Page 181 - Atlas of Histology with Functional Correlations

the ground substance serves as an efficient barrier. It prevents movement of

large molecules and the spread of pathogens from the connective tissue into
the bloodstream. However, certain bacteria can produce hyaluronidase, an
enzyme that hydrolyzes hyaluronic acid and reduces the viscosity of the
gellike ground substance, allowing pathogens to invade the surrounding
tissues.

The density of ground substance depends on the amount of extracellular
tissue fluid or water that it contains. Mineralization of ground substance, as a
result of increased calcium deposition, changes its density, rigidity, and
permeability to diffusion, as seen in developing cartilage models and bones.

In addition to proteoglycans, connective tissue also contains several large
cell adhesive glycoproteins, which have binding sites for cell receptors and
matrix molecules. The adhesive glycoproteins bind cells to the fibers. The
glycoprotein fibronectin binds connective tissue cells, collagen fibers, and
proteoglycans, thereby interconnecting all three components of the

connective tissue. Integral proteins of the plasma membrane, the integrins,
bind to extracellular collagen fibers and to actin filaments in the
cytoskeleton, thus establishing a structural continuity between the

cytoskeleton and the extracellular matrix. Laminin is a large glycoprotein
and a major component of the cell basement membrane. This protein binds
epithelial cells to the basal lamina and has binding sites for integrin, type IV
collagen, and other proteoglycans.

FIGURE 5.10 | Dense Regular Connective Tissue:

Tendon (Longitudinal Section)

Dense regular connective tissue is present in ligaments and tendons. Shown here

is a section of a tendon in the longitudinal plane showing the regular
arrangement of the collagen fibers.

The collagen fibers (2, 5, 8) are arranged in compact, dense parallel bundles
between which are thin partitions of looser connective tissue that contain parallel

rows of fibroblasts (1, 3). The fibroblasts (1, 3) have short processes (not visible
here) and nuclei that appear ovoid when seen in surface view (3) or flat and
rodlike in lateral view (1).

Dense irregular connective tissue with less regular fiber arrangement than in
the tendon surrounds and partitions the collagen bundles as the interfascicular

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