Porth's Essentials of Pathophysiology, 4e

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Cell and Tissue Function

U N I T 1

Continuous tight junction (zonula occludens)

Microfilament bundle (marginal band)

Tonofilament bundle

Desmosome (macula adherens)

Adhesion belt (zonula adherens)

Basement membrane

Channel

Hemidesmosome

Gap junction

FIGURE 1-20. Three types of intercellular junctions found in epithelial tissue: the continuous tight junction (zonula occludens); the adhering junction, which includes the adhesion belt (zonula adherens), desmosomes (macula adherens), and hemidesmosomes; and the gap junction.

Two main classes of extracellular macromolecules make up the extracellular matrix. The first is an amor- phous gel-like material called ground substance . Ground substance is composed of polysaccharide chains of pro- teins called glycosaminoglycans (GAGs), which are usu- ally found linked to protein as proteoglycans. The second type consists of the fibrous proteins (i.e., collagen and elastin) and the fibrous adhesive proteins (i.e., fibronectin and laminin) that are found in the basement membrane. Members of each of these two classes of extracellular macromolecules come in a variety of shapes and sizes. Ground Substance. The proteoglycan and GAG mol- ecules form a highly hydrated, gel-like substance, or tis- sue gel, in which the fibrous proteins are embedded. The polysaccharide gel resists compressive forces; the collagen fibers strengthen and help organize the matrix; the rub- berlike elastin adds resilience; and the adhesive proteins help cells attach to the appropriate part of the matrix. Polysaccharides in the tissue gel are highly hydrophilic, and they form gels even at low concentrations. They also produce a negative charge that attracts cations such as sodium, which are osmotically active, causing large amounts of water to be sucked into the matrix. This cre- ates a swelling pressure, or turgor, that enables the matrix to withstand extensive compressive forces. For example,

the cartilage matrix that lines the knee joint can support pressures of hundreds of atmospheres by this mechanism. Fibrous Proteins. Three types of fibers are found in the extracellular space: collagen, elastin, and reticular fibers. Collagen is the most common protein in the body. It is a tough, nonliving, white fiber that serves as the struc- tural framework for skin, ligaments, tendons, and many other structures. Elastin acts like a rubber band; it can be stretched and then returns to its original form. Elastin fibers are abundant in structures subjected to frequent stretching, such as the aorta and some ligaments. Reticular fibers are extremely thin fibers that create a flexible net- work in organs subjected to changes in form or volume, such as the spleen, liver, uterus, or intestinal muscle layer. Cell Adhesion Molecules Important classes of extracellular macromolecules are the cell adhesion molecules (CAMs). Cell adhesion molecules can be cell-to-cell or cell-to-matrix adhesion molecules. There are four major classes of CAMs: cadherins, selec- tins, integrins, and the immunoglobulin (Ig) superfamily of proteins. These proteins are located on the cell surface where they function as receptors, or they can be stored in the cytoplasm. As receptors, CAMs can bind to similar or