Connective tissues are diverse group of tissues that serve various binding and supportive functions. Connective tissue is composed of smaller cells, large number of fibres suspended in extra cellular ground substance. Cells that are present in connective tissue are fibroblasts and macrophages. Fibroblasts produce protein ingredients of extra cellular fibres. Macrophages are amoeboid cells that move around in the network of fibres and engulf bacteria and debris of dead cells by phagocytosis.
Ground substance is matrix that is secreted by cells of connective tissue and may be liquid, jelly like or solid in nature. The nature and chemical composition of matrix determine the functional properties of various connective tissues. Fibrous network of connective tissue consists of three types of fibres namely collagenous fibre, elastic fibre and reticular fibre. All these fibres are proteins in nature strong flexible collagenous fibre consists of protein collagen that has great tensile strength and is produced by fibroblast. Elastic fibres are long threads made up of protein elastin. These fibres are elastic in nature and resist stretching. Reticular fibres are very thin and branched and are also made up of collagen. All three fibres are tightly interwoven and form a fabric like structure that joins connective tissue with adjacent tissue. Connective tissues may be classified on basis of loose and dense arrangement of these fibres. Major types of connective tissue in vertebrates are loose connective tissue, fibrous connective tissue, adipose tissue, cartilage, bone and blood.
(1) Loose Connective Tissue:
In loose connective tissue strong, flexible fibres of protein collagen that are interwoven with fine, elastic and reticular fibres. Collagen fibres are made up of protein collagen that has great tensile strength and is produced by fibroblast. All these compounds give loose connective tissue its elastic consistency and make it excellent binding tissue. It is widely distributed under the epithelia of human body and binds the skin of underlying muscle tissue. Loose connective tissue also wraps different organs and act as cushions for these organs.
(2) Fibrous Connective Tissue:
In fibrous connective tissue the collagen fibres are densely packed and may lie parallel to one another creating very strong cords. Number of fibroblast in fibrous connective tissue is higher than loose connective tissue. It is located in the dermis of skin, subnucosa of digestive tract, fibrous capsules of organs and joints where it provides structural strength. Tendons and ligaments are two important forms of fibrous connective tissue. The tendons connect muscles to bones or to other muscles while ligaments not only lie between two bones but also help in joint formation.
(3) Adipose Tissue:
Adipose tissue is specialized form of loose connective tissue that consists of large sized cells known as adipocytes or adipose cells distributed throughout matrix. Each adipose cell contains large fat droplets that push the nucleus and cytoplasm towards one side closer to plasma membrane. Adipose cells swell when fat is stored and shrinks when it is used as fuel by the body. These cells accumulate in large number to form material commonly known as fat. It is located in the dermis of skin, in bones, breasts and mesenteries in abdomen and around the kidney and heart. It provides reverse fuel in the form of lipids. It acts as insulating pad and helps in prevention of heat loss. They also support and protect organs.
(4) Cartilage Tissue:
It is hard but flexible tissue comprising of few cells, a ground substance and large number of fibres. In the cartilaginous tissue there are present large numbers of small spaces called lacunae. The rubbery matrix the chondrion which is secreted by chondroblasts surrounds the lacunae. Chondrion is protein carbohydrate complex and it along with collagen fibres gives cartilage its strength and elasticity. Cartilage lacks blood supply; all nutrients and waste material must diffuse through the ground substance from the surrounding tissue.
Fishes like sharks, rays and skates have cartilaginous skeleton other vertebrates also have the cartilaginous skeleton during embryonic development which is replaced by bony skeleton. Cartilagenous skeleton is located as flexible support in outer ear, nose and rings supporting wind pipe, dises that act as cushion between vertebrate and caps on ends of some bones.
(5) Hyaline Cartilage:
In Hyaline cartilage cells are located in lacunae surrounded by inter cellular material containing fine collagen fibres. Hyaline cartilage forms embryonic skeleton, covers the ends of long bones and form cartilage of nose, trachea and larynx. It also maintains the shape of these structures and also provides support.
(6) Elastic Cartilage:
It consists of fine collagenous fibres and many elastic fibres in its inter cellular material. It is located in external ear and epiglottis where it maintains the shape of these structures also provides great flexibility.
(7) Fibro Cartilage:
Fibro cartilage contains many large collagenous fibres in it inter cellular material. Fibroblasts are present in fibro-cartilage. They are present in inter verbal dise, Public symphysis and discs of knee joint. It is responsible for absorbing compression shock.
(8) Bone Tissue:
It is the strongest of vertebrate connective tissue containing mineralized collagen fibres. Bone cells also known as osteocytes are located within lacunae but the matrix around them is heavily impregnated with calcium phosphates? Making this kind of tissue hard and ideally suited for its functions of support and protection.
Bone matrix is deposited in concentric layers around central canal, the osteonic canal, located in bones and support, protects and provides lower system for muscles to act on, stores calcium and fat from blood cells.
(9) Blood:
Blood is also a kind of connective tissue having the fluid matrix in the form of plasma, suspends specialized red blood cells, white blood cells and platelets. Matrix plasma consists of water, salts and variety of dissolved proteins. Suspended in the plasma are red blood cells or erythrocytes which carry oxygen to different parts of the body. Other type of blood cells is called white blood cells or leucocytes that fight against viruses, bacteria and other invaders. Platelets are the cell fragments and are involved in blood clothing. It is located within blood vessels in higher organisms. It transports substances like oxygen, carbon dioxide, nutrients, wastes, hormones, minerals, vitamins etc throughout the body of animal.
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