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Human Body
B. TISSUES Types of Tissues
Groups of cells with a similar structure and function together with the non-living material (inter-cellular substance) between them, form a tissue. Tissues are of many types. They differ in the structure of cells that form them and also in the inter-cellular substance. They can be grouped together into the following categories :
(i) Epithelial or covering tissue (ii) Connective tissue (iii) Bone and cartilage (iv) Muscle tissue, and
(v) Nerve tissue: (i) Epithelial Tissues
Tissues which cover and protect all external and internal surfaces of the body are called epithelia. The outer portion of the skin, the lining of the body cavities and of the digestive, urinary and reproductive tracts are epithelia. Epithelial tissue provides protection from microbes, from physical injury, from various irritants and from drying out. In kidneys, it acts as a membrane for filtration and dialysis, permitting selective passage of certain types of molecules while retaining others. There are many varieties of epithelial tissues, from thick tough skin to the delicate lining of the alveoli in the lungs. They consist of sheets of closely packed cells on a basement membrane of connective tissue. The simplest epithelia are formed of a single layer of flattened cells. They cover the tubes of the kidney, the inner side of the ear-drums, blood vessels, etc. The lining of the alimentary tract, on the other hand, is much thicker, because they have to secrete the enzymes and mucus. Multi-layered epithelia cover the outside of the body, outer ear, mouth, throat, etc. A special water-proof variety covers the internal surfaces of the bladder and other parts of the urinary tract.
The skin is the thickest epithelial tissue in the body. It is an organ of protection as well as heat regulation. It has two parts : tough epidermis and soft dermis.
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