To date, 25 distinct
collagen "a" chains have been identified and each
is encoded by a different gene. Combinations of these genes
are expressed in different tissues. In principle, more than
10,000 types of triple-stranded collagen molecules could be
assembled in the body from various combinations of the 25
but only 15 types of collagen molecules have been identified.
The main type of collagen in connective tissues is Type I,
II, III, V, and IX. Type I is the principle collagen of skin
and bone and, by far, the most abundant in the body (representing
90 per cent of body collagen). Type II is found in the cartilage.
Type III is found in skin, blood vessels and internal organs.
Type V is found in bone, skin, tendons, ligaments, and cornea.
Types IV and VIII are network-forming collagens which polymerize
to form the sheet-like network basal lammae and anchoring
fibril beneath stratified squamous epithelia (epithelium coherent
cell sheets formed from one or more layers of cells covering
an external surface or lining a cavity).
The tissues of
the body are not made solely of cells. A substantial part
of the tissue volume is extracellular space that is filled
with an intricate network of macromolecules that constitute
the extracellular matrix. The matrix is composed of a variety
of versatile proteins and polysaccharides that are secreted
locally and assembled into an organized network in close association
with the cells that produce them. In connective tissue, the
matrix is generally more plentiful than the cells it surrounds
and it determines the tissues physical properties. Variations
in the amounts of the different types of matrix macromolecules
give rise to an amazing diversity of forms. For example, the
matrix can become calcified to become the rock-hard structures
of our teeth and bones, or it can form the transparent matrix
of our cornea, or it can adapt the rope-like helix organization
that give tendons their enormous tensile strength. At the
interface of the epithelium and connective tissue, the matrix
forms a basal lamina, a tough but thin mat that plays a vital
roll in controlling cell behavior. Until very recently, the
extra-cellular matrix was thought to be relatively inactive
scaffolding to stabilize the more physical structure of the
tissues much like the concrete foundation of a house. Recent
research has proven that the matrix plays a very complex and
very active role in regulating the behavior of the cells that
contact it i.e. influencing development, migration,
proliferation, shape, and function. From the new information,
we have learned that the matrix and connective tissue are
message carriers and part of the bodys internal communication
system similar to the interoffice memo.