Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 85
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Not surprisingly, adhesion molecules of animals are evolutionarily ancient and are some of the mosthighly conserved proteins among multicellular (metazoan)organisms. Sponges, the most primitive metazoans, expresscertain CAMs and multiadhesive ECM molecules whosestructures are strikingly similar to those of the correspondinghuman proteins. The evolution of organisms with complextissues and organs has depended on the evolution of diverseCAMs, adhesion receptors, and ECM molecules with novelproperties and functions, whose levels of expression differin different types of cells.The diversity of adhesive molecules arises in large partfrom two phenomena that can generate numerous closely related proteins, called isoforms, that constitute a protein fam-201ily.
In some cases, the different members of a protein familyare encoded by multiple genes that arose from a common ancestor by gene duplication and divergent evolution (Chapter9). Analyses of gene and cDNA sequences can provide evidence for the existence of such a set of related genes, or genefamily. In other cases, a single gene produces an RNA transcript that can undergo alternative splicing to yield multiplemRNAs, each encoding a distinct isoform (Chapter 4).
Alternative splicing thus increases the number of proteins thatcan be expressed from one gene. Both of these phenomenacontribute to the diversity of some protein families such asthe cadherins. Particular isoforms of an adhesive protein areoften expressed in some cell types but not others, accounting for their differential distribution in various tissues.KEY CONCEPTS OF SECTION 6.1Cell–Cell and Cell–Matrix Adhesion: An OverviewCell-adhesion molecules (CAMs) mediate direct cell–celladhesions (homotypic and heterotypic), and cell-surface adhesion receptors mediate cell–matrix adhesions (see Figure6-1). These interactions bind cells into tissues and facilitatecommunication between cells and their environments.■The cytosolic domains of CAMs and adhesion receptorsbind multifunctional adapter proteins that mediate interaction with cytoskeletal fibers and intracellular signalingproteins.■The major families of cell-surface adhesion molecules arethe cadherins, selectins, Ig-superfamily CAMs, and integrins (see Figure 6-2).■Tight cell–cell adhesions entail both cis (lateral or intracellular) oligomerization of CAMs and trans (intercellular) interaction of like (homophilic) or different (heterophilic) CAMs (see Figure 6-3).■The extracellular matrix (ECM) is a complex meshworkof proteins and polysaccharides that contributes to thestructure and function of a tissue.■The evolution of CAMs, adhesion receptors, and ECMmolecules with specialized structures and functions permitscells to assemble into diverse classes of tissues with varying functions.■6.2 Sheetlike Epithelial Tissues:Junctions and Adhesion MoleculesIn general, the external and internal surfaces of organs arecovered 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 intoat least two discrete regions. Typically, the distinct surfacesof a polarized epithelial cell are called the apical (top), basal202CHAPTER 6 • Integrating Cells into Tissues(a) Simple columnarApical surfaceLateralsurfaceBasalsurfaceConnectivetissueBasallamina(b) Simple squamous(c) Transitional(d) Stratified squamous (nonkeratinized)▲ FIGURE 6-4 Principal types of epithelium.
The apical andbasolateral surfaces of epithelial cells exhibit distinctivecharacteristics. (a) Simple columnar epithelia consist of elongatedcells, including mucus-secreting cells (in the lining of thestomach and cervical tract) and absorptive cells (in the lining ofthe small intestine). (b) Simple squamous epithelia, composed ofthin cells, line the blood vessels (endothelial cells/endothelium)and many body cavities.
(c) Transitional epithelia, composed ofseveral layers of cells with different shapes, line certain cavitiessubject to expansion and contraction (e.g., the urinary bladder).(d) Stratified squamous (nonkeratinized) epithelia line surfacessuch as the mouth and vagina; these linings resist abrasion andgenerally do not participate in the absorption or secretion ofmaterials into or out of the cavity. The basal lamina, a thin fibrousnetwork of collagen and other ECM components, supports allepithelia and connects them to the underlying connective tissue.(base or bottom), and lateral (side) surfaces (Figure 6-4). Thebasal surface usually contacts an underlying extracellularmatrix called the basal lamina, whose composition and function are discussed in Section 6.3.
Often the basal and lateralsurfaces are similar in composition and together are calledthe basolateral surface. The basolateral surfaces of most epithelia are usually on the side of the cell closest to the bloodvessels. In animals with closed circulatory systems, bloodflows through vessels whose inner lining is composed of flattened epithelial cells called endothelial cells. The apical sideof endothelial cells, which faces the blood, is usually calledthe luminal surface, and the opposite basal side, the abluminal surface.Epithelia in different body locations have characteristicmorphologies and functions (see Figure 6-4).
Stratified (multilayered) epithelia commonly serve as barriers and protective surfaces (e.g., the skin), whereas simple (single-layer)epithelia often selectively move ions and small moleculesfrom 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 epitheliumlining 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 hasnumerous fingerlike projections (100 nm in diameter) calledmicrovilli (singular, microvillus) that extend from the luminal(apical) surface (see Figure 5-45).
The upright orientation ofa microvillus is maintained by numerous connections between the surrounding plasma membrane and a central bundle of actin microfilaments, which extend into the cell andinteract with keratin intermediate filaments (see Figure5-28). 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 CAMsthat play key roles in the assembly and functioning of epithelial sheets.
In Section 6.3, we consider the components of theextracellular matrix intimately associated with epithelia.Specialized Junctions Help Define the Structureand Function of Epithelial CellsAll epithelial cells in a sheet are connected to one another andthe extracellular matrix by specialized cell junctions consisting of dense clusters of CAMs. Although hundreds of individual CAM-mediated interactions are sufficient to cause cellsto adhere, junctions play special roles in imparting strengthand rigidity to a tissue, transmitting information between theextracellular and the intracellular space, controlling the passage of ions and molecules across cell layers, and serving asconduits 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 alsoFigure 6-1).
Anchoring junctions and tight junctions performthe key task of holding cells together into tissues. These junctions are organized into three parts: adhesive proteins in theplasma 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-6.2 • Sheetlike Epithelial Tissues: Junctions and Adhesion Molecules203(b)(a)ApicalsurfaceMicrovillusMicrovillusTight junctionAdherens junctionActin and myosinfilamentsTight junctionLateralsurfaceAdherens junctionGap junctionIntermediatefilamentsDesmosomeDesmosomeBasalsurfaceHemidesmosomeBasal laminaGap junctionConnective tissue▲ FIGURE 6-5 The principal types of cell junctions thatbetween cells.
Gap junctions allow the movement of smallmolecules and ions between the cytosols of adjacent cells.The remaining three types of junctions—adherens junctions,spot desmosomes, and hemidesmosomes—are critical tocell–cell and cell–matrix adhesion and signaling. (b) Electronmicrograph of a thin section of intestinal epithelial cells, showingrelative locations of the different junctions. [Part (b) C.
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