notable in the dog, but occasionally has been observed in the cat.

 Necrosis may occur as a result of toxicosis, infectious disease, or neoplasia. Cells appear indistinct, with loss of cellular detail.

 Fibrosis is related to increased connective tissue reaction to damage, such as that seen in cirrhosis, post-necrosis hepatopathy, or chronic inflammation.

 Amyloid deposition is an uncommon condition often related to a chronic inflammatory disease. Presence of Congo red positive eosinophilic amorphous material around hepatocytes is diagnostic.

Hepatic Hyperplasia (Regeneration) or Adenoma

Hepatic hyperplasia (regeneration) or adenoma are grouped together since they have a similar cytologic appearance.

 Frequent binucleation is noted within hepatocytes.

 Increased nuclear to cytoplasmic ratio indicates rapid growth.

 Mild to moderate anisocytosis and anisokaryosis is present.

 Increased cytoplasmic basophilia may be noted due to rapid growth.

 Increased frequency of intranuclear crystalline inclusions.

 Conditions to consider include nodular hyperplasia, toxic hepatopathy, hepatocellular adenoma, bile duct adenoma, and cirrhosis.

Hepatic Pigment Abnormalities

Hepatic pigment abnormalities are observed within hepatocytes appearing as shades of blue and green with routine stains. The etiology of these pigments may be differentiated by the cytochemical reactions.

 Biliary stasis within canniculi appears as green casts or granular material between hepatocytes. Conditions associated with bile pigment changes include cholangitis, liver flukes, lipidosis, steroid hepatopathy, toxic hepatopathy, nodular hyperplasia, and cirrhosis.

 Hemosiderosis is an overload condition in which iron appears as blue or blue-green coarse granular material that stains positive with Prussian blue. Chronic hemolysis and excessive iron supplementation are associated with hemosiderosis.

 Lipofuscin appears as blue-green granules on Wright-Giemsa stained preparations which represent degenerated lipids resulting from cellular aging.

 Copper accumulation appears blue-green material which stains positive with rubeanic acid. This may be a primary accumulation or secondary related to liver disease.

Hepatitis/Cholangitis

 Neutrophilic (suppurative) inflammation is associated with necrosis, bacterial infection, and feline suppurative cholangiohepatitis. Degenerate or nondegenerate neutrophils are increased over that found in peripheral blood.

 Lymphocytic or plasmacytic (nonsuppurative) inflammation is common in feline lymphocytic cholangiohepatitis. Lymphoid cells are small, well-differentiated forms associated with chronic disease that may be difficult to distinguish on cytology from a small cell lymphoma.

 Eosinophilic inflammation may be associated with liver flukes or mast cell tumor occurring within the liver.

 Pyogranulomatous inflammation consists of a mixed population of neutrophils and macrophages. This is associated with mycobacteriosis, histoplasmosis, and toxoplasmosis.

Malignant Neoplasms of the Liver

 Primary tumors include: hepatocellular carcinoma, bile duct carcinoma, and hemangiosarcoma.

 Secondary tumors include: myeloid (nonlymphoid) leukemias, intestinal carcinomas, and pancreatic islet cell tumor.

 Lymphoma and mast cell tumor may be primary or secondary.

Hematopoietic Tissue

 Extramedullary hematopoiesis resembles a mixed bone marrow cell population including erythroid, granulocytic, and megakaryocytic precursors. It is often related to a physiologic need, such as in bone marrow disease or hypoxic conditions.

 Myelolipoma is an uncommon tumor resembling extramedullary hematopoiesis, but also contains considerable lipid material. It is benign and often localized.

Spleen

Cytodiagnostic Groups for Spleen Cytology

1.  Normal tissue

2.  Hyperplasia or reactivity

3.  Inflammation

4.  Malignant neoplasia

5.  Hematopoietic tissue

Normal Splenic Tissue

 Sheets of normal mesothelium from the splenic capsular surface are encountered with incisional and excisional biopsies.

