The cell indicated by the arrow is a monocyte. The cell on the left is a lymphocyte and the cell on the right a metamyelocyte. Before discussing the diagnosis of monocytic leukemia, I will, appropriately, demonstrate the various characteristics of monocytes observed in both health and disease.
Monocytes vary in size from 15 microns -30 microns in diameter, and smaller monocytes sometimes may be mistaken for larger lymphocytes. Generally, mature monocytes have irregularly shaped nuclei that may be folded. The fold may extend to the periphery of the nucleus, or it may be seen entirely within the nucleus. Another feature is the presence of straight lines or creaselike structures in the nuclei. The nuclear membranes are thin and flexible, which may account for the irregularity of the nuclei. The nuclear chromatin is fine and lacelike and stains the lightest of all the cell lines. In contrast to lymphocytes, monocytes do not have heavy blocks of chromatin. The cytoplasm has a distinctive gray-blue color. These cells may have small azurophilic granules peppered over the entire surface. Often the granules are so small that they are not within the visible range of a light microscope. The cytoplasm of monocytes tends to project buds or small pseudopods. The cytoplasm may be abundant or scanty. The nuclei are usually more concentric than those of myelocytes and promyelocytes. Vacuoles present in monocytes usually are the result of the blood standing in anticoagulant, although in patients with leukemia and infection, vacuoles may be seen in fresh blood without anticoagulant.
Course Section: 09. Acute Myelomonocytic Leukemia - Monocytic Leukemia - and Erythroleukemia
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