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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.

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.
Loose connective tissue is primarily located beneath the
epithelia that cover the body surfaces and line the internal sur￾faces of the body. It is also associated with the epithelium of
glands and surrounds the smallest blood vessels (Plate 4,
page 192). 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.
Most cell types in loose connective tissue are transient wander￾ing cells that migrate from local blood vessels in response to
specific stimuli. Loose connective tissue is, therefore, the site
of inflammatory and immune reactions. During these reac￾tions, loose connective tissue can swell considerably. In areas
of the body where foreign substances are continually present,
large populations of immune cells are maintained. 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.
Dense irregular connective tissue is characterized by
abundant fibers and few cells.
Dense irregular connective tissue contains mostly colla￾gen fibers. Cells are sparse and are typically of a single type,
the fibroblast. This tissue also contains relatively little ground
substance (Plate 4, page 192). Because of its high pro -
portion 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 submu￾cosa in which the fiber bundles course in varying planes.
This arrangement allows the organ to resist excessive stretch￾ing and distension. Similarly, skin contains a relatively thick
layer of dense irregular connective tissue called the reticular
layer (or deep layer) of the dermis. The reticular layer pro￾vides 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 of dense regular
connective tissue are the prominent feature, and there is little
ECM. However, in dense regular connective tissue, the fibers
are arranged in parallel array and are densely packed to pro￾vide maximum strength. The cells that produce and maintain
the fibers are packed and aligned between fiber bundles.
• Tendons are cordlike structures that attach muscle to
bone. They consist of parallel bundles of collagen fibers.
Situated between these bundles are rows of fibroblasts
called tendinocytes (Fig. 6.4 and Plate 5, page 194).
Tendinocytes are surrounded by a specialized ECM that
separates them from the load-bearing collagen fibrils. In
H&E–stained cross sections of tendon, the tendinocytes
appear stellate. 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. In
most H&E–stained longitudinal sections, however,
tendinocytes appear only as rows of typically flattened ba￾sophilic nuclei. 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. 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. 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 arranged than those of tendons. Ligaments join
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 lig￾aments associated with the spinal column (e.g., ligamenta
flava) contain many more 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, aponeurosis
is a dense regular connective tissue. This orthogonal
array is also found in the cornea of the eye and is respon￾sible 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 connec￾tive 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 the most abundant structural compo￾nents of the connective tissue. They are flexible and have a