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A gland is an organ in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).
Human submaxillary gland. At the right is a group of mucous alveoli, at the left a group of serous alveoli.
Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may in the beginning possess a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated.
As growth proceeds, the column of cells may divide or give off offshoots, in which case a compound gland is formed. In many glands, the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sub-division. As a rule, the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.
Glands are classified according to their shape.
Glands are divided based on their function into two groups:
Endocrine glands secrete substances that circulate through the blood stream. These glands secrete their products through the basal lamina into the blood stream and lack a duct system. These glands often secrete hormones, and play an important role in maintaining homeostasis. The pineal gland, thymus gland, pituitary gland, thyroid gland, and the two adrenal glands are all endocrine glands.
Exocrine glands secrete their products through a duct onto an outer surface of the body, such as the skin or the gastrointestinal tract. Secretion is directly onto the apical surface. The glands in this group can be divided into three groups:
- Apocrine glands a portion of the secreting cell's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands, however it is thought that apocrine sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion. e.g. mammary gland, sweat gland of arm pit, pubic region, skin around anus, lips, nipples.
- Holocrine glands the entire cell disintegrates to secrete its substances (e.g., sebaceous glands), meibomian and zeis glands.
- Merocrine glands cells secrete their substances by exocytosis (e.g., mucous and serous glands). Also called "eccrine". e.g. max sweat gland of humans, goblet cells, salivary gland, tear gland, intestinal glands.
The type of secretory product of exocrine glands may also be one of three categories:
- Serous glands secrete a watery, often protein-rich, fluid-like product, e.g. sweat glands.
- Mucous glands secrete a viscous product, rich in carbohydrates (such as glycoproteins), e.g. goblet cells.
- Sebaceous glands secrete a lipid product. These glands are also known as oil glands, e.g. Fordyce spots and meibomian glands.