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  • Connective tissue is specialized to physically support and connect other tissues and maintain the water required for metabolite diffusion to and from cells.

  • Connective tissues all consist primarily of extracellular material rather than cells.

  • Within most organs connective tissue proper forms the supportive stroma, which supports the organ’s unique functional components or parenchyma.

  • The ECM of connective tissue proper usually consists of both large protein fibers and nonfibrous areas of unstained ground substance rich in various GAGs and water.

  • All adult connective tissues are derived from an embryonic form of connective tissue called mesenchyme, which contains uniformly undifferentiated cells scattered in a gel-like matrix.

Cells of Connective Tissue
  • Fibroblasts (fibrocytes), the major cells of connective tissue proper, are elongated, irregularly shaped cells with oval nuclei that synthesize and secrete most components of the ECM.

  • Adipocytes (fat cells) are very large cells specialized for storage of triglycerides; they predominate in a specialized form of connective tissue called adipose tissue.

  • Macrophages are short-lived cells that differentiate in connective tissue from precursor cells called monocytes circulating in the blood; they function in ECM turnover, phagocytosis of dead cells and debris, and antigen presentation to lymphocytes.

  • Mast cells also originate from blood cell precursors and are filled with granules for the release of various vasoactive agents and other substances during inflammatory and allergic reactions.

  • Plasma cells are short-lived cells that differentiate from B lymphocytes and are specialized for the abundant secretion of specific antibodies (immunoglobulins).

  • Besides macrophages and plasma cells, other leukocytes normally wander through all types of connective tissue proper, providing surveillance against bacterial invaders and stimulating tissue repair.

Fibers of Connective Tissue
  • The most important and abundant fibers of connective tissue are composed of the protein collagen, of which there are some 20 related types.

  • Synthesis of collagen by fibroblasts and certain other cells involves posttranslational modifications in the RER, notably hydroxylation of the numerous prolines and lysines, and formation of helical trimeric subunits of procollagen.

  • Upon exocytosis, the nonhelical ends of the procollagen subunits are removed, forming trimeric collagen molecules that aggregate and become covalently bound together in large collagen fibrils.

  • The highly regular assembly of collagens in the fibrils produces a characteristic pattern of crossbanding visible ultrastructurally along the fibrils of some collagen types.

  • Fibrils of type I collagen are bundled together by other forms of nonfibrillar, linking collagens to produce large collagen bundles.

  • Collagen fibrils are degraded by collagenase enzymes classified as MMPs, produced primarily by macrophages.

  • Type III collagen produces a network of delicate reticular fibers, which stain very dark with silver stains and are abundant in immune and lymphoid tissues.

  • Elastic fibers, or sheets called elastic lamellae, are composed of the proteins elastin and fibrillin, which exist in a stretchable conformation that provides elastic properties to connective tissues rich in this material.

Ground Substance
  • Ground substance...

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