 Small lymphocytes predominate with occasional medium and large lymphocytes present. A few macrophages and plasma cells may be seen along with rare neutrophils and mast cells. Macrophages may contain small amounts of phagocytized debris, compatible with hemosiderin.

 Small amounts of reticular tissue with macrophages and stroma in an aggregated fashion.

Splenic Hyperplasia or Reactive Spleen

 Small lymphocytes still predominate but there is an increase in medium and large lymphocytes.

 Macrophages and plasma cells are commonly observed. Associated with the macrophages may be reticular stroma appearing as basophilic fibrillar or spindle shaped elements.

 Hemosiderosis may be more noticeable with large amounts of coarse dark granules.

 Increased numbers of mast cells and neutrophils may be observed.

 Hyperplasia may result from antigenic reaction to infectious agents or presence of blood parasites.

Splenitis

 An inflammatory response is likely associated with splenic hyperplasia.

 Macrophages often increase in number to systemic fungal infections, e.g., histoplasmosis, protozoal infections e.g., cytauxzoonosis, and leishmaniasis.

Extramedullary Hematopoiesis

 This was the most common cytologic abnormality in one study accounting for 24% of the patients.

 While precursors from all three cell lines may be observed, erythroid cells are the most common with metarubricytes, rubricytes, and prorubricytes present. Care must be taken as erythroid precursors and lymphoid precursors appear very similar.

 Conditions associated with extramedullary hematopoiesis include: chronic hemolytic anemias, myeloproliferative disorders, and lymphoproliferative disorders.

Neoplasia (Primary or Metastatic)

 In myeloproliferative disorders, expect to find immature hematopoietic cell types. Malignant histiocytosis presents with bizarre and immature macrophages, often with evidence of marked erythrophagocytosis.

 Lymphoid neoplasia includes lymphoma and plasmacytoma (extramedullary myeloma). A large granular cell lymphoma arises primarily from the spleen to infiltrate the blood but not typically the bone marrow.

 Mast cell tumor may be primary or secondary.

 Hemangiosarcoma is a common primary or metastatic neoplasm. Cells are large, individual, spindle to stellate with indistinct cytoplasmic borders. The cytoplasm is often vacuolated and basophilic.

 Other mesenchymal neoplasms that occur in the spleen include fibrosarcoma, leiomyosarcoma, and myelolipoma.

Thyroid

Thyroid Tumor

Thyroid tumors occur most frequently in the dog, cat, and horse and often present clinically as a subcutaneous mass on the neck, lateral to the trachea or near the thoracic inlet. Ectopic thyroid tumors are occasionally found at the base of the heart or base of the tongue. Approximately 90% of the thyroid tumors identified clinically in the dog are carcinomas. Aspirates from thyroid carcinomas often contain a large amount of blood contamination. The epithelial cell clusters appear as free nuclei embedded in a background of pale blue cytoplasm with infrequent appearance of cytoplasmic membranes or borders. Sometimes seen within the cytoplasm of epithelial cells is dark blue to black pigment, which is thought to represent tyrosine-containing granules. Amorphous pink material representing colloid may be associated with some clusters.

The nuclei of most endocrine tumors are round to oval with minimal anaplastic or malignant features and generally appear of uniform size. Cytologically, benign and malignant thyroid tumors appear similar. As most canine thyroid tumors are malignant, metastasis is common to the lungs. However, unlike the dog, the vast majority of tumors in the cat are benign adenomas or adenomatous hyperplasia.

References

1.  Baker R, Lumsden JH. Colour Atlas of Cytology of the Dog and Cat: Mosby, St. Louis; 2000

2.  Cowell RL, Tyler RD, Meinkoth JH. Diagnostic Cytology and Hematology of the Dog and Cat: Mosby, St. Louis; 2nd Ed. 1999

3.  Radin MJ, Wellman ML. Interpretation of Canine and Feline Cytology: Ralston Purina Company Clinical Handbook Series. The Gloyd Group, Inc, Wilmington, DE; 2001

4.  Raskin RE, Meyer DJ (eds). Atlas of Canine and Feline Cytology: WB Saunders Co, Philadelphia; 2001