The monocyte is the final stage of monocyte maturation found in the peripheral blood before it migrates into tissues and further develops into a macrophage (histiocyte).
When seen in the bone marrow, a mature monocyte will look identical to its peripheral counterpart. It will have a fine, lacy chromatin pattern with varying degrees of nuclear folding and condensation. The cytoplasm will be blue-gray in color with a slightly grainy texture. The cytoplasm may have a light sprinkling of fine pink cytoplasmic granules. The mature monocyte will be larger than mature segmented neutrophils but not quite as large as promyelocytes or early myelocytes.
The top image to the right shows several monocytes with varying degrees of nuclear folding (see red arrows). Notice that the chromatin clumping is not as dense as that found in neutrophils. Notice also that the cytoplasm is blue-gray and grainy, not the pink/tan of a neutrophil. Observe that the mature monocytes are slightly smaller than the promyelocytes in the image.
The lower image to the right shows a monocyte (red arrow) adjacent to a segmented neutrophil (blue arrow). The monocyte is clearly larger. Notice the increase in size of the two monocytes below (green arrows) as they begin to transform into macrophages (histiocytes). The vacuolation is an indication of this transformation occurring.
