Cytology of Subcutaneous Swellings, Skin Tumours, and Skin Lesions. Part II
Department Clin. Sci. Comp. Anim., Veterinary Faculty, Utrecht University, The Netherlands
Skin tumours and Skin Lesions
Skin tumours are usually recognizable as such. Sometimes the subcutis is included in the process or an inflammation or tumour from the underlying tissues is infiltrated in the skin. The differential diagnosis then becomes more extensive and more difficult. Usually cytological examination can still arrive at the diagnosis, provided that there are enough characteristic cells present.
Tumours of Individualized Round Cells
Several skin tumours are characterized cytologically by a uniform population of round tumour cells that have little or no apparent connection with each other. The FNAB from such tumours is usually cell rich because they, in contrast to mesenchymal tumours, release cells easily during aspiration. To this group of so-celled "discrete cell neoplasms" belong the mast cell tumours, malignant lymphomas, cutaneous plasmacytomas histiocytomas, melanomas, and the transmissible venereal tumours (TVT).
Mast cell tumours are usually immediately recognizable because of the presence of many purple cytoplasmic granules. The cells are large and round. The nucleus of the cell is often difficult to see because it is poorly stained and covered by granules that absorb so much stain. Sometimes large numbers of eosinophils and/or fibroblasts can be seen. In poorly differentiated mast cell tumours the purple granules are often less frequent present or even lacking. Cytology can, however, not be used for grading the tumours.
Differential diagnosis: There are mast cell tumours that contain very few granules. These can be difficult to diagnose. The presence of many eosinophils can give support to the presumptive diagnosis of mast cell tumour. Inflammatory processes can give differential diagnostic problems because they can also contain mast cells and eosinophils. In an inflammation, however, the number of other inflammatory cells is considerably greater than the number of mast cells. Melanomas could also be mistaken for mast cell tumours. Melanoma cells can like mast cells be round to oval and contain pigment granules. However, most melanomas also contain spindle-shaped tumour cells. In addition, the pigment granules in melanomas are variable in size and irregular in shape and they stain, depending on the thickness of the pigment layer, grayish-blue to greenish-black.
Melanomas are tumours composed of cells that produce melanin. The melanomas belong to the group of "round cell tumours without cell interconnections" because they largely meet the characteristic cytological features of these tumours. In addition to round-oval cells, however, there are usually some spindle-shaped cells and sometimes these dominate the picture. Very bizarre cell forms and giant cells can also occur. The amount of pigment in melanoma cells can vary markedly. The nucleus of the melanoma cell is sometimes barely visible because it is covered by melanin granules. These granules are blue to greenish-black, irregular in shape and variable in size. Melanomas can be malignant or benign. If the nucleus is visible it also may show definite malignancy criteria. The melanomas that hardly contain any pigment are almost always malignant. Such amelanotic melanomas are difficult as such to diagnose but a careful search of the preparation will often still reveal a few melanin-containing cells. Macrophages with phagocytized melanin granules may give an indication to the origin of the tumour.
Cutaneous malignant melanomas in cats can also be melanotic or amelanotic. Five types of melanomas can be distinguished: epithelioid, spindle, mixed, signet-ring, and balloon cell. Whereas all epithelioid, spindle, and mixed epithelioid/spindle cell types show pigmentation, signet-ring and balloon cell types are often amelanotic.
The pigmented pathological cells of melanomas are easy to differentiate from normal pigmented epithelial cells. Melanocytes and pigmented squamous cells have very uniform, rod-shaped granules. The nucleus may be degenerated or may have disappeared, but will certainly have no malignancy criteria. The difference between melanomas and mast cells has been discussed in the section on mast cell tumours. Melanocytes must also be differentiated from macrophages that have phagocytized melanin (melanophages) or contain hemosiderin. Melanophages usually contain coarse conglomerates of melanin as well as vacuoles. They are encountered especially in melanomas but also in inflamed lymph nodes and in some disorders of the skin. Hemosiderophages are found in old hematomas. These are macrophages having vacuoles in which iron pigment is stored. Like the pigment of melanoma cells, the color of this hemosiderin is blue to greenish-black. The presence of cells that contain bilirubin crystals and show erythrophagocytosis helps in the differentiation.
