in a stained wet film of pleural effusion. The
cytoplasmic granules of the mast cell are metachromatic and show a delicate
purple-pink staining reaction (toluidine blue x OI).
readily recognized with
Romanowsky stain, but they are not recognizable in Papanico-
laou-stained smears or in cell blocks stained with hematoxylin
and eosin. They can be recognized in toluidine blue-stained wet
films (Fig. 19.42), in which the granules are a delicate purple-
red, quite different from the deep blue of the stain itself.
Mast cells, slightly larger than basophils, are about the size
of small mesothelial cells. Their nuclei are round or oval and
centrally placed, whereas the nuclei of basophils are usually
bilobed. The cytoplasm of mast cells is entirely occupied with
granules; the cytoplasm of basophils contains far fewer.
Key features of basophilic leukocytes and mast cells
• Identifiable in stained wet films;
• Small round cells with cytoplasmic granules; and
• Granules purple-red with toluidine blue stain.
Histiocytes are found in various proportions in almost every
serous effusion. Their size varies considerably, from about 15 to
100 pm in diameter, with most within the range of 20 to 40 pm.
In smears, the typical histiocyte is easily identified by its size,
eccentric round or bean-shaped nucleus, and lightly stained
lacy cytoplasm (Fig. 19.43). Because histiocytes are phagocytic,
their cytoplasm may contain leukocytes, nuclear particles (Fig.
19.44), red blood cells, carbon particles, lipid droplets, melanin,
or hemosiderin. Crystalline cytoplasmic inclusions in histio-
cytes in ascitic fluid from a patient with an indolent plasma cell
dyscrasia resulting in excessive production of immunoglobulin
have been reported.68
Histiocytes tend to be isolated but may coalesce, presumably
because of their long microvilli becoming entangled. This usually
gives a loose, sheet-like appearance to a group of macrophages.
Such sheets do not have the tight cohesiveness and sharp defini-
tion of sheets of mesothelial cells; they tend to straggle at the
periphery, and the overall cohesiveness of the group has a loose
quality, with spaces between individual cells. Furthermore, the
lacy, porous appearance of the cytoplasm contrasts sharply with
A mixed inflammatory picture in a smear of peritoneal effusion
with round to bean-shaped nuclei and lightly
stained lacy cytoplasm (Papanicolaou x HP). Reproduced with permission
from Naylor, B:
Pleural, Peritoneal and Pericardial Fluids, The Manual of
Chicago: American Society of Clinical Pathologists; 1993.
A smear of pleural fluid containing mesothelial cells, neutrophils,
They are enlarged and their cytoplasm
has ingested cellular material (Papanicolaou x HP).
the dense cytoplasm of mesothelial cells. Figure 19.45 exempli-
fies in a smear of pericardial effusion the cohesive property of
histiocytes, in this case to form balls; the patient had fibrinous
Histiocytes may contain large solitary or loculate cytoplasmic
vacuoles that appear to displace the nucleus to the periphery of
the cell (Fig. 19.46), a picture suggestive of signet ring adenocar-
cinoma. In cell-block preparations of a spontaneously formed
clot, groups of macrophages that have become tightly packed
against each other as the clot contracted may be seen; these
compact groups of histiocytes may be mistaken for fragments
Histiocytes usually have a single nucleus, although binucle-
ation is not uncommon. The finding of giant multinucleated
histiocytes formed by fusion of a number of macrophages or
by mitotic division of a nucleus is a rarity, virtually confined
to effusions caused by rheumatoid pleuritis or rheumatoid
pericarditis (see "Rheumatoid Disease," later). Giant multinu-
cleated mesothelial cells are seen with much more frequency,