Dense regular connective tissue-tendon, a. Electron micrograph of a tendon at low magnification, showing tendinocytes (fibroblasts) and their thin processes (arrows) lying between the collagen bundles, x 1,600. b. A tendinocyte with prominent profiles of rough endoplasmic reticulum (rER) is shown at higher magnification. The collagen fibers (C) can be resolved as consisting of very tightly packed colla gen fibrils. The arrows indicate processes of tendinocytes. x9,500. Inset. Photomicrograph of a tendon. Note the orderly and regular alignment of the bundles of collagen fibers. Tendinocytes are aligned in rows between the collagen fibers. x200. (Electron micrographs modified from Rhodin J. Histology. New York: Oxford University Press, 1974.)

tendineum. It contains the small blood vessels and nerves of the tendon.

• Ligaments, like tendons, consist of fibers and fibroblasts arranged in parallel. The fibers of ligaments, however, are less regularly.iiiTa.nged than those of tendons. Ligaments foin bone to bone, which in some locations, such as in the spinal column, requires some elasticity. Although collagen is the major extracellular fiber of most ligaments, some of the ligaments associated with the spinal column (e.g., liga-

menta flava) contain many elastic fibers and fewer collagen fibers. These ligaments are called elastic ligaments.

• Aponeuroses resemble broad, flattened tendons. Instead of fibers lying in parallel arrays, the fibers of aponeuroses are arranged in multiple layers. The bundles of collagen fibers in one layer tend to be arranged at a 90° angle to those in the neighboring layers. The fibers within each of the layers are arranged in regular arrays; thus it is a dense regular connective tissue. This orthogonal ar ray is also found in die cornea of the eye and is believed to be responsible for its transparency.

^ connective tissue fibers

Connective tissue fibers are of three principal types

Connective tissue fibers are present in varying amounts, depending on the structural needs or function of the connective tissue. Each type of fiber is produced by fibroblasts and is composed of protein consisting' of long peptide chains. The types of connective tissue fibers are

• Collagen fibers

• Reticular fibers

• Elastic fibers

Collagen Fibers and Fibrils

Collagen fibers are the most abundant type of connective tissue fiber

Collagen fibers are flexible and have a remarkably high tensile strength. In the light microscope, collagen fibers typically appear as wavy structures of variable width and indeterminate length. They stain readily with eosin and other acidic dyes. They can also be colored with the dye aniline blue, used in Mallory's connective tissue stain, or with the dye light green, used in Masson's stain.

When examined with the transmission electron microscope (TEM), collagen fibers appear as bundles of fine, thread-like subunitsT^These subunits, are collagen fibiils (Fig. 5.5). Within an individual fiber, the collagen fibrils are relatively uniform in diameter. In different locations and at different stages of development, however, the fibrils differ in size. In developing or immature tissues, the fibrils may be as small as 15 or 20 nm in diameter. In dense regular connective tissue of tendons or other tissues that are subject to considerable stress, they may measure up to 300 nm in diameter.

Collagen fibrils have a 68-nm banding pattern

When collagen fibrils stained with osmium or other heavy metals are examined with the TEM, they exhibit a sequence of closely spaced transverse bands that repeat every 68 nm along the length of the fibril (Fig. 5.5, inset). This banding pattern reflects the fibril's subunit structure,

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