Body Tissues

A tissue is a group of similar cells that work together to carry out a specific function. Body tissues can be classified into four basic types according to their function and structure: Epithelial tissue, connective tissue, muscular tissue and nervous tissue.

Epithelial tissue or epithelium consists of tightly packed cells that form a continuous layer. Because the cells are closely packed, and are held tightly together by many cell junctions, there is little intercellular space between adjacent plasma membranes.

The various surfaces of epithelial cells differ in structure and have specialized functions. The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen (interior space) of an internal organ, or a tubular duct that receives cell secretions. Apical surfaces may contain cilia or microvilli. The cilia sweep substances along the surface of the epithelium, for example, ciliated epithelium in the lining of the bronchial tubes sweep impurities along the surface of the bronchial tubes away from the lungs, while microvilli on the surface of epithelial cells increases the surface area of the cell and in the case of the epithelial cells lining the small intestines, the microvilli increase the rate of absorption of nutrients across the cell.

Connective tissue is one of the most abundant and widely distributed tissues in the body. In its various forms, connective tissue has a variety of functions. It binds together, supports, and strengthens other body tissues; protects and insulates internal organs; compartmentalizes structures such as skeletal muscles; serves as the major transport system with in body (blood); is the primary location of stored energy reserves (fat); and is the main source of immune responses.

General characteristics of connective tissue

Connective tissue has cells that are widely separated by a matrix, which is a noncellular material that is made up of a ground substance and fibers. The matrix is usually secreted by the connective tissue cells and determines the tissue’s quality. The ground substance is the component of the connective tissue between the cells and fibers; it may vary in consistency from solid to jellylike to fluid. For example, in cartilage, the ground substance of the matrix is firm but pliable, whereas in bone, it is hard and inflexible. A nonfluid matrix may have fibers of three possible types. Collagen fibers contain collagen, a protein that gives them flexibility and strength. Reticular fibers are very thin collagen fibers that are highly branched and form delicate supporting networks. Elastic fibers contain elastin, a protein that is not as strong as collagen but more elastic.

In contrast to epithelia, connective tissues do not usually occur on body surfaces, and are highly vascular, that is they are a rich blood supply, with the exception of cartilage, which is avascular, and tendons, which have a scant blood supply. Connective tissue also has its own nerves, again with the exception of cartilage.

Muscle Tissue

Muscle tissue consists of elongated cells called muscle fibers that use ATP to generate force. As a result, muscular tissue produces body movements, maintains posture, and generates heat. It also provides protection. There are three types of muscular tissue based on its location, structure and function: skeletal muscle tissue, cardiac muscle tissue and smooth muscle tissue.

Nervous Tissue

Nervous tissue consists of two cell types, neurons and neuroglia, and is present in the brain and spinal cord. A neuron is a specialized cell that has three parts; a cell body, dendrites, and an axon. The cell body contains most of the cytoplasm and the nucleus of the neuron. Dendrites are short, highly branched processes that branch off of the cell body, and they are the major receiving or input part of the neuron, and they conducts signals towards the cell body.