| Some specific features of cytology preparations can provide a more accurate interpretation of the sample. Listed below are some common results and their interpretation. |
| These are seen in benign lipoma and mature body fat, which cannot be differentiated cytologically. Because fat cells are mesenchymal cells, they tend not to exfoliate well and are usually present in low numbers. |
| It is usually not possible to differentiate reactive spindle cells from those of spindle cell neoplasia. Absence of a reactive stimulus (eg, inflammation or hemorrhage) and a higher cell population with more and larger clumps of cells may indicate neoplasia. Greater variation in the neoplastic criteria indicates more aggressive behavior. |
| Keratin includes nucleated and terminally differentiated non-nucleated squamous epithelial cells. It can be a contaminant from the surface of the animal’s skin or the skin of any handlers of the sample and is a common artifact. It can also be sampled from cutaneous keratin-filled cysts, which are always benign and very common, particularly in dogs. Large, densely packed clumps of keratin in restricted areas in the center of the slide suggest that keratin is part of the lesion
rather than an artifact. |
| This is a common artifact of fine-needle aspiration but can also come from blood-filled spaces in the tissue, which can be non-neoplastic, such as hematomas or severe bruising, or neoplastic lesions, such as hemangiomas and hemangiosarcomas. The presence of spindle cells does not adequately differentiate neoplastic from non-neoplastic causes of hemorrhage (see
spindle cells, above). Use of a very fine needle (25 gauge) can help decrease blood contamination. |
| This is a common problem with cytology samples. If proper technique has been used, absence of cells may indicate mesenchymal cell proliferation (including lipomas), as these tightly adherent cells do not exfoliate well. |
| The most important of these is the
mast cell, because mast cell tumors are common in dogs. These are medium-sized cells with round nuclei. With Romanowsky stains, the granules are dark blue or purple, small (similar to bacteria), and usually found in large numbers in the cytoplasm. Less differentiated mast cells have fewer granules, but cytology is a very poor method of grading these tumors. The cells are fragile; large numbers of granules are released from damaged cells. In dogs in particular,
eosinophils are often present and occasionally may be the dominant cell type. Eosinophils are less commonly seen in other species. Mast cell tumors in horses have similar cytology to those in dogs, but mast cell tumors in cats often have cells that are smaller and more uniform, with less distinct granulation. |
|
Thyroid cells can also have dark granules, usually blue or black, called tyrosine granules. These are relatively small, low in number, and can be difficult to see. Black granules are associated with melanocytes and are seen in
melanomas. Cytology cannot differentiate benign from malignant melanomas but, in dogs, melanomas on haired skin are usually benign and those on nonhaired areas, such as the lips, feet, and mouth, are usually malignant. However, basal cell tumors also often contain cells with melanin. These tumors are usually benign and cytologically can sometimes be difficult to distinguish from melanomas. Melanin can also be seen in macrophages, sometimes in large amounts, but it is
usually in much larger clumps within the cytoplasm rather than fine granules that are found in melanocytes. Very fine magenta granules can be seen in
osteoblastic cells from osteosarcoma. Golden, granular material accumulates in the cytoplasm of macrophages following hemorrhage into soft tissues. |
| A large, single vacuole is seen in fat cells. In normal or benign cells, the nuclei are small and often indistinct; the cells are often folded like a collapsed ball. Smaller cells with larger, more prominent nuclei and some cytoplasm in addition to clear vacuoles are more suggestive of malignancy. Cells with multiple, small vacuoles with a foamy appearance include macrophages, sebaceous glandular cells, and salivary cells. These can be very difficult to differentiate. Biopsy
site and other clinical features can be a deciding factor in interpretation. |
| These cells include mast cells, plasma cells, lymphocytes, histiocytes, and cells of transmissible venereal tumors. Mast cells have very distinctive cytoplasmic granules and are usually easily distinguished except in the small number of cases that have few or no cytoplasmic granules. |
| Lymphoid cells classically have a very high nucleus:cytoplasm ratio, which is found in few other cells. When neoplastic,
lymphoma cells are medium to large in size, with a nucleus at least 1½ times the size of RBC. Nucleoli are often multiple and may be prominent. |
| Cells of a
histiocytoma are not especially histiocytic cytologically. They are round cells with a moderate amount of pale-staining cytoplasm. They are fairly uniform and have nuclei that are eccentric within the cell. Nucleoli are not prominent. Histiocytic cells are slightly more problematic. They are part of the cell line that includes macrophages and inflammatory and reactive cells, as well as highly malignant round cell tumors. Histiocytic cells tend to be larger than other
round cells and may be vacuolated, with more cytoplasm and nuclei that are oval or indented. Infiltrates of histiocytic cells are often problematic, even with full tissue biopsy examination, and always require histologic examination to determine malignancy. |
| Neoplastic plasma cells include
myeloma cells. These are usually very well differentiated and have most of the characteristics of normal plasma cells but, when neoplastic, are present in very large numbers, along with few other cell types. Benign nodular proliferations of plasma cells—
plasmacytomas—show more marked pleomorphism and often differ markedly from normal plasma cells. Many have a slightly histiocytic appearance and can be difficult to distinguish cytologically. |
| Even histologically, it can be very difficult to distinguish cells of transmissible venereal tumors. They tend to have a moderate amount of cytoplasm (more than lymphoblasts), often with small vacuoles. Nuclear chromatin is coarse with 1 or 2 fairly prominent nucleoli. Mitotic figures are often numerous, unlike most cytology preparations of neoplastic lesions, and they tend to show moderate variation in the nucleus:cytoplasm ratio. |
