Ground substance
Ground substance is found in all
cavities and clefts between the fibres and cells of connective tissues. Water,
salts and other low molecular substances are contained within the ground
substance, but its main structural constituent are proteoglycans.
Ground substance is soluble in most of the solvents used
to prepare histological sections and therefore not visible in ordinary
sections.
Proteoglycans are responsible
for the highly viscous character of the ground substance.
Proteoglycans consist of proteins (~5%) and polysaccharide chains (~95%), which
are covalently linked to each other. The polysaccharide chains belong to one of
the five types of glycosaminoglycans, which form
the bulk of the polysaccharides in the ground substance.
Hyaluronan
(or hyaluronic acid) is the dominant
glycosaminoglycan in connective tissues. The molecular weight (MW) of hyaluronic acid is very high (~ MW 1,000,000 ). With a length of about 2.5 µm
hyaluronan is very large. Hyaluronan serves as a
"backbone" for the assembly of other glycosaminoglycans in connective
and skeletal tissue, which results in even larger molecule complexes (MW 30,000,000 - 200,000,000).
Hyaluronan is also a major component of the synovial
fluid, which fills joint cavities, and the vitreous body of the eye.
The remaining four major
glycosaminoglycans are chondroitin sulfate, dermatan
sulfate, keratan sulfate and heparan sulfate.
These glycosaminoglycans attach via core- and link-proteins to a backbone
formed by the hyaluronic acid. The coiled
arrangement of the hyaluronan and other attached glucosaminoglycans fills a
roughly spherical space with a diameter of ~0.5 µm. This space is called a domain. Neighbouring domains overlap and form a more or
less continuous three-dimensional molecular sieve
in the interstitial spaces of the connective tissues.
The large polyanionic
carbohydrates of the glycosaminoglycans bind large amounts of water and
cations. The bound water in the domains forms a medium for the diffusion of
substances of low molecular weight such as gases, ions and small molecules,
which can take the shortest route, for example, from capillaries to connective
tissue cells. Large molecules are excluded from the domains and have to find
their way through the spaces between domains.
The restricted motility of
larger molecules in the extracellular space inhibits the spread of
microorganisms through the extracellular space. A typical bacterium ( 0.5 x 1
µm) is essentially immobilised in the meshwork formed by the domains. The
pathogenicity of a bacterium is indeed to some extent determined by its ability
to find its way through the mesh, and some of the more invasive types produce
the enzyme hyaluronidase, which depolymerises
hyaluronic acid.
The components of the ground
substance, collagen, elastic and reticular fibres are synthesised by cells of
the connective tissues, the fibrocytes.
![thumbnail]( "Ground substance")
Title:
Ground substance
by:
om
at
2013-02-15T02:51:00+07:00
Rating: 4.8
of 5 Reviews
Ground substance