Figure 519

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, x 1,250.

block numerous coagulation factors. Based on its anticoagulant properties, heparin is useful for treatment of thrombosis. -- "

In addition, several secondary mediators, namely, leukotrienes and prostaglandin D, are released during mast cell activation. These mediators are not stored in granules but are synthesized by the cell and released immediately into the extracellular matrix.

Mast cells are especially numerous in connective tissues of skin and mucous membranes but are not present in the brain and spinal cord

Mast cells are distributed chiefly in the vicinity of small blood vessels, a target of histamine and SRS-A. Mast cells are also present in the capsules of organs and the connective tissue that surrounds the blood vessels of organs. A notable exception is the central nervous system. Although the meninges (sheets of connective tissue that surround the 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 disrupting effects of the edema characteristic of allergic reactions. Mast cells are also numerous in fire "thymus and, to a lesser degree, in other lymphatic organs, but they are not present in the spleen.

In certain immune reactions, basophils leave the circulation and function in the connective tissue

Basophils are also characterized by the presence of intensely basophilic secretory granules in the cytoplasm.

Like that of mast cells, the basophil cell membrane exhibits specific receptors for the Fc fragment of IgE immunoglobulin, which is produced in response to allergens. In allergic reactions, IgE immunoglobulins become bound to the surface Fc receptor of the basophil. This binding triggers the rapid exocytosis of the basophil secretory granules. The release of histamine, heparan sulfate, ECF, NCF, and peroxidase contained in the granules enhances the vascular response in dermal hypersensitivity reactions, such as those that follow insect bites and stings. In highly sensitive individuals, the antigen injected by an insect can trigger a massive discharge of basophil granules. This often-explosive, life-threatening reaction, known as anaphylactic shock, is characterized by a decreased volume of circulating blood (leaky vessels) and constriction of smooth muscle cells in blood vessels and in the bronchial tree. The individual has difficulty breathing and may exhibit a rash as well as nausea and vomiting. Symptoms of anaphylactic shock usually develop within 1 to 3 minutes, and they require immediate treatment with vasoconstrictors such as epinephrine.

Adipose Cells

The adipose cell is a connective tissue cell specialized to store neutral fat

Adipose cells differentiate from undifferentiated mesenchymal cells and gradually accumulate fat in their cytoplasm. They are located throughout loose connective tissue as individual cells and groups of cells. When they~accumu-late in large numbers, they are called adipose tissue. This specialized connective tissue is discussed in Chapter 6.

Undifferentiated Mesenchymal Cells and Pericytes

Many researchers have postulated the existence of cells in loose connective tissue of the adult that retain the multiple potentials of embryonic mesenchymal cells. These cells, called undifferentiated mesenchymal cells, are thought to give rise to differentiated cells that function in repair and formation of new tissue, as in wound healing, and development of new blood vessels (neovascularization).

The pericyte may serve as one type of undifferentiated mesenchymal cell

Pericytes, also called adventitial cells or perivascular cells, are found around capillaries and venules (Fig. 5.20). They are surrounded by basal lamina material that is continuous with the basal lamina of the capillary endothelium; thus, they are not truly located in the connective tissue compartment. The pericyte is typically wrapped, at least partially, around the capillary, and its nucleus takes on a shape similar to that of endothelial cells, i.e., flattened but curved to conform to the tubular shape of the vessel.

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