Monocytes are the largest of the normal peripheral blood cells, ranging from 14-20µm in diameter with an N:C ratio of approximately 3:1 to 2:1. Monocytes have abundant blue-gray cytoplasm containing many fine lilac granules. These give the cytoplasm a "ground glass" appearance. However, these granules may be difficult to see if the blood film is poorly stained. Frequently, cytoplasmic vacuoles are present. These vacuoles appear as unstained areas or "holes" in the cytoplasm, an example of which can be found in the lower image to the right.
Because monocytes are extremely motile cells, blunt pseudopods may be seen. These should not be confused with the apparent cytoplasmic projections produced when large lymphocytes are indented by surrounding cells.
Monocytes generally have lighter staining nuclei than other leukocytes. The nucleus stains a pale bluish-violet, and the chromatin is fine. Overall, the nucleus has a soft, spongy, three-dimensional appearance, in contrast to the hard, flat nucleus of the large lymphocyte and the densely clumped nucleus of the band. The nucleus may be round, kidney-bean shaped, folded, indented, or horseshoe, and may show "brain-like" convolutions.
