Porth's Essentials of Pathophysiology, 4e

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Cell Structure and Function

C h a p t e r 1

different molecules in other cells, providing for interaction between the same cell types or different cell types. Cadherins. Cadherins are the major CAMs responsible for calcium-dependent cell-to-cell adhesion junctions. The word cadherin is derived from the term “calcium- dependent adhesion protein.” There are over 90 members of the cadherin superfamily. The first three cad- herins that were discovered were named according to the main tissues in which they were found: the E-cadherins, which are present in many types of epithelial cells; the N-cadherins, which are present in nerve and muscle cells; and the P-cadherins, which are found on cells in the placenta and epidermis. All are found in various other tissues; N-cadherin, for example, is expressed in fibro- blasts, and E-cadherin is expressed in parts of the brain. Most cadherins function as transmembrane adhesive proteins that indirectly link the actin cytoskeletons of cells they join together, an arrangement that occurs in adhering junctions. Cadherins also form desmosomes that interact with intermediate filaments of the cyto- skeleton, rather than the actin filaments. Cell-to-cell interactions mediated by cadherins play a major role in regulating cell motility, proliferation, and differentiation. transient, cell-to-cell adhesion interactions in the blood- stream. They are found on activated endothelial cells of blood vessels, on leukocytes, and on platelets. Selectins, together with integrins and immunoglobulins, partici- pate in leukocyte movement through the endothelial lin- ing of blood vessels during inflammation. Integrins. Integrins usually assist in attaching epi- thelial cells to the underlying basement membrane. Extracellularly, they are attached to fibronectin and lam- inin, the two major components of the basement mem- brane. Like the cadherins, their intracellular portion is linked to actin. One group of integrins is associated with hemidesmosomes, whereas others are associated with the surface of white blood cells, macrophages, and platelets. Integrins usually have a weak affinity for their ligands unless they are associated with cellular focal contacts and hemidesmosomes. This allows some movement between cells except where a firm attachment is required to attach epithelial cells to the underlying connective tissue. Certain integrins play an important role in allowing white blood cells to pass through the vessel wall, a process called transmigration . Persons affected with leukocyte adhesion deficiency are unable to synthesize appropriate integrin molecules. As a result, they experience repeated bacterial infections because their white blood cells are not able to transmigrate through vessel walls. Immunoglobulin Superfamily. The Ig superfamily consists of a group of one or more immunoglobulin- like adhesion proteins that are similar structurally to those of antibody molecules. They have many func- tions outside the immune system that are unrelated to immune defenses. The best-studied example of Ig Selectins. Selectins are cell surface carbohydrate-binding proteins (lectins) that mediate a variety of

superfamily proteins are the neural cell adhesion mol- ecules (N-CAMs), which are expressed in a variety of cells, including most nerve cells. During early develop- ment, N-CAMs play an important role in connecting the neurons of the developing nervous system.

SUMMARY CONCEPTS

■■ Body cells are organized into four basic tissue types: epithelial, connective, muscle, and nervous.The epithelium covers and lines the body surfaces and forms the functional components of glandular structures. Epithelial tissue is classified into three types according to the shape of the cells and the number of layers that are present: simple, stratified, and pseudostratified. ■■ Connective tissue supports and connects body structures; it forms the bones and cartilage, the joint structures, the dermis of the skin, the sheaths of blood vessels and nerves, adipose tissue, lymphatic tissues, and blood. Fibroblasts are the most abundant connective tissue cells. They are responsible for the synthesis of collagen, elastic, and reticular fibers and the gel-like ground substance that fills the intercellular spaces. ■■ Muscle tissue is a specialized tissue designed for contractility.Three types of muscle tissue exist: skeletal, cardiac, and smooth. Actin and myosin filaments interact to produce muscle shortening, a process activated by the presence of calcium. In skeletal muscle, calcium is released from the sarcoplasmic reticulum in response to an action potential. Smooth muscle is often called involuntary muscle because it contracts spontaneously or through the activity of the autonomic nervous system. It differs from skeletal muscle in that its sarcoplasmic reticulum is less defined and it depends on the entry of extracellular calcium ions for muscle contraction. ■■ Nervous tissue is designed for communication purposes and includes the neurons, the supporting neural structures, and the ependymal cells that line the ventricles of the brain and the spinal canal. ■■ Within tissues, cells are held together by cell junctions, which are especially plentiful in epithelial tissues.There are three basic types of cell junctions: tight junctions, adhering or adhesive-type junctions, and gap junctions.The ability of cells to adhere together (cell to cell) or to components of the extracellular matrix (cell to matrix) is mediated by cell adhesion molecules (cadherins, selectins, integrins, and the immunoglobulin superfamily of proteins).

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