Classification

The descriptive terms of different types of connective tissue are assigned according to the correlation between cells and extracellular substance and to the fibers, which are loosely or densely packed, and their direction.

Thus they distinguish loose and dense connective tissue proper. In the loose connective tissue the ground substance predominates over the other components, while there is predominance of the fibers in the dense connective tissue and less amount of the ground substance and cells.

Within the second category it indicates the fibers have an ordered or a disordered arrangement.

The loose connective tissue always has disordered fibers and due to that is named loose irregular connective tissue.

The dense connective tissue, which has disordered fibers, is named dense irregular connective tissue and is present in the skin and some septa and capsules. In the second type of the dense connective tissue the fibers form parallel bundles as in the tendons, ligaments, aponeuroses. This type is called dense regular connective tissue.

We begin to consider different types of proper connective tissue from the most numerous and important – loose irregular connective tissue.

Loose irregular connective tissue (areolar)

Just loose irregular connective tissue is widely distributed and the most abundant in the human body because forms the stroma and capsules of each organ, and is located under the epithelia and everywhere accompanies blood and lymphatic vessels and carries out all of enumerated connective tissues functions.

Loose irregular connective tissue has all types of cells, which are characteristic for the connective tissue proper, big amount of ground substance and all types of fibers are arranged in different directions, which form network.

Embryogenesis and construction of the loose irregular connective tissue. Loose irregular connective tissue, as all other connective tissues, consists of cells and extracellular matrix. Connective tissue cells make all components of the extracellular substance.

The cells of the loose irregular connective tissue (as in the whole connective tissue proper) are predominantly developed from mesenchyme. There are two types of the connective tissue cells:

I.A relatively stable population of fixed cells, connective tissue cells properly or intrinsic cells. They are fibroblasts, adipose (fat) cells, reticular cells, pericytes, adventitial (perivascular) cells. All of them are of mesenchymal origin.

Pigment cell – is of a neural crest origin.

II. A population of mobile, wondering (migratory) cells or extrinsic cells. They come from blood and are of mesenchymal origin too.

The mostnumerous isfibroblast. It is fixed cell, that elaborates the extracellular fibers and amorphous components of the connective tissue. It is a stellate cell with several processes. Its cytoplasm contains all of organelles, particularly well developed rER, Golgi apparatus are necessary for the synthesis and secretion. The elliptical nucleus is usually smoothly contoured, but may be sometimes slightly folded. There are 1 or 2 nucleoli and the chromatin is sparse. The cytoplasm usually contains few inclusions. So, fibroblast is a 'fiber-forming cell'.

And there are the fibrocytes in the connective tissue. They are the mature form of fibroblast, smaller cells without cytoplasmic projections and they cannot produce fibers.

Adipose cells (fat cell, lipocyte) specialise in the synthesis and storage of lipids. Their functions are as follows: 1) they act as a storehouse of nutriment and energy, 2) mechanical function, 3) conservation of body heat.

Adipose cell accumulates fat until the nucleus is flattened and displaced to one side and the cytoplasm becomes thinner. That is why this cell has a thin rim of cytoplasm around of the single large lipid drop. In the slide of the usual histological section the lipid droplet is dissolved out during making up the preparation and we can see only a thin layer of cytoplasm, slightly thickened in the area of the nucleus. So, the assemblage of the fat cells is visible as a delicate meshwork of irregular polygonal profiles.

Reticular cells are the typical basal supportive cells of the hemopoietic organs and are the small cells with cytoplasmic processes.

Pericytes convey blood capillaries and participate in the regulation of the capillaries lumen.

Adventitial cells are undifferentiated cells that are located near blood vessels and can participate in the blood vessels regeneration.

Pigment cells originate from the neural crest. They have irregular shape with small cytoplasmic projections, ovoid light nucleus and more or smaller clumps of pigment in cytoplasm. These pigment clumps are melanin and pigment cells can absorb ultra-violet – that is why there are many pigment cells in a skin. Also they can synthesize melanin from the waste of adrenaline (the product of adrenal).

The second group of cells is formed of migratory cells that migrate from blood.

- Monocyte - in the connective tissue called macrophage.

- Plasma cell - is the B-lymphocyte of blood.

- All types of T-lymphocytes.

- Neutrophilic leukocyte - named in the connective tissue as microphage.

- Eosinophilic leukocyte - named commonly tissue's eosinophil.

- Basophilic leukocyte - as mast cell of connective tissue.

Those cells live and work in the connective tissue (particularly in the loose connective tissue). They participate in the protective function of the connective tissue.

Monocyte also belongs to the mononuclear phagocytic system, comprises a system of cells, characterised by their phagocytic capability and common derivation from monocyte.

Fig. 1. Mononuclear phagocytic system

Connective tissue extracellular substance consists of ground substance and fibers

Ground substance. The formed elements of the connective tissue are situated in a matrix of amorphous ground substance, having the properties of a viscous solution or thin gel.

This gel consists of glycosaminoglycans (GAGs) or mucopolysaccharides, such as hyaluronic acid and different sulfates: keratan-sulfate, chondroitin-sulfate. All GAGs are bound with protein core, forming proteoglican molecules. In a light microscope ground substance looks homogenous, but examination in an electron microscope shows that it contains network of fibrils and small granules.

Ground substance functions by diffusion of nutrients, fluids, metabolites, waste products between the cells of the different tissues and the blood vessels. Ground substance also functions as a barrier to the penetration of bacteria and foreign particles.

Water component of ground substance comes from blood passing through the capillary wall. Under certain pathologic conditions the quantity of tissue fluid may increase, causing the oedema in result.

Fig. 2. Components of Connective Tissue matrix.

There are two types of fibers - collagenous and elastic.

Collagenous fibers are thick, slightly wavy. They are present in all types of the connective tissue and are responsible for the tissue tensile strength.

At a high magnification faint longitudinal striations can be detected in the collagen fibers. They consist of the bundles of smaller parallel fibrils. In EM the unit fibrils of collagen are seen to be cross striated because each fibril consists of molecules are called tropocollagen (protein).

Tropocollagen consists of three polypeptide chains, which coiled around one another, that is why we can see striations.

At present many types of collagen fibers - about 14 - are known. For example, the reticular fibers, they are immature collagen fibers and sometimes termed as precollagen fibers (or type III collagen, or IV).

Collagen fibers are very durable, but swell and become soft, when treated with water, especially boiled or acid. On boiling collagen is converted into gelatine.

Elastic fibers called yellow fibers are rubber-like material. The normal function of some organs requires that their components be able to move to one another. Due to the elastic fibers, connective tissue in each organ has resilience to restore the original state after deformation. The elastic fibers in the connective tissue stroma and septa of organs provide the elasticity.

These fibers are smaller, thinner and less variable in size than collagen fibers and they branch and anastomose to form a network.

Now it is known that the elastic fibers have two components - microfibriles, aggregated in bundles, that are embedded in a more abundant amorphous component - elastin (protein). The elastic fibers can stretch and relax.

Notes, all components of the extracellular substance are produced by fibroblasts. This process begins inside the cell and ends outside, where collagen and elastic fibers connect with amorphous substance.

The elaboration of collagen fibers begins inside the fibroblast with the entrance of amino acids. At the ribosomes of the rER the polypeptide subunits are formed from entered amino acids. Next step is the synthesis of tropocollagen molecules which released at the cell surface with the subsequent formation of collagen fibrils outside the fibroblast.

The orientation of the synthesized fibrils is in response to mechanical stresses in the tissue and wound not be influenced by the cells.

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