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 and 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 the 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.
