Definition
A sebaceous gland is a special type of gland located in the dermis, which secretes a fatty secretion (sebum) on body surfaces via a holocrine mechanism. A holocrine mechanism describes a form of glands that secrete the secretion and die in the process. Sebaceous glands are found in varying concentrations all over the body except on the inside of the hands and feet. Most of the sebaceous glands present in the body are closely associated with hair, but at the transitions from skin to mucous membrane (eyelids, lips, genitals, anus, nipples, nasal and ear openings) there are so-called free sebaceous glands without any relation to the hair root. Around the nipple, 10-15 particularly large free sebaceous glands, which are called Glandulae areolares or Montgomery glands, usually sit in a circular arrangement.
Anatomy of the sebaceous gland
Sebaceous glands are the only holocrine glands in the human body and therefore have a characteristic structure. A holocrine gland is a gland which completely perishes when the secretion is released. This is called apoptosis, a form of programmed cell death.
The end pieces of the glands have a pear or bulb shape and usually occur in groups that share a short common excretory duct. In contrast to all other glandular forms, these end pieces are completely filled with glandular cells, the sebocytes, and have no cavity. This is due to the fact that the cells themselves “become secretions” by migrating from the outermost edge of the end piece to the excretory duct, thereby changing their shape and finally dying.
At the base, i.e. on the very outside, there are basal cells from which new glandular cells are constantly being created by cell division. The maturation of the sebocytes consists of two microscopically well visible changes: On the one hand, the cell stores fats, which makes it grow and become lighter. In addition, the oval cell nucleus, which is at first rather coarse-spotted, first becomes round and then shrinks through compression and finally disintegrates.
The other cell organelles also perish. This process is typical for apoptotic (= controlled dying) cells and is called pycnosis. When the cell reaches the apical end (lat.
apex = tip), i.e. the interface to the excretory duct, it is expelled from the cell assembly and decays. The excretory duct consists of another particularly flat cell shape and, in most sebaceous glands, opens up after a short time into the hair follicle with which it shares an outlet. New studies have found that some cells of the Montgomery glands are able to secrete a chemical odorant (so-called pheromone).
However, the mechanism is different from sebum secretion, as the corresponding cells do not completely die off, but secrete the scent in the form of larger vesicles. The sebum itself is a yellowish, low viscosity mixture and consists mainly of triglycerides (approx. 43%), i.e. of fatty acids bound to glycerine, free fatty acids (approx.
15%), waxes (approx. 23%), squalenes (approx. 15%) and cholesterol (approx. 4%), i.e. very fat-loving compounds. In addition, there are also proteins, as well as other cell debris and skin scales, which are carried along with the skin during the secretion.
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