Exocrine Glands

Structural classification

Functional classification

Glands in the human body are classified as exocrine or endocrine. The secretions of exocrine glands are released through ducts onto an organ’s surface, while those of endocrine glands are released directly into the blood. The secretions of both types of glands are carefully regulated by the body. The pancreas is both an exocrine gland and endocrine gland; it produces digestive enzymes that are released into the intestine via the pancreatic duct, and it produces hormones, such as insulin and glucagon, which are released from the islets of Langerhans directly into the bloodstream.

Exocrine glands are made up of glandular epithelial tissue arranged in single or multilayered sheets. Exocrine gland tissue does not have blood vessels running through it; the cells are nourished by vessels in the connective tissue to which the glands are attached. Gland cells communicate with each other and nerves via channels of communication, which run through the tissue. Exocrine gland secretions include saliva, perspiration, oil, earwax, milk, mucus, and digestive enzymes.

Structural classification

Exocrine glands have two structural classifications: unicellular (one cell layer) and multicellular (many cell layers). Goblet cells are unicellular exocrine glands; so named for their shape, these glands secrete mucus and are found in the epithelial lining of the respiratory, urinary, digestive, and reproductive systems. Multicellular exocrine glands are classified by their shape of secretory parts and by the arrangement of their ducts. A gland with one duct is a “simple,” whereas a gland with a branched duct is a “compounc” gland. The secretory portions of simple glands can be straight tubular, coiled tubular, acinar, or alveolar (flasklike). The secretory portions of compound glands can be tubular, acinar, or a combination: tubulo-acinar.

Functional classification

Exocrine glands can also be classified according to how they secrete their products. There are three categories of functional classification: holocrine glands, merocrine (or eccrine) glands, and apocrine glands. Holocrine glands accumulate their secretions in each cell’s cytoplasm and release the whole cell into the duct. This destroys the cell, which is replaced by a new growth cell. Most exocrine glands are merocrine (or eccrine) glands. Here, the gland cells produce their secretions and release it into the duct, causing no damage to the cell. The secretions of apocrine cells accumulate in one part of the cell, called the apical region. This part breaks off

KEY TERMS

Duct— A tube-like passage for secretions and excretions.

Epithelial tissue— Tissue that forms glands, the outer layer of the skin, which lines blood vessels, hollow organs, and body passageways.

Interstitial— Interspaces of a tissue.

from the rest of the cell along with some cytoplasm, releasing its product into the duct. The cells repair themselves quickly and soon repeat the process. An example of apocrine exocrine glands are the apocrine glands in the mammary glands and the armpits and groin.

Exocrine glands perform a variety of bodily functions. They regulate body temperature by producing sweat; nurture young by producing milk; clean, moisten, and lubricate the eye by producing tears; and begin digestion and lubricate the mouth by producing saliva. Oil (sebum) from sebaceous glands keeps skin and hair conditioned and protected. Wax (cerumen) from ceruminous glands in the outer ear protects ears from foreign matter. Exocrine glands in the testes produce seminal fluid, which transports and nourishes sperm. Exocrine gland secretions also aid in the defense against bacterial infection by carrying special enzymes, forming protective films, or by washing away microbes.

Humans are not the only living beings that have exocrine glands. Exocrine glands in plant life produce water, sticky protective fluids, and nectars. The substances necessary for making birds’ eggs, caterpillars’ cocoons, spiders’ webs, and beeswax are all produced by exocrine glands. Silk is a product of the silkworm’s salivary gland secretion.

See also Endocrine system.

Christine Miner Minderovic