Mast cells are oval or irregularly shaped cells of connective tissue, between 7 and 20 μm in diameter, filled with basophilic secretory granules that often obscure the central nucleus (Figure1). These granules are electron dense and of variable size, ranging from 0.3 to 2.0 μm in diameter. Because of the high content of acidic radicals in their sulfated GAGs, mast cell granules display metachromasia, which means that they can change the color of some basic dyes (eg, toluidine blue) from blue to purple or red. The granules are poorly preserved by common fixatives, so mast cells may be difficult to identify in routinely prepared slides.
Fig1. Mast cells.
Mast cells function in the localized release of many bioactive substances important in the local inflammatory response, innate immunity, and tissue repair. A partial list of molecules released from these cells’ secretory granules includes the following:
■ Heparin, a sulfated GAG that acts locally as an anticoagulant
■ Histamine, which promotes increased vascular permeability and smooth muscle contraction
■ Serine proteases, which activate various mediators of inflammation
■ Eosinophil and neutrophil chemotactic factors, which attract those leukocytes
■ Cytokines, polypeptides directing activities of leukocytes and other cells of the immune system
■ Phospholipid precursors, which are converted to prostaglandins, leukotrienes, and other important lipid mediators of the inflammatory response.
Occurring in connective tissue of many organs, mast cells are especially numerous near small blood vessels in skin and mesenteries (perivascular mast cells) and in the tissue that lines digestive and respiratory tracts (mucosal mast cells); the granule content of the two populations differs somewhat. These major locations suggest that mast cells place themselves strategically to function as sentinels detecting invasion by microorganisms.
Release of certain chemical mediators stored in mast cells promotes the allergic reactions known as immediate hypersensitivity reactions because they occur within a few minutes after the appearance of an antigen in an individual previously sensitized to that antigen. There are many examples of immediate hypersensitivity reaction; a dramatic one is anaphylactic shock, a potentially fatal condition. Anaphylaxis consists of the following sequential events (Figure2). The first exposure to an antigen (allergen), such as bee venom, causes antibody-producing cells to produce an immunoglobulin of the IgE class that binds avidly to receptors on the surface of mast cells. Upon a second exposure to the antigen, it reacts with the IgE on the mast cells, triggering rapid release of histamine, leukotrienes, chemokines, and heparin from the mast cell granules that can produce the sudden onset of the allergic reaction. Degranulation of mast cells also occurs as a result of the action of the complement molecules that participate in the immunologic reactions.
Fig2. Mast cell secretion.
Like macrophages, mast cells originate from progenitor cells in the bone marrow, which circulate in the blood, cross the wall of small vessels called venules, and enter connective tissues, where they differentiate. Although mast cells are in many respects similar to basophilic leukocytes, they appear to have a different lineage at least in humans.
