Alan H. Rebar, DVM, PhD, DACVP
Before considering pathologic cytology of lymph nodes, it is necessary to define normal lymph node cytology. Aspirates from normal lymph nodes contain mixed cell populations in which small lymphocytes are the predominant cell (>80 percent of all cells). Small lymphocytes are round cells approximately 8 to 10 µm in diameter that contain round, densely stained nuclei with a scant rim of pale basophilic cytoplasm. Nucleoli are rarely visible. Prolymphocytes constitute the second most prevalent cell type in normal nodes. Prolymphocytes (10 to 15 µm in diameter) are larger, more vesicular nuclei and more abundant basophilic cytoplasm. As with small lymphocytes, nucleoli are not seen. Lymphoblasts are even less prevalent than prolymphocytes, constituting 1 percent or less of all cells seen. Lymphoblasts are large cells (up to 30 µm in diameter) with large pale vesicular nuclei in which single to multiple nucleoli are visualized. Cytoplasm is relatively scant and basophilic.
It should be emphasized that not all aspirated lymphoid cells can be classified. Aspiration and impression smear techniques are somewhat traumatic and a number of cells are ruptured. These ruptured cells are seen as naked nuclei, the origin of which cannot be determined. Consequently, only intact cells with clearly recognized cytoplasmic boundaries should be evaluated.
In addition to lymphocytic cells, other cell types may be seen in lymph node aspirates in small numbers. Plasma cells, mast cells and macrophages may all be seen in low numbers. Macrophages are often very active, containing cytoplasmic vacuoles field with cellular debris or brown-black hemosiderin (iron) granules. Rare neutrophils may also be seen.
Lymph Node Hyperplasia
Hyperplastic lymph node aspirates are similar morphologically to aspirates from normal nodes in that all the aforementioned populations are seen. However there is a shift in the relative numbers of the different cell types. Small lymphocytes continue to predominate, but in general, the cell populations are 'left-shifted'; increased numbers of prolymphocytes and lymphoblasts are present. In addition, mitotic figures, which are rarely encountered in normal lymph node aspirates, are observed with some frequency.
Simple lymph node hyperplasia is rarely seen. A much more commonly observed phenomenon is reactive lymph node hyperplasia. Reactive hyperplasia implies antigenic stimulation of the involved node. Cytologically reactive hyperplasia exhibits all the features of simple hyperplasia. However, the striking feature is the presence of large numbers of plasma cells and plasmacytoid prolymphocytes and lymphoblasts; that is, precursors that are obviously differentiating into plasma cells. In some instances, plasma cells containing numerous vacuoles (presumably filled with immunoglobulin) are observed. These cells are known as Mott cells; cells containing immunoglobulin crystals may also be observed. In the author's experience, reactive hyperplasia has been most commonly seen in superficial lymph nodes that drain macrophagic inflammatory dermal lesions. For example, reactive hyperplasia of superficial lymph nodes is a common accompaniment of canine demodicosis and fleabite dermatitis.
Inflammatory lesions of lymph nodes (lymphadenitis) may be classified as neutrophilic, mixed, macrophagic, or granulomatous on the basis of the predominant inflammatory cell type infiltrating the node. Neutrophilic lymphadenitis aspirates contain large numbers of neutrophils and suggest severe irritation. Bacterial agents should always be suspected and in some cases may even be seen. Mixed lymphadenitis exhibits increased numbers of both neutrophils and macrophages and is usually associated with less severe irritation than is neutrophilic inflammation (or a more prolonged time course). Mixed lymphadenitis is usually accompanied by reactive hyperplasia of lymphoid elements. Macrophagic lymphadenitis is characterized by a marked increase in nodal macrophage numbers. Granulomatous lymphadenitis is a special form of macrophagic inflammatory response characterized by the presence of either inflammatory giant cells or epithelioid cells or both. In both macrophagic and granulomatous lymphadenitis, the inciting irritation is low grade and etiologic agents such as systemic mycotic agents or foreign bodies should be sought.
Neoplastic processes in lymph nodes may be either primary or metastatic. In primary neoplastic disease (malignant lymphoma), the striking cytologic feature is the presence of a remarkably uniform population of round cells, which may be classified as either bizarre lymphoblasts or prolymphocytes. This uniformity of cell type contrasts sharply with the heterogeneous cell populations seen in normal nodes and non-neoplastic reactions. Examination of lymphomatous aspirates at high magnification reveals many nuclear and cytoplasmic criteria of malignancy including variable cell size, variable nuclear size, variable nuclear/cytoplasmic ratio, large prominent nucleoli, and prominent cytoplasmic basophilia and vacuolation.
Metastatic neoplastic disease is characterized cytologically by the presence of an aberrant cell population in lymph node aspirates; metastatic foci may consist of spindle-shaped cells (sarcoma), clusters or sheets of round or oval cells (carcinoma), or individual round or oval cells (discrete cell tumors such as mastocytoma). As in primary neoplastic disease, several criteria of malignancy should be identified in the aberrant cell population. Reactive and hyperplastic changes may also be identified in the lymphoid elements aspirated from the more normal portions of the node.