Malignant lymphomas can be primary or secondary tumours in the skin. The primary, often epitheliotropic lymphomas are usually characterized by the presence of large number of T lymphoid tumour cells. These cells may look more differentiated, with some nuclear indentations, and with pale cytoplasm. This in contrast to the B-lymphoid tumours, which are more often characterized by the presence of large, blastic cells, with dark blue cytoplasm and round nuclei with prominent nucleoli.
(Muco) cutaneous plasmacytomas are in principle benign tumours. In contrast to their malignant counterpart, the multiple myeloma, they are not combined with paraneoplastic syndromes. Aspirates normally yield large amount of tumour cells. The cells sometimes look like typically plasma cells, while in other cases they are less well differentiated. Most often there are discrete cells with distinct cytoplasmic borders and a blue cytoplasm. The nucleus can be eccentric, a Golgi apparatus can sometimes be seen, and there can be several cells with two or more nuclei.
Histiocytomas are usually benign. They occur primarily in young dogs and can disappear spontaneously. A population of cells with somewhat variable shapes characterizes them cytological. Small histiocytes resemble lymphocytes but have finer nuclear chromatin and more cytoplasm. The larger histiocytes resemble epithelial cells but have no tissue organization. The cytoplasm is grey to light blue; a few cells are clear. The nuclei are mainly round but may be indented. The nuclei contain a few small, not very obvious nucleoli.
Transmissible venereal tumours (TVT) in the dog occur in the genital area, but on the head as well. They are seldom seen in the northern parts of Europe. The cells resemble histiocytes but the tumour exfoliates more easily and thus the preparations are richer in cells. The cytoplasm is also more sharply outlined and sometimes contains readily visible vacuoles. The round to oval nucleus is eccentric, seldom indented, and can have large, noticeable nucleoli. The cell size, nuclear size, and the N/C ratio vary much more than in histiocytomas. Usually many mitotic figures are seen. TVTs can contain remarkably many plasma cells and macrophages in addition to tumour cells.
Different types of epithelial cells can be encountered in cytological preparations from skin tumours, skin lesions, and subcutaneous swellings. They may be squamous cells but they can also be of glandular origin.
FNABs of lesions in the skin and the mucosa contain squamous cells in all stages of development, in addition to inflammatory cells. A cell-rich preparation can contain many individualized epithelial cells, but what is characteristic of normal epithelial cells is their appearance in groups or clusters that are often composed of a single layer of cells (so-called "monolayers"). Depending on the depth of the lesion, more of less immature epithelial cells will be seen. These basal cells and parabasal cells are round, deep blue, small in relation to mature epithelial cells, and have a higher N/C ratio. The mature squamous cells, which are usually much more numerous, appear in different stages of keratinization. The largest cells have often already lost the nucleus (keratin flakes or dandruff) or still contain a shrunken, pyknotic nucleus. The N/C ratio is very low and the cells are rectangular and often folded double. As the cell becomes more keratinized the cytoplasm staining changes from dark blue to sky blue. The cells can also contain vacuoles as signs of keratinization. Mature but not yet keratinized epithelial cells are lightly basophilic, round or oval, and have a centrally located nucleus with a well-defined chromatin structure that resembles a fine network. Just as for normal epithelial cells, the arrangement of the cells in clusters is characteristic of epithelial tumours. Although in FNAB preparations from epithelial tumours many loose tumour cells can be seen, usually several definite clusters can be found. If the epithelial tumour is of glandular origin, the cell clusters also have an acinar structure (arrangement in a group or in a circle around a usually invisible duct). In some preparations there are predominantly clusters or monolayers of normal epithelial cells but careful searching also reveals definitely malignant cells.
