Your skin's oil glands work by destroying their own cells
Tiny glands beside almost every hair follicle coat your skin and hair in sebum, a waxy mix found nowhere else in the body. They make it in a strikingly self-destructive way: the producing cells swell with oil, burst and are shed along with it, while fresh cells keep dividing at the base to replace them.
Sebaceous glands appear everywhere on human skin except the palms and soles, clustering most densely on the face and scalp. Most open into a hair follicle, forming a unit with the hair and the tiny muscle that makes it stand up, but some exist on hairless skin too. Those on the eyelids, called meibomian glands, spread an oily film that slows tear evaporation and keeps lids from sticking together, while Montgomery's glands around the nipple release scents thought to guide a newborn. Earwax is partly sebum as well.
Human sebum is roughly 41% triglycerides, 26% wax esters, 12% squalene and 16% free fatty acids. The wax esters and squalene are not made as end products anywhere else in the body, and one fatty acid, sapienic acid, is unique to humans. Sebum has no smell of its own; odour comes when bacteria break it down. It also helps form the acid mantle, a slightly acidic film with a pH between 4.5 and 6.2 that discourages microbes, and in hot weather it mixes with sweat into a sheet that clings to the skin rather than dripping away, delaying dehydration.
The glands keep an unusual schedule. They show up between the 13th and 16th weeks of fetal life and, late in pregnancy, produce vernix caseosa, the white waxy coating that protects a baby's skin from amniotic fluid. After birth they nearly shut down from about age two to six, then surge at puberty as androgens rise, which is why acne tends to strike teenagers when excess sebum and dead cells plug a pore.
Other animals have repurposed the same equipment. Mice and rats use enlarged versions to make territorial pheromones, and Mexican free-tailed bats carry a scent gland on the throat, more often in males.
Source: Sebaceous gland
