Monocytes are the leukocyte that is the most problematic for identification, because they can be fairly variable in size and appearance. They are often larger than neutrophils and are usually the largest leukocyte. The nucleus can be round to kidney-shaped to pseudo-lobulated (can mimic a neutrophil). It can even occasionally be band-shaped, especially in dogs, and may be confused with band neutrophils. However, the chromatin of monocytes is less dense than neutrophils, and is described as lacey to slightly clumped. The cytoplasm is usually smooth and blue-grey and may contain a few variably-sized vacuoles, and occasionally a few very fine pink-red granules. They are also larger than neutrophils
Monocytes share a common committed stem cell with neutrophils. They are produced in marrow, circulate briefly in blood, and migrate into tissues where they differentiate further to become macrophages.
There is no storage pool of monocytes in marrow; their numbers in marrow at a given time are very small. Monocytes in blood are unevenly distributed between a marginated and circulating pool. Corticosteroid affect differs among species. Factors produced at sites of inflammation can increase monocyte production.
Monocytopenia may be difficult to document because monocyte reference intervals often go down to zero, and has no clinical significance.
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