Key Concepts Of Section

Cell-Cell and Cell-Matrix Adhesion: An Overview

■ Cell-adhesion molecules (CAMs) mediate direct cell-cell adhesions (homotypic and heterotypic), and cell-surface adhesion receptors mediate cell-matrix adhesions (see Figure 6-1). These interactions bind cells into tissues and facilitate communication between cells and their environments.

■ The cytosolic domains of CAMs and adhesion receptors bind multifunctional adapter proteins that mediate interaction with cytoskeletal fibers and intracellular signaling proteins.

■ The major families of cell-surface adhesion molecules are the cadherins, selectins, Ig-superfamily CAMs, and integrins (see Figure 6-2).

■ Tight cell-cell adhesions entail both cis (lateral or in-tracellular) oligomerization of CAMs and trans (intercellular) interaction of like (homophilic) or different (hetero-philic) CAMs (see Figure 6-3).

■ The extracellular matrix (ECM) is a complex meshwork of proteins and polysaccharides that contributes to the structure and function of a tissue.

■ The evolution of CAMs, adhesion receptors, and ECM molecules with specialized structures and functions permits cells to assemble into diverse classes of tissues with varying functions.

Sheetlike Epithelial Tissues: Junctions and Adhesion Molecules

In general, the external and internal surfaces of organs are covered by a sheetlike layer of epithelial tissue called an epithelium. Cells that form epithelial tissues are said to be polarized because their plasma membranes are organized into at least two discrete regions. Typically, the distinct surfaces of a polarized epithelial cell are called the apical (top), basal

▲ FIGURE 6-4 Principal types of epithelium. The apical and basolateral surfaces of epithelial cells exhibit distinctive characteristics. (a) Simple columnar epithelia consist of elongated cells, including mucus-secreting cells (in the lining of the stomach and cervical tract) and absorptive cells (in the lining of the small intestine). (b) Simple squamous epithelia, composed of thin cells, line the blood vessels (endothelial cells/endothelium) and many body cavities. (c) Transitional epithelia, composed of several layers of cells with different shapes, line certain cavities subject to expansion and contraction (e.g., the urinary bladder). (d) Stratified squamous (nonkeratinized) epithelia line surfaces such as the mouth and vagina; these linings resist abrasion and generally do not participate in the absorption or secretion of materials into or out of the cavity. The basal lamina, a thin fibrous network of collagen and other ECM components, supports all epithelia and connects them to the underlying connective tissue.

(base or bottom), and lateral (side) surfaces (Figure 6-4). The basal surface usually contacts an underlying extracellular matrix called the basal lamina, whose composition and function are discussed in Section 6.3. Often the basal and lateral surfaces are similar in composition and together are called the basolateral surface. The basolateral surfaces of most ep-ithelia are usually on the side of the cell closest to the blood vessels. In animals with closed circulatory systems, blood flows through vessels whose inner lining is composed of flattened epithelial cells called endothelial cells. The apical side of endothelial cells, which faces the blood, is usually called the luminal surface, and the opposite basal side, the ablumi-nal surface.

Epithelia in different body locations have characteristic morphologies and functions (see Figure 6-4). Stratified (mul-tilayered) epithelia commonly serve as barriers and protective surfaces (e.g., the skin), whereas simple (single-layer) epithelia often selectively move ions and small molecules from one side of the layer to the other. For instance, the simple columnar epithelium lining the stomach secretes hydrochloric acid into the stomach lumen; a similar epithelium lining the small intestine transports products of digestion (e.g., glucose and amino acids) from the lumen of the intestine across the basolateral surface into the blood (Chapter 7). The simple columnar epithelium lining the small intestine has numerous fingerlike projections (100 nm in diameter) called microvilli (singular, microvillus) that extend from the luminal (apical) surface (see Figure 5-45). The upright orientation of a microvillus is maintained by numerous connections between the surrounding plasma membrane and a central bundle of actin microfilaments, which extend into the cell and interact with keratin intermediate filaments (see Figure 5-28). Microvilli greatly increase the area of the apical surface and thus the number of proteins that it can contain, enhancing the absorptive capacity of the intestinal epithelium.

Here we describe the various cell junctions and CAMs that play key roles in the assembly and functioning of epithelial sheets. In Section 6.3, we consider the components of the extracellular matrix intimately associated with epithelia.

Specialized Junctions Help Define the Structure and Function of Epithelial Cells

All epithelial cells in a sheet are connected to one another and the extracellular matrix by specialized cell junctions consisting of dense clusters of CAMs. Although hundreds of individual CAM-mediated interactions are sufficient to cause cells to adhere, junctions play special roles in imparting strength and rigidity to a tissue, transmitting information between the extracellular and the intracellular space, controlling the passage of ions and molecules across cell layers, and serving as conduits for the movement of ions and molecules from the cytoplasm of one cell to that of its immediate neighbor.

Three major classes of animal cell junctions are prominent features of the intestinal epithelium (Figure 6-5; see also Figure 6-1). Anchoring junctions and tight junctions perform the key task of holding cells together into tissues. These junctions are organized into three parts: adhesive proteins in the plasma membrane that connect one cell to another cell (CAMs) or to the extracellular matrix (adhesion receptors); adapter proteins, which connect the CAMs or adhesion re-

Microvillus

Tightjunction-

Adherens junction

Actin and myosin -filaments

Gap junction-

Intermediate-

filaments

Desmosome-

Hemidesmosome

Basal lamina-

Connective tissue

Tightjunction-

Adherens junction

Actin and myosin -filaments

Gap junction-

Intermediate-

filaments

Desmosome-

Hemidesmosome

Basal lamina-

Connective tissue

Apical surface

Lateral surface

Basal surface

Apical surface

Lateral surface

Basal surface

Microvillus

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