Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport, and detection of sensation. Epithelia can also be organized into clusters of cells that function as exocrine and endocrine glands. Exocrine and endocrine epithelial cells are highly vascular.
Secretion is one major function of epithelial cells Figure 1. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classifications of glands: endocrine glands and exocrine glands.
An exocrine gland is distinguished by the fact that it excretes its essential product by way of a duct to some environment external to itself, be it either inside the body or on a surface of the body. Examples of exocrine glands include the sweat glands, salivary glands, mammary glands, the pancreas, and the liver.
Endocrine glands secrete their product into the extracellular space where it is rapidly taken up by the blood vascular system. An endocrine gland is its counterpart. It secretes its essential product without the use of a duct directly into the bloodstream or else by diffusion into its surrounding tissue (paracrine signaling) where it often affects only target cells near the release site. The exocrine glands secrete their products into a duct that then delivers the product to the lumen of an organ or onto the free surface of the epithelium. Examples of endocrine glands include the adrenal glands, located atop the kidneys and responsible for the secretion of certain hormones such as adrenaline, cortisol, and others. The testes, sing: testicle, in males and ovaries in females are not only gonads, organs which generate male and female germ cells respectively, but are also endocrine glands in that they produce various androgens and estrogens together known as steroidal sex hormones.