Connective tissue develops from the mesoderm and is widely distributed in the body. There are four main classes of connective tissues.They are connective tissue (which includes fat and the fibrous tissue of ligaments), cartilage, bones and blood. Major functions of connective tissues are binding and support, protection, insulation and transportation of substances.
All connective tissues consist of three main componentsnamelyfibres, groundsubstance and cells. The ‘Fibres’ of connective tissue provide support. Three types of fibres are found in the connective tissue matrix. They are collagen, elastic and reticular fibres. Connective tissue are of two types namely, Loose connective tissues (Areolar, Adipose and Reticular) and Dense connective tissues (dense regular, dense irregular and elastic). Specialized connective tissues include cartilage, bone and blood.
In this tissue the cells and fibres are loosely arranged in a semi fluid ground substances. For example the Areolar connective tissue beneath the skin acts as a support framework for epithelium and acts as a reservoir of water and salts for the surrounding body tissues, hence aptly called tissue fluid. It contains fibroblasts, macrophages, and mast cells (Figure 3.5).
Adipose tissue is similar to areolar tissue in structure and function and located beneath theskin. Adipocytes commonly called adipose or fat cells predominate and account for 90% of this tissue mass. The cells of this tissue store fats and the excess nutrients which are not utilised immediately are converted to fats and are stored in tissues. Adipose tissue is richly vascularised indicating its high metabolic activity. While fasting, these cells maintain life by producing and supplying energy as fuel. Adipose tissues arealso found in subcutaneous tissue, surrounding the kidneys, eyeball, heart, etc. Adipose tissue is called ‘white fat’ or white adipose tissue. The adipose tissue which contains abundant mitochondria is called ‘Brown fat’ or Brown adipose tissue. White fat stores nutrients whereas brown fat is used to heat the blood stream to warm the body. Brown fat produces heat by non-shivering thermogenesis in neonates.
Reticular connective tissue resembles areolar connective tissue, but, the matrix is filled with fibroblasts called reticular cells. It forms an internal framework (stroma) that supports the blood cells (largely lymphocytes) in the lymph nodes, spleen and bone marrow.
Fibres and fibroblasts are compactly packed in the dense connective tissues. Orientation of fibres show a regular or irregular pattern and is called dense regular and dense irregular tissues. Dense regular connective tissues primarily contain collagen fibres in rows between many parallel bundles of tissues and a few elastic fibres. The major cell type is fibroblast. It attaches muscles and bones and withstands great tensile stress when pulling force is applied in one direction. This connective tissue is present in tendons, that attach skeletal muscles to bones and ligaments attach one bone to another. Dense irregular connective tissues have bundles of thick collagen fibres and fibroblasts which are arranged irregularly. The major cell type is the fibroblast. It is able to withstand tension exerted in many directions and provides structural strength. Some elastic fibres are also present. It is found in the skin as the leathery dermis and forms fibrous capsules of organs such as kidneys, bones, cartilages, muscles, nerves and joints. Elastic connectivetissue contains high proportion of elastic fibres. It allows recoil of tissues following stretching. It maintains the pulsatile flow of blood through the arteries and the passive recoil of lungs following inspiration. It is found in the walls of large arteries; ligaments associated with vertebral column and within the walls of the bronchial tubes.
Specialised connective tissues are classified as cartilage, bones and blood. The intercellular material of cartilage is solidand pliable and resists compression. Cells of this tissue (chondrocytes) are enclosed in small cavities within the matrix secreted by them (Figure 3.6). Most of the cartilages in vertebrate embryos are replaced by bones in adults. Cartilage is present in the tip of nose, outer ear joints, ear pinna, between adjacent bones of the vertebral column, limbs and hands in adults.
Bones have a hard and non-pliable ground substance rich in calcium
salts and collagen fibres which gives strength to the bones.
It is the main tissue that provides structural frame to the body. Bones support and protect softer tissues and organs. The bone cells (osteocytes) are present in the spaces called lacunae. Limb bones, such as the long bones of the legs, serve weight-bearing functions. They also interact with skeletal muscles attached to them to bring about movements. The bone marrow in some bones is the site of production of blood cells.
Blood is the fluid connective tissue containing plasma, red blood cells (RBC), white blood cells (WBC) and platelets. It functions as the transport medium for the cardiovascular system, carrying nutrients, wastes, respiratory gases throughout the body.