Intercellular ground substance.

It consists of fibers and amorphous substance. The fibers can be of three types: collagen, elastic and reticular. The intercellular ground substance is produced by cells of LICT. The main cells producing ground substance are fibroblasts. They produce both fibers and amorphous substance. Mast cells also produce some of the components. Partially, the ground substance is filtrated from blood plasma.

COLLAGEN FIBERS. They are made of collagen protein. Today, the 19 collagen types have been described. But only five of them are of great importance:

First type is in the connective tissue of skin, bone, artery wall.

Second type can be found in cartilages.

Third type is located in fetal dermis, in the wall of large vessels, in reticular fibers.

Fourth type is included to the structure of basement membranes and eye lens.

Fifth type also participates in basement membrane formation as well as in formation of walls of vessels, main substance of a cornea.

Collagens of I, II, III, and V types are fibrillar because they can form filaments and fibrils. Other collagens are amorphous because they have no such ability. VI-XIX types of collagen are incompletely studied.

Molecules of the collagens are made from three coiled around each other α-chains. The biochemical composition of the chains is also specific. Here is the prevalence of such amino acids as glycine, proline, lysine, hydroxyproline and hydroxylysine.

The collagen molecule is made by fibroblasts. Beside them the following cells may produce collagen: osteoblasts, chondroblasts, cementoblasts, dentinobalsts, reticular cells, smooth myocytes, cells of perynervium. The process of collagen biosynthesis is similar in all listed above cells. The process has two stages: intracellular and extracellular.

Intracellular stage. First which has to be done is synthesis of polypeptide chain. It takes place in rough EPR. The produced α-chains are accumulated in cisterns of EPR. Here they are subjected to modification: the hydroxilation of some lysine and proline molecules with help of vitamin C and formation of disulfide bonds. Insufficiency of vitamin C results in formation insufficient amount of hydroxilysine and hydroxiproline. The chains made by this way are unable to coil into threefold spiral. The next is formation of procollagen chains having 3 polypeptide chains. Each of them is shifted along others on ¼ of length. As result of this the collagen molecules have striations. Then the collagen molecules move to Golgi complex in secretory vesicles. Here they subject to terminal glicosilation. Molecules of procollagen in Golgi complex are packed to excretory vesicles. Then, they release to the extracellular matrix.

Extracellular stage. The terminal regions of procollagen molecules are cut off after seretion to the matrix. It results in formation of tropocollagen. This process takes place only in fibrillar collagens. It enables them to make fibrils. Next step is polymerization of tropocollagen molecules. They are joined “side by side” and “end to end”. They make protofibrilles with diameter 3-5 nm. Five-six protofibrilles make microfibrilles with diameter 10-20 nm. Then, microfibrilles merge with each other making fibrilles with diameter 100 nm. Several fibrilles are united to visible in light microscope fiberes with diameter 1-100 mcm. So, the collagen fiber has following levels of organization: polypeptide chain → procollagen molecule → tropocollagen molecule → protofibrilles → microfibrilles → fibrilles → collagen fibers.

The collagen fibers of LICT are mainly of I type. They are visible on histological slides as oxyphilic crimped fibers going in many directions along or forming bundles of various width. In polarizing microscope the collagen bundles have double refraction. In electronic microscope they look like parallel striated fibers.

Functions of collagen fibers: 1) support; 2) providing tissue strength; 3) regulatory – participation in morphogenesis, differentiation, regeneration of cells and tissues, regulation of migration, secretory and synthetic activity of cells, participation in cell adhesion, in platelets adhesion and in clot formation; 4) setting the tissue architecture of connective tissue.

ELASTIC FIBERS. They are less in LICT than collagen fibers. They are made of amorphous elastin and fibrillar fibrillin. Both are made in rough EPR and then are modified in Golgi complex. Elastin as well as collagen contains many glicine and proline molecules. It also contains two unique amino acids: desmosin and isodesmosin. The elastin molegules are globules. Being secreted to extracellular matrix they are united to the chains and form elastin protofibrilles with width 3nm.

Later, protofibrilles make central latex-like network of molecules. It is so called central component of elastic fiber. Outside of it there is so called fibrillar component of elastic fiber. The elements of it are made of fibrillin. The process of elastic fiber formation can be described as following. At the beginning of the fiber formation fibrillin make microfibrilles (oxytantal fibers). They serve as a framework for further elastin accumulation. Elastin accumulates centrally in the form of amorphous component (or so called elaunin fibers). Accumulated elastin shifts fibrillar component to the periphery.

The elastic fibers can be visible in light microscope only if special staining techniques have been used (iron hematoxilin, orsein and etc.). They look like thin straight branched lines connecting to each other and by this making three-dimensional network. They are elastic. This means that they can return to primary position after being stretched or compressed.

There are immature elastic fibers in LICT beside mature elastic fibers: oxytantal and elaunin fibers. The first type contains only oxytantal fibrilles and nothing else. In elaunin fibers there are both types of fibrilles, but oxytantal fibrilles are located centrally with a small amount of amorphous component. In mature fibers, as it was said above, the fibrillar component is located peripherally.

Apart to fibroblasts, the following cells can produce elastic fibers: chondroblasts, chondrocytes and smooth myocytes.

Functions of elastic fibers: 1) providing reversible deformability of tissues – elasticity; 2) setting the tissue architecture of connective tissue.

RETICULAR FIBERS. They may be reffere as collagen fibers because they are made of collagen type III. They are invisible in slides stained by routine hematixilin-eosine. They consist of microfibrilles and there are binding glicoproteins and proteoglicans between them. Perhaps, the reticular fibers can be impregnated by siver salts and they have positive Schiff base reaction. Reticular fibers are located in reticular tissue of hemopoietic and immune organs. However, they can be found in other types of connective tissue. The cells which can produce reticular fibers are fibroblasts, reticular cells, adipocytes, smooth myocytes, cardiomyocytes, neurolemmocytes.

The main function of the reticular fibers is support.

AMORPHOUS SUBSTANCE.

It is the second component of ground substance. It is transparent if it is studied by light microscope. In electronic microscope it has low electronic density.

It is made of water (90% of total volume), proteins (glicoproteins, proteoglicans, plasma proteins – albumines, globulines, fibrinogen), lipida, carbohydrates (af first glucosaminoglicans), inorganic substances. Glucoseaminoglican molecules are large. They make three dimensional network. They are hydrophilic and by this they bind a lot of water making gel. Glicoproteins and proteoglicans provide the interaction of the intercellular ground substance with cells, participate in substance transportation, accumulate growth factors and perform some of other functions. There are two functional states of the amorphous substance: sol and gel. Gel is colloid state, where as sol is more liquid state. The changing of functional state is controlled by hyaluronidase enzyme.

Functions f amorphous substance are regulatory, metabolic, providing environment for cells being.

Дата добавления: 2015-08-05