Mast cells are connective tissue cells that contain granules that stain metachromatically. The granules are rich in histamine and other pharmacologically active substances. In addition, these cells also contain heparin proteoglycans that serve to store preformed chemical mediators and are responsible for the metachromatic staining of the granules.
Mast cells are widely distributed in the body but are particularly concentrated in places which are in contact with antigens such as the subepithelial connective tissue of the respiratory, intestinal and urinary systems. Free. mast cells are found in the peritonial cavities of rats and mice.
The free mast cells have been widely used in studying the mechanism of the anaphylactic histamine release. They provided the first evidence showing that the anaphylactic antibody-antigen (Ab-Ag) reaction occurs on the membrane of these cells. More recently, techniques have been developed for the isolation of the mast cells by enzymatic or mechanical dissociation of intact tissues such as the lungs.
Mammals contain two distinct types of mast cells that differ in their morphology, histochemical staining and location. One type, abundant in the normal connective tissue of most organs, can be obtained from the peritoneal cavity of rodents and forms the basis of our classical knowledge of mast cells.
The other type is usually referred to as mucosal mast cells because in normal rats it has been observed only in mucosal tissue. Infection with helminth parasites induces an extensive accumulation of mast cells and eosinophils in the tissues.
Parasites of the mucous surface, in particular, stimulate a rapid hyperplasia of mucosal mast cells. Both mucosal mast cells and connective tissue mast cells derive from a bone marrow precursor. However, ‘unlike connective mast cells, mucosal mast cells require the thymus for differentiation.
Apparently, progenitors of mucosal mast cells leave the bone marrow and locate in the intestinal mucosa where they proliferate and differentiate under the effect of a growth-stimulating factor produced in this tissue by T lymphocytes which have reached the place from mesenteric lymph nodes. IL-3, a lymphokine produced by activated T lymphocytes is a absolute requirement for the growth of a number of haemopoietic cell lines including the mast cell progeny.