Mast cell

Mast cells (mastocytes, labrocytes) are sometimes called tissue basophils as they resemble the basophils of blood (a kind of granulocytes). This type of loose connective tissue cells derives from hematopoietic stem cells but exhibits a proliferative activity (though basophils do not divide) and a longer lifespan (while blood basophils are not so long-lived). Two basophil types are of different lineages.

TEM image, 12,400X (photo courtesy of G. N. Bulanova). The nucleus of a mast cell is not segmented, contains a lot of euchromatin and anucleolus. The cytoplasm is filled by granules, which differ by their size and density. Mature granules are small, surrounded by a membrane and filled by a dense matrix. The granules swell #1, #2, #3 prior to secreting accumulated products; the matrix density decreases, and it acquires a granular structure.

In a physiological state, mast cells secrete bioactive substances (heparin, histamine, dopamine, prostaglandins, chemotactic factors, interleukins, etc.) constantly but at a slow rate so as to exert their homeostatic, protective, and regulatory functions.

TEM image, 6,000X. Upon activation of a mast cell, for example in case of a new interaction between an antigen and a body already sensitized to it, the granule contents are released intensively and relatively fast (the image depicts a peripheral portion of the cytoplasm in a mast cell). Groups of swollen #1, #2 or denser #1, #2 granules are removed from the cell by exocytosis #1, #2, which is accompanied by formation of numerous ridges and projections on the plasma membrane surface.

Anti-inflammatory factors secreted by mast cells upon their active degranulation can cause an immediate hypersensitivity reaction (allergy, anaphylaxis).