Basal cell tumours are infrequent epithelial tumours that arise from the basal cell layer of the epidermis. Histologically, they can be divided into different varieties. The cytological diagnosis can be difficult due to the variable cytological features. Basal cells are round or elongated cells with one nucleus in a central or basal position and fine granular chromatin with a single and, in most cases, poorly detectable nucleolus. The N/C ratio is 1:1, with a uniform size of cells within cell aggregates. The basal cells can be organized in fragments of tissue, often with a typical linear or palisade-like arrangement. In some cases well-differentiated fibrocytes and fibroblasts can be found, as well as pigmented basal cells or melanocytes. In addition, the presence of some squamous cells in some basal cell tumours with basosquamous areas can confuse the examiner even further. In most tumours also some inflammatory cells, such as neutrophils, can be detected. Despite the fact that in several tumours a significant proportion of the basal cells might reveal a few malignancy criteria, such as anisokaryosis, anisonucleoliosis and clumped chromatin pattern, giving the tumour a less differentiated appearance, basal cell tumours are considered benign.
Squamous cell carcinomas are quite common tumours, both in the cat and the dog. Preparations from squamous cell carcinomas are usually cell rich and contain loose tumour cells as well as clusters. The characteristics of malignancy can be very pronounced or not very obvious. Squamous cell carcinomas easily become ulcerated and then contain many inflammatory cells. Because aspirates from squamous cell carcinomas usually contain neoplastic as well as normal, non-neoplastic squamous cells in all stages of development, the malignancy criteria of well-differentiated squamous cell carcinomas can be less obvious, the diagnosis of these tumours is sometimes difficult. One of the characteristic features of the squamous cell carcinoma is the discrepancy between maturation of the nucleus and the cytoplasm. The tumour cells can contain a large amount of vacuolated cytoplasm and at the same time a completely intact, non-pyknotic nucleus having a detailed structure that sometimes also has malignancy criteria. Sometimes the vacuoles have become confluent into one large vacuole that causes a noticeable clear space around the nucleus, so that the cell resembles a bull's-eye. This is a strong indication of malignancy. Also, the occurrence side by side in a single cluster of mature and immature cells, or cells of markedly different basophilia, is very suspicious.
It must be emphasized that skin lesions can become secondarily infected and inflamed. The confirmation of septic inflammation is thus not very informative in case of a skin lesion and certainly not if this is demonstrated by an impression smear, unless a very specific infective agent is demonstrated. The microorganisms that can play a role in inflammatory processes will be discussed in a later chapter. Inflammation and necrosis can make it very difficult to determine the primary cause of a process. Hence it is advisable not to rely on an impression smear but also to obtain a FNAB from a peripheral, not yet inflamed, part of the swelling. If the cytology of a skin lesion does not lead to a diagnosis, a dermatologist should be consulted and/or histological examination should be performed.
Pemphigus foliaceus is an autoimmune process that is directed against keratinocyte desmosomal cadherins, and in which interference occurs with the adhesive function of these molecules. When the epidermal cells lose their cohesion due to degeneration of the intercellular bridges, intra-epidermal clefts, vesicles, and bullae are formed. The isolated epidermal cells are called acantholytic cells. They are characterized by a basophilic cytoplasm and have well defined, round borders. Several cells have a perinuclear halo. The nucleus is somewhat enlarged and has a coarse, irregular chromatin pattern. Often a clear nucleolus can be seen. Acantholysis is most often associated with the pemphigus complex. The acantholytic cells are usually surrounded by many neutrophils and/or eosinophils. No bacteria are present, unless the lesion has been ulcerated.
(Lympho) plasmacytic gingivitis/stomatitis is a disease with an unclear etiology can present itself in many different ways in cats of all ages. Cytologic specimens can be obtained by brush methods and might reveal a combination of plasma cells, lymphocytes and sometimes also neutrophils. The morphology of the cells is normal. The ratios between the different cell types can largely vary among cases.