Glands
In addition to covering surfaces and lining spaces epithelia serve a secretory function. The simplest form of the secretory epithelium may be represented as the "unicellular gland". It has been seen as the goblet cell in simple and pseudostratified columnar epithelia.
Glands increase in complexity from simple tubular structures to elaborately branched organs. Their secretory products also vary; they commonly secrete a viscous, mucous product, or a watery proteinaceous, serous product, and frequently some combination thereof.
Glands that release their products through ducts are called exocrine, while those that have lost their ducts during development secrete their hormones into capillaries and are referred to as endocrine.