Figure 53

Loose and dense irregular connective tissue. Photomicrograph comparing loose and dense irregular connective tissue from the mammary gland stained with Masson's trichrome. In the center, loose connective tissue surrounds the glandular epithelium. The loose connective tissue is composed of a wispy arrangement of collagen fibers with many cells. Note the large number of nuclei visible at this low magnification. On the upper left and lower right of the figure is dense irregular connective tissue, in contrast, few nuclei are revealed in the dense connective tissue. However, collagen is considerably more abundant and is composed of very thick fibers. X100.

membranes, such as those of the respiratory and alimentary systems, contains large numbers of these cells.

Dense irregular connective tissue is characterized by abundant fibers and few cells

Dense hregular connective tissue contains mostly collagen fibers. Cells are sparse and are typically~(5f^ar single type, the fibroblast- Tills tissue also contains relatively little ground substance. Because of its high proportion of collagen fibers, dense irregular connective tissue provides significant strength. Typically, the fibers are arranged in bundles oriented in various directions (thus the term "irregular") that can withstand stresses on organs or structures. Hollow organs (e.g., the intestinal tract) possess a distinct layer of dense irregular connective tissue, called the submucosa, in which the fiber bundles course in varying planes. This arrangement allows the organ to resist excessive stretching andjiistension. Similarly, skin contains a relatively thick layer of dense irregular connective tissue in the dermis, called the reticular or deep layer of the dermis. It provides resistance to tearing as a consequence of stretching forces from different directions.

Dense regular connective tissue is characterized by ordered and densely packed arrays of fibers and cells

Dense regular connective tissue is the main functional component of tendons, ligaments, and aponeuroses. As in dense irregular connective tissue, the fibers ofciense regular connective tissue are the prominent feature, and there is little ground substance. However, in dense regular connective tissue, the fibers are arranged in parallel array and are densely packed to provide maximum strengthTThe cells that produce and maintain the fibers are packed and aligned between fiber bundles.

• Tendons are cord-like structures that attach muscle to bone. They consist of parallel bundles of collagen^bers. Situated between these bundles are rows of fibroblasts called tendinocytes (Fig. 5.4). In H&E-stained cross sections of tendon, the tendinocytes appear stellate. In transmission electron micrograph sections parallel to the long axis of tendons, the cytoplasmic projections of the cell are seen to lie between the fibers and appear as thin cytoplasmic sheets. In most H&E-stained longitudinal sections, however, tendinocytes appear only as rows of typically flattened basophilic nuclei. The cytoplasmic sheets that extend from the body of the tendinocytes are not usually evident in longitudinal H&E-stained sections because they blend in with the collagen fibers.

• The substance of the tendon is surrounded by a thin connective tissue capsule, the epitendineum, in which the collagen fibers are not nearly as orderly. Typically, the tendon is subdivided into fascicles by endo-tendineum, a connective tissue extension of the epi-

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