Section 15: Endocrine Glands

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

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These cellular masses, designated as the ductless glands or glands of internal secretion, have during development lost their original connection with the epithelium of the free surface. Their secretions are called hormones. The gland cells produce specific chemical substances, which are secreted in a rich capillary bed and carried by the blood to another part, or parts, of the body where they have a distinctive function. The endocrine glands are essentially a vertebrate development, and any one of these hormonal substances has a similar action in all vertebrates with little or no species-specific function.

Endocrine glands may appear as distinct organs (e.g., the hypophysis and adrenal glands), may be found associated with exocrine glands (e.g., pancreatic islets and the interstitial cells of the testes), may appear as mixed endocrine glands (e.g., the thyroid and parathyroid glands), or may have cells so diffusely distributed that they are not usually considered as organs (e.g., argentaffin cells of the digestive system).

Some endocrine glands are essential for life; these include the adrenal cortex, pancreatic islets, and the parathyroid glands. The other endocrine glands, although not essential for life, determine to a great extent the quality of ones life and the ability to adapt to stress. The endocrine glands, separately and in conjunction with the nervous system, are coordinators of body functions that maintain the organism in a viable homeostatic state.

The structural/functional organization of the endocrine glands is diverse but distinctive. In general, all endocrine glands store their secretory products either within the cells of origin or within cellular follicles or sacs. The cells of the adrenal cortex contain minimal amounts of stored hormone, whereas in the pancreas and pituitary gland (hypophysis), secretory granules (if preserved) are usually evident. In the thyroid gland, the hormone is stored extracellularly in a pool surrounded by epithelial gland cells (a follicle). In this case, the release of the hormone into the blood stream involves the reabsorption and transfer of the hormone through the cells of origin into the extracellular space, where it enters the capillaries.

An essential feature of the endocrine glands is the manner in which the secretory activity is regulated by a feedback mechanism. As an example, the beta cell of the anterior lobe of the hypophysis secretes adrenocorticotropic hormone (ACTH), which stimulates the secretion of certain hormones from the adrenal cortex. As the level of adrenal cortical hormones rises in the blood stream, the secretion of ACTH is inhibited. Declining levels of the hormones of the adrenal cortex result in an increased secretion of ACTH by the pituitary. In this manner, appropriate levels of adrenal cortical hormones are maintained in the blood stream.

Specific details of the structure and function of the endocrine glands will be found in this section and, as appropriate, in the sections concerned with the digestive, urinary, male reproductive, female reproductive, and nervous systems.

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