Three ages of monocytes are demonstrated in this slide. The large cell on the left is a mature monocyte. Its characteristics are an irregularly folded nucleus, a moderate amount of gray-blue cytoplasm, and many small azurophilic granules. The cell in the upper center of the field is a monoblast. The nucleus-to-cytoplasm ratio (N:C) is high, the nuclear chromatin is fine and homogeneous, and the cytoplasm is basophilic. The cell on the right is a promonocyte. The nucleus is more regular than that of the mature monocyte and the N:C ratio is higher. How do we know that the center cell is a monoblast? We don't know for sure because it is extremely difficult to differentiate monoblasts from myeloblasts in Romanowsky's stained preparations. If the slide contains mainly recognizable monocytes and promonocytes, then it can be assumed that the blasts are monoblasts. It becomes more difficult to identify the type of blasts if both promyelocytes and promonocytes are present, and often it is as difficult to distinguish promonocytes from promyelocytes as it is to distinguish the blasts.
Course Section: 09. Acute Myelomonocytic Leukemia - Monocytic Leukemia - and Erythroleukemia
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