The principal characteristics of mesothelial cells are described in great detail in Chapter 25. In the context of the peritoneal fluid, the mesothelial cells are fairly easy to recognize when well preserved: the cells are of medium size, comparable to small parabasal squamous cells, and have a generally basophilic, delicate cytoplasm, wherein the outer zone is often lighter than the inner, perinuclear zone (Fig. 16-1A). However, depending on the technique used in processing the material and speed of fixation, the mesothelial cells may vary in size and staining properties among specimens. When in flat sheets, the mesothelial cells are often separated from each other by narrow clear gaps or “windows” (Fig. 16-1B,C). Sometimes, long strips of mesothelial cells, forcibly removed from their setting, may be observed in cell blocks (Fig. 16-1D). The cells have central, round, but sometimes slightly indented nuclei, occasionally containing visible small nucleoli (Fig. 16-1A,B). The presence of nucleoli may be troublesome to an inexperienced observer, who may confuse such cells with cancer cells; the fairly monotonous size of the mesothelial cells and their nuclei should prevent the erroneous diagnosis. Through the courtesy of Dr. Wai-Kuen Ng of the Pamela Youde Nethersole Eastern Hospital in Hong Kong, I was privileged to see a very unusual variant of mesothelial cells in peritoneal washings characterized by lobulated nucleus. The term “daisy cells” has been appended to these cells, which were apparently observed before in peritoneal washings. A transition between normal mesothelial cells and “daisy cells” is shown in Figure 16-2. Such cells have not been observed by us in any other fluid and, hence, appear to be a unique feature of peritoneal mesothelial cells.

It is not uncommon, however, to see mesothelial cells in large, densely packed sheets wherein the individual cells are difficult to study (Fig. 16-3A,B). As a rule, the diagnosis of cancer should not be made unless the characteristics of cells and their nuclei can be studied in detail; the thick sheets or dense clusters of mesothelial cells are no exception to this rule.

Benign epithelial cells of various types may be observed in peritoneal washings, especially after a vigorous irrigation. These may represent a variety of structures, from ruptured benign inclusion cysts, commonly found on the surfaces of tubes and ovaries, foci of endosalpingiosis and endometriosis, or benign ciliated tubal epithelium. The cells may be of cuboidal or columnar configuration (Figs. 16-3C,D and 16-4A,B) and may appear singly or may form sheets or even papillary clusters, composed of small cuboidal or columnar cells with inconspicuous nuclei and nucleoli. Some of the cells may be ciliated. Detached ciliated tufts of tubal origin may also be observed (Poropatich and Ehya, 1986). Sidawy et al (1987) correlated the presence of the ciliated tufts with stages of menstrual cycle and observed them only in the secretory stage (Fig. 16-4C,D).

In “first look” specimens and in the absence of cancer, the leukocytes are usually few in number, except in the presence of an inflammatory process. In the latter condition, macrophages accompanied by leukocytes of various types, fibrin and necrotic material may be observed. Cancerous processes may also be accompanied by an inflammatory reaction. In “second look” procedures, a marked inflammatory reaction is often present (see below).

Macrophages may vary in size: most are mononucleated
cells, comparable in size to mesothelial cells, but having finely vacuolated cytoplasm and peripheral, spherical or kidney-shaped nuclei of similar size. Large cytoplasmic vacuoles may occur, pushing the nucleus to the periphery. Evidence of phagocytosis in the form of ingested particles of pigment, such as hemosiderin, helps in the identification of these cells, although sometimes cancer cells are also capable of phagocytosis. Macrophages may also form large, either mono- or multinucleated giant cells; the latter are commonly observed in patients with chronic inflammatory processes or as a reaction to foreign bodies, such as powder, usually observed after a surgical intervention. Macrophages are more common in “second look” procedures.

Under this term, Wojcik and Naylor (1992) analyzed the frequency and origin of peculiar homogenous structures, lined by a single layer of cuboidal cells that may be observed in about 5% of pelvic fluids and lavage specimens (Fig. 16-5A,B). In one such case, the structures could be traced to small collagenous excrescences on the surface of an ovary, hence, the conclusion that the small cuboidal cells represent ovarian epithelium. We have observed the collagen balls in benign and malignant peritoneal lavage specimens and, therefore, they have no diagnostic significance.

During cul-de-sac aspirations performed with a needle-syringe system, a loop of bowel may be inadvertently penetrated. Epithelial cells of intestinal origin or bowel contents in the form of plant cells may be observed. The enteric cells are usually columnar and may occur in large clusters. The plant cells may be identified by a thick, transparent cellulose wall and fine, refractile cytoplasmic granules (see Chapters 8 and 19). Rare findings in aspirates include large ganglion cells, with abundant granular cytoplasm and peripheral nuclei, inadvertently removed from presacral ganglia (Fig. 16-5C,D).