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In the promonocyte stage of development, the nucleolus is still visible while the nucleus begins to indent and fold. This may be observed as pleated or creased-looking chromatin or as a definite flattening or indenting of the nucleus. The chromatin will begin to condense but will still be finer and more "lacy" than what is found in a mature monocyte. The cytoplasm of the promonocyte will begin to mature as well as the color begins to shift toward the blue-gray, grainy texture found in mature monocytes. The fine pink granules found in mature monocytes will also begin to appear.

The image on the right is from a patient with monoblastic leukemia. This slide permits the observation of several promonocytes in one image. These cells would only rarely be seen in a normal bone marrow . Notice the folded and indented nuclei of the promonocytes (see red arrows). Note that as the promonocyte matures, the cell size decreases and the complexity of the nucleus increases . Notice the fine pink granules, which increase in number as the cell size decreases.