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
