Loose or areolar connective tissue is very widely distributed in the body. It forms the superficial and of the deep fascia; forms part of the frame work (stroma) of most of the organs; or surrounds blood vessels and nerves and fills in any otherwise unoccupied spaces.
Areolar tissues contain cells, fibres and ground substance.
CELLS
- Cells constantly present are:
- Fibroblasts
- Macrophages or histiocytes
- Mast cells
- Plasma cells
- Wandering cells
- Pigment cells
- Fat cells
- Of these fibroblast and histiocytes are the most numerous.
Fibroblasts
- These cells are larger flat branching cells with extensive processes which may join the processes of other fibroblasts. The cell membrane is delicate and usually not seen. The nucleus is spherical or oval and in ordinary preparations is lightly stained. In section, the fibroblast nuclei are usually shrunken and stain deeply with basic dyes. It contains dusk like chromatin and one of two nucleoli. The cytoblasm is homogenous or finely granular and stains very lightly.
- Fibroblasts are present in all connective tissue such as tendon. They are responsible for formation of fibres. The name fibroblasts instead of fibrocytes indicate that after injury they are active and form new fibres. Fibrocytes on the other are inactive. Their cell bodies are less irregular and nucleus is condensed to a long cylinder or flattened oval. Chromatin is dense.
Histiocytes or Macrophages
- These cells are irregular cells with short processes. The nucleus is smaller and darkly staining than fibroblasts: cytoplasm coarsely granular and shows vacuoles. They are studied by vital staining. They are present in all fasciae and in the stroma of organs. They also occur in sinusoids of liver in lymphoid organs and in bone marrow. They show amoeboid movement and phagocytosis is pathological conditions.
- When activated, the histiocytes become larger and more rounded and the cytoplasm will be filled with granules of ingested material. They form part of the macrophage system or reticulo endothelial system.
Mast cells
- Occur in most loose connective tissue especially along the course of blood vessels. They are large oval or round cells with pale-staining nucleus and coarse cytoplasmic granules which stain with certain basic dyes. They resemble basophils of blood and contain an anticoagulant heparin, histamine and serotonin.
Plasma cells
- Plasma cells are comparatively rare in loose connective tissue, but are numerous in the alimentary mucous membrane and the great omentum and also in pathological conditions. They are smaller than macrophages of round or irregular shape, with basic staining homogenous cytoplasm. The nucleus is small and eccentrically placed. Large granules of chromatin radially arranged in a regular manner give a cart wheel appearance to the nucleus. There is a characteristic unstained or lightly stained area in the cytoplasm at the side of the nucleus, where the cytoplasm is more abundant. They are derived from lymphocyte like cells and are active in forming antibodies.
Wandering cells
- These cells are lymphocytes, eosinophils and neutrophils, migrated from blood.
Three types of fibres are seen in adult connective tissue.
- White or collagenous fibres
- Elastic or yellow fibres
- Reticular fibres
- White or collagenous fibres
- These vary in size and are really bundles of extremely fine fibrillae, which lie parallel to one another giving the fibre a longitudinally striated appearance and are united by a small amount of intercellular cement substance. Fibrillae do not branch. The fibres is usually wavy, soft and flexible. They are non-extensile. They stain with acid dyes. Chemically they contain an albuminous substance called collagen which on boiling yields gelatin. The fibres shrink slightly in water; swell enormously in dilute acids or alkalies. In the dilute acids or alkali the fibres break up into fibrils, become individual the inter-fibrillar cement substance is dissolved. Gastric juice rapidly digests the fibres, but alkaline trypsin has no effect on them. Collagen fibres are birefringent or aniostropic throughout their length.
- Elastic or yellow fibres
- These are homogenous highly refractive fibres which are thinner than the white fibres. They branch and anastomose freely forming networks. Smaller fibres are round in cross section and larger are flat or polygonal. They are highly elastic. Chemically they contain elastin which does not yield gelatin and has a remarkable resistance to most agents. It is not affected by heat or cold water, by dilute acids or gastric juice. It is however rapidly dissolved by pancreatin. The elastic fibres are lightly stained by ordinary methods, but deeply stained by orcein and resorcin fuchsin.
- Reticular fibres
- These are smaller branching fibres which form netlike supporting frame work or reticulum. They become black in silver impregnation methods. Hence they are also called argyrophilic fibres. In most areolar tissue these reticular fibres are few. In glandular organs and between musclefibres they are numerous. In lymphoid organs and in red bone marrow, the reticular fibres are associated with a special type of cells called reticular cells. Reticular fibres with reticular cells form a type of tissue called the reticular tissue. In other situation reticular fibres have the same relationship to fibroblasts as do collagenous or elastic fibres. Chemically though related to collagenous fibres the reticular fibres are not dissolved by pepsin and on boiling yield reticulin and not gelatin.
Ground substance
- It is the background material within which all other connective tissue elements are embedded. It varies from a fluid-like to a gel like state. It is one of the two components of non-fibrillar intercellular substance. The other is the tissue fluid. In ordinary connective tissue, the ground substance consists mainly of water whose major role is to provide a route for communication and transport (by diffusion) between tissues. This water is stabilized by a complex of glycosaminoglycans ( GAGs ), proteoglycans , and glycoproteins , all of which comprise only a small fraction of the weight of the ground substance.
- Ground substance may be highly modified in the special forms of connective tissue.
- In blood, the ground substance lacks stabilizing macromolecules. We call this free-flowing ground substance as plasma .
- In skeletal tissue, the ground substance may become mineralized by deposition of calcium salts. We call this rigid ground substance as bone .
- In cartilage, the ground substance is much more solid than in ordinary connective tissue but still retains more resiliency than bone.
Functions of areolar tissue
It binds structures or holds them in position.
It acts as padding and serves as a pathway for blood vessel and nerves. Nutrient substances to body cells and metabolites from the cells traverse connective tissue.
Spread of localized infections is limited by connective tissue.
