Xix

Connective tissue of skin, bone, tendon, ligaments, dentin, sclera, fascia, and organ capsules (accounts for 90% of body collagen)

Cartilage (hyaline and elastic), notochord, and intervertebral disc

Connective tissue of organs (uterus, liver, spleen, kidney, lung, etc.); smooth muscle; endoneurium; blood vessels; and fetal skin

Basal laminae of epithelia, kidney glomeruli, and lens capsule

Distributed uniformly throughout connective tissue stroma; may be related to reticular network

Forms part of the cartilage matrix immediately surrounding the chondrocytes

Present in anchoring fibrils

Product of endothelial cells

Found in cartilage associated with type II collagen fibrils

Produced by chondrocytes in the zone of hypertrophy of normal growth plate

Produced by chondrocytes; associated with type II collagen fibrils

Isolated from skin and placenta; abundanf in tissues where mechanical strain is high

An unusual transmembrane collagen detected in bone, cartilage, intestine, skin, placenta, and striated muscles

Isolated from placenta; also detected in the bone marrow

Present in tissues derived from mesenchyme

Localized in close association with fibroblasts and arterial smooth muscle cells, but not associated with type I collagen fibrils

Another unusual transmembrane collagen found in epithelial cell membranes

Found in epithelial and vascular basement membrane

Discovered from the sequence of rhabdomyosarcoma cDNA

Provides resistance to force, tension, and stretch

Provides resistance to intermittent pressure Provides structural support and elasticity

Provides support and filtration barrier

Localized at the surface of type I collagen fibrils along with type XII and XIV collagen to modulate biomechani-cal properties of the fibril

Attaches the chondrocyte to the matrix

Secures basal lamina to connective tissue fibers

Facilitates movement of endothelial cells during angio-genesis

Stabilizes network of cartilage type II collagen fibers by interaction with proteoglycan molecules at their intersections

Contributes to the bone mineralization process by forming hexagonal lattices necessary to arrange types II, IX, and XI collagen within cartilage.

Regulates size of type II collagen fibrils; it is essential for cohesive properties of cartilage matrix

Localized at the surface of type I collagen fibrils along with type V and XIV collagen to modulate biomechani-cal properties of the fibril

Associated with the basal lamina along with type VII collagen

Localized at the surface of type I collagen fibrils along with type V and XII collagen to modulate biomechanical properties of the fibril; has a strong cell-cell binding property

Involved in adhesion of basal lamina to the underlying connective tissue

Contributes to structural integrity of connective tissue

Interacts with integrins to stabilize hemidesmosome structure

Represents a basement membrane heparan sulfate proteoglycan thought to inhibit endothelial cell proliferation and angiogenesis

The pronounced vascular and stromal interaction suggests involvement in angiogenesis

"Fibrillar collagens are indicated by blue type. Nonfibrillar collagens are indicated by black type.

"Each collagen molecule is composed of three polypeptide a chains intertwined in a helical configuration. The Roman numerals in the parentheses in the Composition column indicate that the a chains have a distinctive structure that differs from the chains with different numerals. Thus, collagen type I has two identical al chains and one a2 chain; collagen type II has three identical al chains.

plasma membrane to produce the collagen molecule, followed by assembly of the molecules into fibrils in the extracellular matrix under guidance by the cell (Fig. 5.7).

Collagen synthesis involves a number of intracellular events

• Polypeptide chains are produced by polyribosomes of the rough endoplasmic reticulum (rER). This synthesis (translation) is directed by information provided by messenger RNA (mRNA), and newly synthesized polypeptides are simultaneously discharged into the cis-ternae of the rER.

• Within the cisternae of the rER and the Golgi apparatus, a number of posttranslational modifications of the polypeptide chains occur. These include

1. Cleavage of the signal peptide

2. Hydroxylation of proline and lysine residues while the polypeptides are still in the nonhelical conformation

3. Addition of O-linked sugar groups to some hydroxy-lysine residues (glycosylation) and N-linked sugars to the two terminal positions

4. Formation of a triple helix by three polypeptide chains, except at the terminals where the polypeptide chains remain uncoiled

5. Formation of intrachain and interchain hydrogen bonds that influence the shape of the molecule and stabilize the interactions of the polypeptides.

The resultant molecule is procollagen. Note that ascor-bate (vitamin C) is required for the function of prolyl-hydroxylase and lysylhydroxylase; without posttranslational hydroxylation of proline and lysine, the hydrogen bonds essential to the final structure of the collagen molecule cannot form. This explains why wounds fail to heal and bone formation is impaired in vitamin C deficiency (scurvy).

• The procollagen moves to the exterior of the cell by means of exocytosis of secretory vesicles. Microtubules are involved in the movement of the secretory vesicles from the region of the Golgi apparatus to the cell surface. If the microtubules are disrupted with agents such as colchicine or vinblastine, the secretory vesicles accumulate in the Golgi region.

Collagen synthesis also involves extracellular events

• As procollagen is secreted from the cell, it is converted to a collagen molecule by procollagen peptidase associated with the cell membrane, which cleaves the uncoiled ends of the molecule (Fig. 5.8).

• The aggregated collagen molecules then align to form the final collagen fibrils. The cell controls the orderly array of the newly formed fibrils by directing the secretory vesicles to a localized surface site for discharge. The cell simultaneously creates a "cove" or indentation at its surface to allow molecules to concentrate where assembly will occur (see Fig. 5.7). Within the cove, the collagen fibril self-assembles: The collagen molecules align in rows and then cross-link by covalent bonds that form between the lysine and hydroxylysine aldehyde groups.

Reticular Fibers

Reticular fibers provide a supporting framework for the cellular constituents of various tissues and organs

Reticular fibers and collagen fibers share a prominent feature: They both consist of collagen fibrils. Unlike collagen fibers, however, reticular fibers are composed of type III collagen. The individual fibrils that constitute the reticular fiber exhibit a 68-nm banding pattern (the same as the fibrils of type I collagen). The fibrils have a narrow diameter (about 20 nm) and typically do not bundle to form thick fibers.

In routinely stained H&E preparations, reticular fibers cannot be identified positively. When visualized in the light microscope with special techniques, the reticular fibers have a thread-like appearance. Because they contain a greater relative content of sugar groups than collagen fibers, reticular fibers are readily displayed by means of the periodic acid-Schiff (PAS) reaction. They are also revealed with special silver-staining procedures, such as the Gomori and Wilder methods. After silver treatment, the fibers appear black; thus, they are said to be argyrophilic (Fig. 5.9). The thicker collagen fibers in such preparations are colored brown.

Reticular fibers are named for their arrangement in a mesh-like pattern or network

In loose connective tissue, networks of reticular fibers are found at the boundary of connective tissue and epithelium, as well as surrounding adipocytes, small blood vessels, nerves, and muscle cells. Reticular fibers also function as a supporting stroma in hemopoietic and lymphatic tissues (but not in the thymus). In these tissues, the collagen of the reticular fiber is produced by a special cell type, the reticular cell. This cell maintains a unique relationship to the fiber; it surrounds the fiber with its cytoplasm, thus isolating the fiber from other tissue components.

In most other locations, the reticular fiber is produced by fibroblasts. Important exceptions to this general rule include the endoneurium of peripheral nerves, where Schwann cells secrete reticular fibers, and the reticular and other collagen fibers secreted by smooth muscle cells of the tunica media of blood vessels and the muscularis of the alimentary canal.

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