Gland

Lore & Background

Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may initially possess a tubular structure, or in other instances glands may start as a solid column of cells that subsequently becomes tubulated. As growth proceeds, the column of cells may split or give off offshoots, forming a compound gland. In many glands the number of branches is limited, while in others such as the salivary glands and pancreas, a very large structure is formed by repeated growth and subdivision. As a rule, the branches do not unite with one another, except in the liver, where a reticulated compound gland is produced.

Reader's Guide

Glands are fundamental to animal physiology, serving as the body's secretory organs. They are divided into two main types based on function: endocrine glands, which are ductless and secrete hormones directly into the bloodstream via interstitial spaces, and exocrine glands, which secrete products through a duct onto an outer or inner surface. Endocrine glands include the pineal gland, thymus, pituitary gland, thyroid gland, and adrenal glands, and their hormones play an important role in maintaining homeostasis. Exocrine glands are further categorized by their method of secretion—merocrine (exocytosis), apocrine (loss of part of the cell), and holocrine (disintegration of the entire cell)—and by their product, such as serous (watery), mucous (viscous), or sebaceous (lipid). Glands are mostly composed of epithelial tissue with a supporting framework of connective tissue and a capsule. Their development begins as an ingrowth from an epithelial surface, and they are classified by shape as tubular or alveolar (saccular). Clinically, adenosis refers to any disease of a gland involving abnormal formation or development of glandular tissue, sometimes tumorous.

Did You Know?

• Endocrine glands secrete hormones directly into interstitial spaces to be taken up into the bloodstream.
• Exocrine glands secrete through a duct onto an outer or inner surface, such as the skin or gastrointestinal tract.
• The liver is an exception to the rule that gland branches do not unite, forming a reticulated compound gland.
• Holocrine glands, such as sebaceous glands, secrete by the entire cell disintegrating.