Monocytes are the largest leucocytes. They vary considerable in size and shape and may measure about 12-20 µm in diameter. The have a lobulated nucleus that is centrally placed with a fine chromatin. The nucleus has been classically described as kidney shaped but may take other lobulated forms. The cytoplasm is abundant, grey or light-blue grey. Fine azurophilic granules that are seen on staining on wright’s stain giving the cytoplasm a ground glass appearance (evident in the monocyte on the left in figure 1). The granules may become prominent in patients with marrow stimulation (bacteraemia, marrow recovery from aplasia or following the use of G-CSF)
Monocytes show typical granules in some inherited disorders. They are phagocytic and may show ingested red cells, malarial pigment or microorganisms.
The monocyte needs to be differentiated from a neutrophilic metamyelocyte, a cell with a similar size and indented nucleus. The monocyte has a fine chromatin. The chromatin of the metamyelocyte is coarser and clumped. The cytoplasm of a monocyte is grey or light grey blue cytoplasm and fine azurophilic granules whereas the cytoplasm of a metamyelocyte has is pink with fine pink granules.