The Structure of Tissues

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The theme of this chapter is summarized by a paragraph from Trelstad and Silver (1981): “The processes of morphogenesis, growth, repair, adaptation and aging are all reflected in tissues by the processes of either changing the ratio of their constituents or the properties of their constituents. The capacity of cells in tissues to produce composite extracellular matrices that assemble into multiple diverse forms reflects a successful stratagem of multicellular organisms to segregate cells into functional units of tissues and organs able to contend with the forces of gravity and work in unison to transmit the forces necessary for movement. The shape and size of higher organisms are defined by spaces, partitions, and unique forms of the matrix. In the embryo, the extracellular matrix is the scaffolding that helps determine tissue patterns, and in the adult it serves to stabilize these same patterns. The mineralized matrices of the bones and teeth are stiff, hard structures, whereas the nonmineralized cartilages are flexible and compressible and serve as joint cushions during compression and translation of joints. The ropelike organization of tendons and ligaments provides them with the capacity to withstand large forces without stretching more than a few percent of their length, making movement possible. Elastic blood vessels transiently store pulse pressures generated by the heart and thus ensure a relatively continuous flow of blood. The stretchable, tough, and tight-fitting skin is a collagenous shield that serves to keep undesired materials out and desired materials in, whereas its counterpart covering the eye, the cornea, is a transparent lattice of collagen fibrils serving both barrier and optical functions.”