Exocrine Gland Organization and Secretion

Exocrine glands deliver their secretory products through a duct system onto an epithelial surface, such as the skin, the lining of the gut, or the lumen of an organ. Their architecture is described by two features: the branching pattern of the duct (simple versus compound) and the shape of the secretory end-piece (tubular, acinar, or tubuloacinar).

Definition

An exocrine gland is a secretory structure that discharges its product through one or more ducts onto an epithelial surface, classified by its duct-branching pattern and the morphology of its secretory units.

Scope

The entry covers how exocrine glands are classified by duct branching and secretory-unit shape, the cells of the secretory unit and its supporting elements (such as myoepithelial cells), the three modes of release (merocrine, apocrine, holocrine), and the chemical nature of the product (serous, mucous, or mixed). It treats exocrine glands as a structural topic and does not give clinical guidance.

Core questions

• How are exocrine glands classified by duct branching and end-piece shape?
• What cells make up a secretory unit, and what is the role of myoepithelial cells?
• What distinguishes merocrine, apocrine, and holocrine release?
• How do serous and mucous secretory cells differ histologically?

Key concepts

• Duct system and surface delivery
• Simple versus compound glands
• Tubular, acinar (alveolar), and tubuloacinar secretory units
• Serous, mucous, and mixed secretory cells
• Myoepithelial cells
• Merocrine, apocrine, and holocrine secretion
• Branching (duct) morphogenesis during development

Mechanisms

In a merocrine exocrine cell, secretory protein is made on the rough endoplasmic reticulum, transported through the Golgi, concentrated into granules, and released by exocytosis without loss of cytoplasm, the pathway worked out in the pancreatic acinar cell (Jamieson & Palade, 1968; Palade, 1975). Contractile myoepithelial cells wrapped around secretory units and small ducts help expel the product. Apocrine release sheds the apical cytoplasm with the secretion, and holocrine release discharges the disintegrated whole cell. The characteristic branched architecture of compound glands is laid down during development by branching (duct) morphogenesis (Sternlicht, 2005).

Clinical relevance

The classification of exocrine glands and the appearance of serous and mucous cells provide the descriptive vocabulary used to identify glandular tissue in histological sections and to characterize glandular tumours. This is reference background on normal structure and is not a basis for diagnosis or treatment of any individual.

Evidence & guidelines

Statements rest on standard histology texts (Ross & Pawlina, 2020; Mescher, 2018), foundational secretory-pathway studies (Jamieson & Palade, 1968; Palade, 1975), and a review of branching morphogenesis (Sternlicht, 2005); they reflect established descriptive science rather than clinical guidelines.

History

The structural classification of exocrine glands by duct and end-piece shape is a long-standing element of descriptive histology, while the cellular route of secretion was defined by Palade and Jamieson's mid-twentieth-century studies of the pancreatic acinar cell.

Key figures

• George Palade
• James Jamieson

Seminal works

• jamieson-palade-1968
• palade-1975

Frequently asked questions

What does 'compound' mean when describing an exocrine gland?

A compound gland has a branched duct system, whereas a simple gland has a single unbranched duct. The term refers to the duct, not to the shape of the secretory end-pieces.

What is a myoepithelial cell?

A myoepithelial cell is a contractile, basket-like cell that sits between the secretory cells and their basement membrane; its contraction helps squeeze the secretory product out of the gland and into the duct.