Holocrine glands are glands that secrete whole cells that have completely broken down for elimination from the body. This is unique because other forms of secretion do not decimate entire cells. Only three methods of glandular secretion exist in the human body: apocrine, merocine and holocrine. Unlike holocrine secretion, secretions from apocrine glands consist only of the cell membranes — not the whole cell. Merocrine glands secrete a combination of mucus and serum, leaving the rest of the cell intact.
Sebaceous glands are the only holocrine glands that exist in the body. They are a type of exocrine gland, which means they use ducts to transport secretions to a specific location outside the body; the prefix “exo” means “outside” or “on top of.” Since sebaceous glands are located on the epidermal layer of the skin, they transport secretions directly to the skin’s surface.
They are located parallel to hair follicles and are usually between hair follicles and the arrector pili muscles that support the follicles, allowing them to contract and tighten around the hair. Before secretion, the whole cells of the sebaceous glands first swell with lipids and other moisturizing agents. Then they break down, die and ooze out to engulf the surface of the skin.
When sebaceous glands disintegrate, they are secreted as a substance called “sebum.” This sebum, though a form of waste, is beneficial since it provides lubrication for hair follicles, reducing hair breakage and giving moisture to dry scalp and dry skin all over the body. In coating the body, the sebum discharged by holocrine glands prevents the excessive evaporation of water, thereby staving off dehydration. Adequate sebum can also allow good bacteria to thrive while serving as a buffer against malicious fungi and bacteria which tend to enter the body through dry, cracked skin. One drawback of this secretion from holocrine glands is that sebum, because it harbors good and bad bacteria, can lead to an unpleasant stench if not washed off regularly.
Holocrine glands consist of small clusters of cells called acini. Inside one acinus is a duct for secretion surrounded by three different layers of cells. The outer layer of cells, known as cuboidal cells, is never shed for secretion; neither is the second layer. Only the innermost layer of cells right next to the duct, known as centroacinar cells, are destroyed for secretion, leaving most of the acinus intact. New cells are generated to replace the shed cells.