Exocrines

Exocrines, or exocrine glands, are glands that secrete their products onto an epithelial surface or into a body cavity through ducts. This contrasts with endocrine glands, which release hormones directly into the bloodstream. Exocrines range from single-celled goblet cells to complex multicellular organs with branching duct systems. Their secretory units are usually acini or tubules that produce enzymes, mucus, sweat, or other substances, which are carried through ducts to surfaces such as the skin, gastrointestinal tract, respiratory tract, or ocular surface.

Secretions can be serous (watery and enzyme-rich), mucous (viscous mucins), or mixed. The predominant secretory mechanism is merocrine secretion, in which secretory products are released by exocytosis. Some glands employ apocrine secretion, in which part of the cell’s apical cytoplasm is shed, while others perform holocrine secretion, where the entire cell disintegrates to release its product.

Key examples include salivary glands, lacrimal glands, sweat glands, sebaceous glands, pancreatic exocrine tissue, and goblet cells in the airways and intestine. Exocrine glands can be stimulated or regulated by the autonomic nervous system and local mediators; for instance, parasympathetic activity increases saliva production, while sweat gland function is influenced by sympathetic innervation.

Clinical relevance includes disorders of exocrine secretion, such as pancreatic exocrine insufficiency in cystic fibrosis and reduced lacrimal or salivary output in Sjögren’s syndrome. Understanding exocrines highlights the integration of secretion, ductal transport, and surface interfaces in physiology and disease.