All connective tissues have two common characteristics; they are all developed from embryonic mesoderm, and have a large amount of intercellular substance. The types of connective tissues are tabulated overleaf.
What are Connective Tissues?
Connective tissues are a collection of living cells characterized by the possession of considerable numbers of fibres in the intercellular substance which, In life, is of a fluid or semi fluid nature. Areolar tissue is the characteristic type, the others being specialized forms; in many parts of the body various transitional types may be found.
Areolar Connective Tissue
This contains a great proportion of transparent semi-fluid matrix which contains abundant mucin. Scattered throughout the matrixare numerous bundles of wavy white fibres cemented together by mucin. The fibres largely consist of protein collagen which can be dissolve in the stomach by peptic digestion. Interspersed among these wavy bundles in an anastomosing network of simple fine fibre made of the protein elastin, which can be dissolve by trytic digestion. Careful examination of the tissues will reveal a number of types of cells. Fibro-blasts often lie along side the fibres which they secrete. They are flattened cells, often branched, with oval nuclei. Wandering phagocytic cells known as histiocytes are often found. They may be rounded or amoeboid and have small spherical nuclei. Many small oval mast cells with granular cytoplasm may be seen; these produce the matrix. Small plasma cells derive by division of migratory lymphocytes from the blood, are common; in inflammatory conditions they may be plentiful. In the areolar tissue of some regions, pigment cells are present. They are much-branched, contain numerous melanin granules, and the especially plentiful in the skin, the eye, and the pia matter enveloping the brain and spinal cord.
Areolar tissue is almost ubiquitous in the body, packing in all organs connecting the skin to the under-lying structures, surrounding blood-vessels and nerves wherever they penetrate into organs, fastening together the two sheets of squamous epithelium which form the mesenteries, and securing the peritoneum firmly to the body-wall muscle and the muscle of the gut. While being essentially connective it allows of considerable stretching and recovery. The limit of stretch of the skin, for example, is reached when the white fibre bundles are straightened out, and recovery to the normal position is due to the elasticity of the yellow fibres.
White Fibrous Tissue
White fibrous tissue consists almost entirely of closely-packed white collagen fibres occasionally interspersed with continuous rows of fibroblasts.The bundles of white fibres are bound together by areolar tissue. It is found in locations where great strength with limited flexibility are desirable. Such are the tendons of muscles, the dura mater of the brain and other sheets covering organs such as the sclerotic and cornea of the eye, and the kidney capsule. The perichondrium of cartilage and the periosteum of bone also consist mainly of this tissue.
This consists mainly of parallel, yellow elastic fibres, with interspersed fibroblasts and intervening bundles of very fine white fibres. It combines considerable strength with great elasticity and is found in the cords of the neck, in many ligament joining the bones, in the walls of arteries and in the bronchioles. There is a great deal of elastic tissue in the lung; it is largely responsible for recovery of the lung after distension. Elastic tissue reaches its greatest development in the ligamentum nuchae, extending from the rear of the skull to the neural spines of the cervical vertebrae.
In several regions of the body, areolar tissue is found with the matrix largely filled up with fat cells. These are large spherical cells with a central globule of fat which has squeezed the cytoplasm and nucleus to the periphery. This adipose tissue is arranged in lobules encased in areolar connective tissues. In mammals, it is mainly located in the dermis of the skin, above the kidneys and sometimes, in older animals around the heart.
All the connective tissues proper are derived from a soft jelly with few fibres in the embryo animal. The only remnant of this embryonic jelly is found in the vitreous humour of the eye.
Cartilage and bone provide firm areas for attachment of the tendons of the muscles. They are a specialized form of connective tissues. They give a large measure of support to the body, forming a framework to which soft parts can be fastened. Further, they provide a system of levers with fulcra at the joints, and in conjunction with the muscles, enable locomotion.