Exocrine glands are glands that secrete substances onto an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one of two types of glands in the human body, the other being endocrine glands, which secrete their products directly into the bloodstream. The liver and pancreas are both exocrine and endocrine glands; they are exocrine glands because they secrete products—bile and pancreatic juice—into the gastrointestinal tract through a series of ducts, and endocrine because they secrete other substances directly into the bloodstream.
Exocrine glands contain a glandular portion and a duct portion, the structures of which can be used to classify the gland.
- The duct portion may be branched (called compound) or unbranched (called simple).
- The glandular portion may be tubular or acinar, or may be a mix of the two (called tubuloacinar). If the glandular portion branches, then the gland is called a branched gland.
Method of secretion
Exocrine glands are named apocrine glands, holocrine glands, or merocrine glands based on how their products are secreted.
- Merocrine secretion – cells excrete their substances by exocytosis into a duct; for example, pancreatic acinar cells, maximum sweat glands of humans, goblet cells, intestinal glands, tear glands, etc.
- Apocrine secretion – a portion of the cell membrane that contains the excretion buds off. Examples are sweat glands of arm pits, pubic region, skin around anus, lips and nipples; mammary glands, etc.
- Holocrine secretion – the entire cell disintegrates to excrete its substance; for example, sebaceous glands of the skin and nose, meibomian gland, zeis gland, etc.
- Serous cells secrete proteins, often enzymes. Examples include gastric chief cells and Paneth cells
- Mucous cells secrete mucus. Examples include Brunner's glands, esophageal glands, and pyloric glands
- Seromucous glands (mixed) secrete both protein and mucus. Examples include the salivary glands: although the parotid gland (saliva secretion 25%) is predominantly serous, the sublingual gland (saliva secretion 5%) mainly mucous gland, and the submandibular gland (saliva secretion 70%) is a mixed, mainly serous gland.
- Sebaceous glands secrete sebum, a lipid product. These glands are also known as oil glands, e.g. Fordyce spots and Meibomian glands.
Section of the human esophagus.