Exocrine Gland

Exocrine glands are specialised structures that release their secretions onto epithelial surfaces through a network of ducts. These glands are essential for numerous physiological processes in the human body, producing substances such as enzymes, mucus, sweat, oils and various protective secretions. Unlike endocrine glands, which discharge hormones directly into the bloodstream, exocrine glands rely on ductal pathways to deliver their products to specific external or internal surfaces.

General Characteristics and Examples

Exocrine glands are widely distributed across the body, contributing to multiple organ systems. Sweat glands, salivary glands, mammary glands, ceruminous glands, lacrimal glands, sebaceous glands, the prostate and various mucous glands are notable examples. Some organs, such as the liver and pancreas, have both exocrine and endocrine roles. The liver releases bile into the gastrointestinal tract, while the pancreas transports pancreatic juice through an elaborate duct system. Simultaneously, both organs secrete hormones directly into the bloodstream, demonstrating dual functionality.
Sweat glands, components of the integumentary system, are major exocrine structures and exist in two primary forms: eccrine glands, which regulate thermoregulation by secreting watery sweat, and apocrine glands, which produce thicker secretions associated with areas such as the axillae and groin.

Structural Classification

The architecture of exocrine glands provides an important basis for classification. Each gland consists of a secretory, glandular portion and a duct portion that facilitates the transport of secreted substances. The pattern of duct branching is a key criterion:

• Simple glands possess unbranched ducts.
• Compound glands feature branched duct systems.

The form of the glandular portion further refines classification:

• Tubular glands, in which the secretory unit is tube-shaped.
• Acinar (or alveolar) glands, where the secretory region is rounded or sac-like.
• Tubuloacinar glands, which combine tubular and acinar features.

A gland may also be described as branched if multiple secretory units drain into a single duct.

Methods of Secretion

Exocrine glands employ three principal mechanisms to release their products, each reflecting differences in cellular activity and the fate of the secreting cell.

• Merocrine secretion involves the release of substances via exocytosis, with no loss of cellular material. This is the most widespread mechanism and is observed in pancreatic acinar cells, eccrine sweat glands, salivary glands, goblet cells, intestinal glands and lacrimal glands.
• Apocrine secretion occurs when the apical portion of the cell pinches off, carrying the secretory product with it. Examples include sweat glands of the axillae, pubic region, perianal skin and areolar regions, as well as mammary glands.
• Holocrine secretion requires complete disintegration of the entire cell to release its contents. The sebaceous glands of the skin, Meibomian glands of the eyelids and glands of Zeis are characteristic holocrine structures.

Types of Products Secreted

Exocrine glands may be categorised according to the composition of their secretions, reflecting functional differences in various tissues.

• Serous glands produce protein-rich watery secretions, often containing digestive enzymes. Gastric chief cells and Paneth cells are significant examples, synthesising pepsinogen and antimicrobial peptides respectively.
• Mucous glands secrete viscous mucus that provides lubrication and protection. These include Brunner’s glands of the duodenum, oesophageal glands and pyloric glands.
• Seromucous (mixed) glands generate both serous and mucous products. This group includes the major salivary glands, although their secretions differ in proportion: the parotid gland is predominantly serous, the sublingual gland predominantly mucous and the submandibular gland a mixed gland with a largely serous output.
• Sebaceous glands secrete sebum, a lipid-rich product that lubricates the skin and hair. These glands, also termed oil glands, include Fordyce spots and Meibomian glands, which contribute to tear film stability.

Functional Significance and Physiological Roles

Exocrine glands play vital roles in maintaining homeostasis, protecting body surfaces and enabling specific biochemical processes. Digestive exocrine glands such as the pancreas and salivary glands allow for efficient breakdown of nutrients. Sweat glands contribute to temperature regulation, while sebaceous glands prevent desiccation of the skin. Lacrimal and ceruminous glands protect sensory structures by producing tears and earwax respectively.