ground substance, however, is abundant; in fact, it occupies
more volume than the fibers do. It has a viscous to gel-like
consistency and plays an important role in the diffusion of
oxygen and nutrients from the small vessels that course
through this connective tissue as well as in the diffusion of
carbon dioxide and metabolic wastes back to the vessels.Collagen fibers are the most abundant structural compo?nents of the connective tissue.Hollow organs (e.g., the intestinal tract) possess a distinct
layer of dense irregular connective tissue called the submu?cosa in which the fiber bundles course in varying planes. Although collagen is the
major extracellular fiber of most ligaments, some of the lig?aments associated with the spinal column (e.g., ligamenta
flava) contain many more elastic fibers and fewer collagen
fibers.Typically, the fibers are arranged
in bundles oriented in various directions (thus, the term
irregular) that can withstand stresses on organs or structures.In transmission electron micrograph
(TEM) sections parallel to the long axis of tendons, the
cytoplasmic projections of the cell are seen to lie be tween
the fibers and appear as thin cytoplasmic sheets.The cytoplasmic sheets that extend from
the body of the tendinocytes are not usually evident in lon?gitudinal H&E-stained sections because they blend in
with the collagen fibers.Similarly, skin contains a relatively thick
layer of dense irregular connective tissue called the reticular
layer (or deep layer) of the dermis.This tissue is thus the initial site where pathogenic
agents such as bacteria that have breached an epithelial surface
are challenged and destroyed by cells of the immune system.For
example, the lamina propria, the loose connective tissue of
mucous membranes, such as those of the respiratory and ali?mentary systems, contains large numbers of these cells.Because of its high pro -
portion of collagen fibers, dense irregular connective tissue
provides significant strength.In
most H&E-stained longitudinal sections, however,
tendinocytes appear only as rows of typically flattened ba?sophilic nuclei.The substance of the tendon is
surrounded by a thin connective tissue capsule, the epi?tendineum, in which the collagen fibers are not nearly as
orderly (Plate 5, page 194).Typically, the tendon is subdi?vided into fascicles by endotendineum, a connective tis?sue extension of the epitendineum.Connective tissue fibers are present in varying amounts,
 depending on the structural needs or function of the connec?tive tissue.Most cell types in loose connective tissue are transient wander?ing cells that migrate from local blood vessels in response to
specific stimuli.Dense irregular connective tissue contains mostly colla?gen fibers.Dense regular connective tissue is the main functional
component of tendons, ligaments, and aponeuroses.Each type of fiber is produced by fibroblasts and is
composed of protein consisting of long peptide chains.