Article Outline
Tissue, group of associated, similarly structured cells that perform specialized functions for the survival of the organism (see Physiology). Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm (see Embryology: Normal Development in Animals). Through further cell differentiation, or histogenesis, groups of cells grow into more specialized units to form organs made up, usually, of several tissues of similarly performing cells. Animal tissues are classified into four main groups.
These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epithelium is made up of tightly packed cells with little intercellular substance between them.
About 12 kinds of epithelial tissue occur. One kind is stratified squamous tissue found in the skin and the linings of the esophagus and vagina. It is made up of thin layers of flat, scalelike cells that form rapidly above the blood capillaries and are pushed toward the tissue surface, where they die and are shed. Another is simple columnar epithelium, which lines the digestive system from the stomach to the anus; these cells stand upright and not only control the absorption of nutrients but also secrete mucus through individual goblet cells. Glands are formed by the inward growth of epithelium—for example, the sweat glands of the skin and the gastric glands of the stomach. Outward growth results in hair, nails, and other structures. See Epithelium.
These tissues, which support and hold parts of the body together, comprise the fibrous and elastic connective tissues, the adipose (fatty) tissues, and cartilage and bone. In contrast to epithelium, the cells of these tissues are widely separated from one another, with a large amount of intercellular substance between them. The cells of fibrous tissue, found throughout the body, connect to one another by an irregular network of strands, forming a soft, cushiony layer that also supports blood vessels, nerves, and other organs. Adipose tissue has a similar function, except that its fibroblasts also contain and store fat. Elastic tissue, found in ligaments, the trachea, and the arterial walls, stretches and contracts again with each pulse beat. In the human embryo, the fibroblast cells that originally secreted collagen for the formation of fibrous tissue later change to secrete a different form of protein called chondrion, for the formation of cartilage; some cartilage later becomes calcified by the action of osteoblasts to form bones. Blood and lymph are also often considered connective tissues. See Bone; Connective Tissue.
These tissues, which contract and relax, comprise the striated, smooth, and cardiac muscles. Striated muscles, also called skeletal or voluntary muscles, include those that are activated by the somatic, or voluntary, nervous system. They are joined together without cell walls and have several nuclei. The smooth, or involuntary muscles, which are activated by the autonomic nervous system, are found in the internal organs and consist of simple sheets of cells. Cardiac muscles, which have characteristics of both striated and smooth muscles, are joined together in a vast network of interlacing cells and muscle sheaths. See Muscle.
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