Exokrin

Exokrin, or exocrine, refers to glands that secrete substances onto an epithelial surface through ducts. This distinguishes them from endocrine glands, which release hormones directly into the bloodstream. Exocrine secretions serve functions such as digestion, lubrication, and protection of body surfaces.

Multicellular exocrine glands consist of a secretory portion connected to a duct system. The secretory units may be tubular, acinar (alveolar), or tubuloalveolar, and glands are classified as simple (unbranched ducts) or compound (branched ducts). Examples include the salivary glands, pancreas (exocrine portion), lacrimal glands, sweat glands, and gastric glands. The arrangement of ducts and secretory units influences the volume and type of secretion produced.

Secretory methods vary. Merocrine secretion involves exocytosis of secretory products and is the most common mode. Apocrine secretion releases a portion of the cell, while holocrine secretion involves the disintegration of whole cells. The chosen method affects the composition and maintenance of the gland.

Secretions are categorized by their content: serous secretions are watery and enzyme-rich; mucous secretions are rich in mucins; mixed secretions contain both components. For example, pancreatic juice is mostly a serous mix of digestive enzymes and bicarbonate, while saliva can be mixed, containing both enzymes and mucins.

Regulation occurs mainly through neural and hormonal signals. Parasympathetic activity generally increases secretion, while hormones such as secretin and cholecystokinin modulate specific glands (notably the pancreas and stomach). Local factors and feedback from target surfaces can also influence secretion.

Clinically, exocrine dysfunction can cause malabsorption (pancreatic insufficiency), thickened secretions in cystic fibrosis, or impaired lubrication in salivary and lacrimal systems.