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 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 arm pits 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, caterpillar cocoons, spiders’ webs, and beeswax are all produced by exocrine glands. Silk is a product of the silkworm’s salivary gland secretion.