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Exocrine Glands: Unicellular and Multicellular Glands

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Exocrine glands produce and discharge secretions onto their exterior surfaces. For example, sweat glands release sweat on the skin to maintain body temperature, while salivary glands secrete saliva inside the mouth for lubrication.

Exocrine glands are unicellular or multicellular.

Unicellular glands are individual secretory cells like the goblet cells of the intestinal epithelia. Goblet cells secrete a complex glycoprotein called mucin onto the epithelial surface, forming a slimy protective mucus coat.

Multicellular exocrine glands consist of a group of cells which form the secretory units and produce secretions that are delivered by ducts onto the surface.

Multicellular glands are further classified based on the morphology of the ducts and the secretory units.

Based on morphology of ducts, multicellular glands are simple with unbranched ducts or compound with branched ducts.

Based on the morphology of the secretory units, they may form straight or coiled tubes and are called tubular glands, as seen in intestinal glands, or sweat glands.

In contrast, alveolar or acinar glands such as sebaceous and mammary glands, contain flask-shaped hollow secretory sacs.

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Exocrine glands are classified as unicellular and multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestines. On the other hand, multicellular exocrine glands develop as secretory sheets, like the internal lining of the abdomen or chest. Such secretory sheets release their secretions directly into the lumen of these organs. In addition, some multicellular glands have deep-seated secretory units to produce secretions and a hollow duct to carry glandular contents to the epithelial surface. Depending on the structure of ducts and secretory units, multicellular glands can be classified into different types.

The glands with single, continuous ducts are called simple glands, while those with branched ducts are called compound glands. Simple and compound glands can have secretory units of two different shapes and are accordingly classified as tubular and alveolar glands.

Tubular Glands

Tubular glands consist of tubular secretory units with either straight or coiled tubes. One tubular unit with a single duct forms simple tubular glands, such as intestinal and sweat glands. Sometimes multiple straight tubes open into a single duct, forming a simple branched tubular gland, such as gastric glands. In others, several clusters of tubular secretory units attach to the branched ducts. These compound tubular glands are found in organs such as the testes and other glands of the male reproductive system.

Alveolar Glands

In contrast, alveolar or acinar glands have secretory units in the shape of a hollow sac. When two or more secretory sacs are attached to a single duct, it is called a simple branched alveolar gland. A common example is the oil-producing sebaceous gland. Conversely, several groups of secretory sacs connected to the branched ducts are compound alveolar glands. Examples include mammary glands.

Some glands, like the salivary glands, have both tubular and alveolar sacs with branched ducts. Such glands are called compound tubuloalveolar glands.

This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial Tissues

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Exocrine Glands

Unicellular Glands

Multicellular Glands

Compound Glands

Alveolar Glands

Secretory Units

Gland Classification