This triggers mast cell activation, which results in granule exocytosis (degranulation) and the release of granule content into the ECM. Mast cells can also be activated by the IgEindependent mechanism during complement protein activation.
Two types of human mast cells have been identified based on morphologic and biochemical properties. Most mast cells in the connective tissue of the skin and intestinal submucosa and breast and axillary lymph nodes contain cytoplasmic granules with a latticelike internal structure. These cells contain granule-associated tryptase and chymase and are referred to as MCTC mast cells. In contrast, mast cells in the lungs and intestinal mucosa have granules with a scroll-like internal structure. These cells produce only tryptase and are termed MCT mast cells. Nearly equivalent concentrations of each type are found in nasal mucosa.
Mast cells are especially numerous in the connective tissues of skin and mucous membranes, but are not present in the brain and spinal cord.
Mast cells are distributed chiefly in the connective tissue of skin (MCTC mast cells) in the vicinity of small blood vessels, hair follicles, sebaceous glands, and sweat glands. Mast cells are also present in the capsules of organs and the connective tissue that surrounds the blood vessels of internal organs. A notable exception is the central nervous system. Although the meninges (sheets of connective tissue that surround the
FIGURE 6.23 • The mast cell. a. Photomicrograph of a mast cell stained with H&E. The granules stain intensely and, because of their numbers, tend to appear as a solid mass in some areas. The nucleus of the cell is represented by the pale-staining area.
1,250. b. This electron micrograph shows the cytoplasm of a mast cell that is virtually filled with granules. Note a small lymphocyte present in the upper left of the figure. 6,000.
brain and spinal cord) contain mast cells, the connective tissue around the small blood vessels within the brain and spinal cord is devoid of mast cells. The absence of mast cells protects the brain and spinal cord from the potentially disruptive effects of the edema of allergic reactions. Mast cells are also numerous in the thymus and, to a lesser degree, in other lymphatic organs, but they are not present in the spleen.
Most mast cell secretory products (mediators of inflammation) are stored in granules and are released at the time of mast cell activation.
chemical substances known as mediators of infammation.
Mediators produced by mast cells are divided into two cate gories: Preformed mediators that are stored in secretory thesized mediators (mostly lipids and cytokines) that are often absent in the resting cells, although they are produced and secreted by activated mast cells. Preformed mediators found inside mast cell granules are the following:  Histamine is a biogenic amine that increases the permeability of small blood vessels, causing edema in the surrounding tissue and a skin reaction demonstrated by an itching sensation. In addition, it increases mucus production in the bronchial tree and prompts contraction of smooth muscle in the pulmonary airways. Histamine’s effects can be blocked by antihistaminic agents. These competitive inhibitors have a similar chemical structure and bind to histamine receptors without initiating histamine’s effects.
 Heparin is a sulfated GAG that is an anticoagulant. Its expression is limited essentially to the granules of mast cells and basophils. When heparin unites with antithrombin III and platelet factor IV, it can block numerous coagulation factors. On the basis of its anticoagulant properties, heparin is useful for treatment of thrombosis. It also interacts with FGF and its receptor to induce signal transduction in the cells.