Monocytes: Overview, Quantity, Morphology

Overview

Origin

Monocytes originate in the bone marrow.

> Unlike granulocytes, they are released into the peripheral blood as immature cells and are transported to the tissues where they can differentiate into macrophages, epithelioid cells, or multinucleated inflammatory giant cells.

> The circulating monocyte is comparable in degree of differentiation to the neutrophilic myelocytes found in the bone marrow.

There is no bone marrow storage pool for monocytes.

> Monocytes and their precursors (monoblast, promonocyte) are present in low numbers and may be difficult to recognize.

> Increased monocyte production is reflected by increased numbers of circulating monocytes.

Function

The monocyte/macrophage continuum represents the second major branch of the circulating phagocyte system (neutrophils being the first). Specific macrophage functions include:

• Phagocytosis
• Regulation of the inflammatory response via release of inflammatory mediators (chemotactic factors, prostaglandins, complement fragments, etc.)
• Antigen processing for presentation to lymphocytes; involved in the initiation of the immune response
• Participation in the regulation of body iron stores

Quantity

Monocytopenia - reduced numbers of circulating monocytes - is not a recognized clinical entity.

Monocytosis is defined as increased numbers of circulating monocytes.

Monocytosis indicates:

• The presence of inflammation
• Demand for phagocytosis
• Tissue necrosis

Mild monocytosis can be associated with a "stress response" induced by high circulating glucocorticoids. This response is nonspecific.

Morphology

Normal

Monocyte morphology is similar for dogs and cats.

Normal circulating monocytes have the following features.

> Size - 15 - 20 m

> Nucleus - irregular shape, lacy reticulated chromatin pattern (Fig. 8-1a and Fig. 8-1b)

> Cytoplasm - abundant, grey to blue-grey

Figure 8-1a. Canine monocytes. Monocytes are slightly larger than a neutrophil and have a blue-grey cytoplasm, lobulated nucleus, and a lacy chromatin pattern. A segmented neutrophil (left) and a monocyte (right).

Figure 8-1b. Canine monocytes. Monocytes are slightly larger than a neutrophil and have a blue-grey cytoplasm, lobulated nucleus, and a lacy chromatin pattern. The monocyte (left) contains a few round clear cytoplasmic vacuoles. A band neutrophil and an NRBC (arrow) are on the right (100x).

Variations in normal morphology can be related to sample handling.

> Monocytes in films made from fresh non-coagulated blood usually are round and contain no cytoplasmic vacuoles

> Monocytes in films made from EDTA-treated blood may have irregular cell margins with pseudopodia. They often contain cytoplasmic vacuoles.

In Disease

Circulating monocytes can differentiate into macrophages if there is demand for phagocytosis in the blood.

> Nuclei become round to oval (Fig. 8-2).

> Cytoplasm becomes more abundant.

> Vacuoles become more prominent and may contain phagocytized material of diagnostic significance (Fig. 8-3):

• Red blood cells in cases of immune-mediated hemolytic anemia.
• Yeasts in diseases such as systemic histoplasmosis.
• Protozoal forms in diseases such as leishmaniasis.

Figure 8-2. Fine needle aspirate of a granulomatous skin lesion. Neutrophils are adjacent to large macrophages that have abundant vacuolated cytoplasm and round to oval nuclei. Macrophages are derived from blood monocytes (100x).

Figure 8-3. Buffy coat film from a dog. A large macrophage contains phagocytized Histoplasma organisms.

Whenever monocytosis is marked, buffy coat smears should be made to concentrate monocytes to facilitate their evaluation for phagocytized material.

> Use freshly collected blood and make buffy coat smears promptly.

> If blood is allowed to stand in EDTA, some monocytes differentiate into macrophages and phagocytize damaged red cells and even other leukocytes. Such a finding is not indicative of disease.