H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 81
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1998. Methods in Cell Biology, Vol.56: Video Microscopy. Academic Press.5.1 • Last A Head1976INTEGRATING CELLSINTO TISSUESModel of inflammatory bowel disease in whichcultured flat colonic smooth muscle cells were inducedto secrete cables of hyaluronan (green) that bind tospheroidal mononuclear leukocytes via their CD44receptors (red).
Nuclei are stained blue. [Courtesy ofC. de la Motte et al., Lerner Research Institute.]In the development of complex multicellular organismssuch as plants and animals, progenitor cells differentiateinto distinct “types” that have characteristic compositions, structures, and functions. Cells of a given type oftenaggregate into a tissue to cooperatively perform a commonfunction: muscle contracts; nervous tissues conduct electricalimpulses; xylem tissue in plants transports water. Differenttissues can be organized into an organ, again to perform oneor more specific functions. For instance, the muscles, valves,and blood vessels of a heart work together to pump bloodthrough the body.
The coordinated functioning of manytypes of cells within tissues, as well as of multiple specialized tissues, permits the organism as a whole to move, metabolize, reproduce, and carry out other essential activities.The adult form of the roundworm Caenorhabditis eleganscontains a mere 959 cells, yet these cells fall into 12 differentgeneral cell types and many distinct subtypes. Vertebrateshave hundreds of different cell types, including leukocytes(white blood cells), erythrocytes, and macrophages in theblood; photoreceptors in the retina; adipocytes that store fat;secretory and cells in the pancreas; fibroblasts in connective tissue; and hundreds of different subtypes of neurons inthe human brain. Despite their diverse forms and functions,all animal cells can be classified as being components of justfive main classes of tissue: epithelial tissue, connective tissue,muscular tissue, nervous tissue, and blood.
Various cell typesare arranged in precise patterns of staggering complexity togenerate the different tissues and organs. The costs of suchcomplexity include increased requirements for information,material, energy, and time during the development of an individual organism. Although the physiological costs of complex tissues and organs are high, they provide organismswith the ability to thrive in varied and variable environments, a major evolutionary advantage.The complex and diverse morphologies of plants andanimals are examples of the whole being greater than thesum of the individual parts, more technically described as theemergent properties of a complex system.
For example, theroot-stem-leaf organization of plants permits them to simultaneously obtain energy (sunlight) and carbon (CO2) fromOUTLINE6.1 Cell–Cell and Cell–Matrix Adhesion:An Overview6.2 Sheetlike Epithelial Tissues: Junctionsand Adhesion Molecules6.3 The Extracellular Matrix of Epithelial Sheets6.4 The Extracellular Matrix of NonepithelialTissues6.5 Adhesive Interactions and Nonepithelial Cells6.6 Plant Tissues6.7 Growth and Use of Cultured Cells197198CHAPTER 6 • Integrating Cells into Tissuesthe atmosphere and water and nutrients (e.g., minerals) fromthe soil. The distinct mechanical properties of rigid bones,flexible joints, and contracting muscles permit vertebratesto move efficiently and achieve substantial size.
Sheets oftightly attached epithelial cells can act as regulatable, selective permeability barriers, which permit the generation ofchemically and functionally distinct compartments in an organism (e.g., stomach, bloodstream). As a result, distinct andsometimes opposite functions (e.g., digestion and synthesis)can efficiently proceed simultaneously within an organism.Such compartmentalization also permits more sophisticatedregulation of diverse biological functions.
In many ways, theroles of complex tissues and organs in an organism are analogous to those of organelles and membranes in individualcells.The assembly of distinct tissues and their organizationinto organs are determined by molecular interactions at thecellular level and would not be possible without the temporally and spatially regulated expression of a wide array of adhesive molecules. Cells in tissues can adhere directly to oneanother (cell–cell adhesion) through specialized integralmembrane proteins called cell-adhesion molecules (CAMs)that often cluster into specialized cell junctions (Figure 6-1).Cells in animal tissues also adhere indirectly (cell–matrixAdaptersTight junctionCell adhesionmolecules (CAMs)Apical surfaceCELL1CELL4CELL-CELL ADHESIONS6AdapterGap junctionIntermediatefilament8 Adherens junction79 DesmosomeHemidesmosome210ConnexonActinBasalsurface▲ FIGURE 6-1 Schematic overview of major adhesiveinteractions that bind cells to each other and to theextracellular matrix.
Schematic cutaway drawing of a typicalepithelial tissue, such as the intestines. The apical (upper) surfaceof these cells is packed with fingerlike microvilli 1 that projectinto the intestinal lumen, and the basal (bottom) surface 2 restson extracellular matrix (ECM). The ECM associated with epithelialcells is usually organized into various interconnected layers (e.g.,the basal lamina, connecting fibers, connective tissue), in whichlarge, interdigitating ECM macromolecules bind to one anotherand to the cells 3 . Cell-adhesion molecules (CAMs) bind toCAMs on other cells, mediating cell–cell adhesions 4 , andadhesion receptors bind to various components of the ECM,mediating cell–matrix adhesions 5 .
Both types of cell-surfaceadhesion molecules are usually integral membrane proteinswhose cytosolic domains often bind to multiple intracellularadapter proteins. These adapters, directly or indirectly, link theCAM to the cytoskeleton (actin or intermediate filaments) and toCELL5CELL-MATRIXADHESIONSAdhesionreceptorsBasal lamina3Extracellularmatrix (ECM)ECMintracellular signaling pathways. As a consequence, informationcan be transferred by CAMs and the macromolecules to whichthey bind from the cell exterior into the intracellular environment,and vice versa. In some cases, a complex aggregate of CAMs,adapters, and associated proteins is assembled.
Specific localizedaggregates of CAMs or adhesion receptors form various types ofcell junctions that play important roles in holding tissues togetherand facilitating communication between cells and theirenvironment. Tight junctions 6 , lying just under the microvilli,prevent the diffusion of many substances through theextracellular spaces between the cells. Gap junctions 7 allowthe movement through connexon channels of small molecules andions between the cytosols of adjacent cells. The remaining threetypes of junctions, adherens junctions 8 , spot desmosomes9 , and hemidesmosomes 10 , link the cytoskeleton of a cell toother cells or the ECM. [See V.
Vasioukhin and E. Fuchs, 2001, Curr.Opin. Cell Biol. 13:76.]6.1 • Cell–Cell and Cell–Matrix Adhesion: An Overviewadhesion) through the binding of adhesion receptors in theplasma membrane to components of the surrounding extracellular matrix (ECM), a complex interdigitating meshworkof proteins and polysaccharides secreted by cells into thespaces between them. These two basic types of interactionsnot only allow cells to aggregate into distinct tissues but alsoprovide a means for the bidirectional transfer of information between the exterior and the interior of cells.In this chapter, we examine the various types of adhesivemolecules and how they interact.
The evolution of plants andanimals is thought to have diverged before multicellularorganisms arose. Thus multicellularity and the molecularmeans for assembling tissues and organs must have arisen independently in animal and plant lineages. Not surprisingly,then, animals and plants exhibit many differences in the organization and development of tissues. For this reason, wefirst consider the organization of epithelial and nonepithelial tissues in animals and then deal separately with plant tissues.
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