Reticular fibers in the lymph node. Photomicrograph of a lymph node silver preparation showing the connective tissue capsule at the top and a trabecula extending from it at the left. The reticular fibers (arrows) form an irregular anastomosing network. x650.

Elastic Fibers

Elastic fibers allow tissues to respond to stretch and distension

Elastic fibers are typically thinner than collagen fibers and are arranged in a branching pattern to form a three-dimensional network. The fibers are interwoven with collagen fibers to limit the distensibility of the tissue and to prevent tearing from excessive stretching.

Elastic fibers stain with eosin, but not well; therefore, they cannot always be distinguished from collagen fibers in routine H&E preparations. Because elastic fibers become somewhat refractile with certain fixatives, they may be distinguished from collagen fibers in specimens stained with H&E when they display this characteristic. Elastic fibers can also be selectively stained with special dyes such as orcein or resorcin-fuchsin, as shown in Figure 5.10.

Elastic fibers are produced by most of the same cells that produce collagen and reticular fibers, particularly fibroblasts and smooth muscle cells. In contrast to collagen fibers, however, elastic fibers are composed of two struc tural components: a central core of elastin and surrounding fibrillin microfibrils.

• Elastin is a protein that, like collagen, is rich in proline and glycine. Unlike collagen, it is poor in hydroxyproline and completely lacks hydroxy lysine. Elastin forms fibers of variable thickness or lamellar layers (as in elastic arteries).

• Fibrillin is a glycoprotein that forms fine microfibrils measuring 10 to 12 nm in diameter. During the early stages of elastogenesis, these microfibrils are formed first; elastin material is then deposited on the surface of the microfibrils. Elastin-associated fibrillin microfibrils play a major role in organizing elastin into fibers. The absence of fibrillin microfibrils during elastogenesis results in the formation of elastin sheets or lamellae, as found in blood vessels.

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