Tissue
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 ). 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. EPITHELIAL TISSUES 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 stratifi