University of Virginia Health System

Histiocyte
(Macrophage, Phagocytic Reticular or Reticulum Cell)

Bone marrow aspirate smear, Wright-Giemsa stain, 1000x

Description:

Histiocytes (macrophages) are phagocytic cells and most are thought to be terminally differentiated monocytes, although some may derive directly from the granulocyte-monocyte precursors (GMP) or from mitosis of other histiocytes. They are found in essentially all organs and tissues, and are referred to by several different designations depending on which location they are found:

• Macrophages- bone marrow, lymph nodes (e.g. tingible body macrophages in the germinal centers), spleen, cerebrospinal fluid, serosal cavities (pleural and peritoneal)
• Littoral cells- venous sinuses
• Kupffer cells- hepatic littoral cell
• Microglia cells- brain
• Alveolar and interstitial macrophages- lung
• Osteoclasts- bone marrow
• Langerhan's cells- epidermis
• Veiled cells- afferent lymph
• Type A synovial cells- joints
• Epithelioid cells and giant cells- granulomas

Histiocytes vary markedly in size, shape, and appearance, determined to some degree by location. Most histiocytes are large with round, oval, or reniform nuclei (some have two or more nuclei), and abundant cytoplasm. The nuclear chromatin usually is fine to slightly coarse in texture with variable parachromatin, often taking on a vesicular appearance. One or more small sky-blue nucleoli may be visible. The appearance of the cytoplasm is quite variable, but in most instances it is grey to pale blue in color. Cytoplasmic projections are frequently seen. Evidence of the phagocytic function of these cells includes the presence of vacuoles and engulfed cells or other material in various stages of degradation. Giant cells are histiocytes with multiple nucleoli thought to result from either endomitosis or fusion of several histiocytes.

Function:

Histiocytes (macrophages) have many functions that, to some extent, also depend on their location.

• Phagocytosis and breakdown of senescent cells, especially erythrocytes and platelets, but also other cells.
• Engulfment of and further degradation of cellular debris (e.g. cell nuclei).
• Phagocytosis of antibody- and complement- coded cells and microorganisms.
• Phagoctyosis of microorganisms and tumor cells via activation of the Toll-like receptors by IFN-γ. Phagocytosis also occurs through other stimuli (e.g. other cytokines, nitric oxide) depending on the nature of the material being engulfed.
• Antigen presentation.
• Iron storage.

Trafficking:

Histiocytes are either tissue-based or reside in venous sinusoids in the spleen, liver, lymph nodes, and bone marrow and are rarely seen in the peripheral blood. The only morphologically recognizeable histiocyte precursors in the peripheral blood is the monocyte. The lifespan of histiocytes is quite variable depending on their location. Most live 3-6 weeks and some probably longer.

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