«Apocrine glands
The sebaceous glands, as the name implies, are responsible for the production of sebum. In our body they are located on the entire skin surface, with the exception of the palms of the hands and soles of the feet.
From the histological point of view they are composed alveolar glands, formed by clusters of berries, which give them the characteristic cluster shape.
The sebaceous glands are generally associated with a hair follicle; however in certain areas they open directly to the skin surface, just like the sweat glands. In these isolated cases, which we find, for example, in the upper lip and in the wings of the nose, the sebaceous glands take on a larger than normal size.
While the sweat and apocrine secretion is intermittent, the sebaceous one is continuous. The secretory mechanism provides for a progressive accumulation of sebum inside the secreting cells, which increase more and more in size, until they burst. For this reason, not only the sebum, but also the residue of the cells is poured into the hair follicle. that produced it. This necrosis is compensated by the continuous production of new cell populations, originating from groups of undifferentiated cells that have retained the ability to divide repeatedly. Once formed, the new cells detach from the berry wall and begin to produce sebum, migrating close to the follicle, where they unravel releasing their contents.
The activity of the sebaceous glands is influenced by numerous factors.
After giving birth, sebum production decreases rapidly and remains dormant until puberty. For this reason, in infancy the liposoluble fraction of the hydrolipidic film is made up exclusively of skin lipids.
The massive activation of the sebaceous glands occurs only in puberty; subsequently it remains stable throughout adulthood, only to decrease with aging, especially in women.
The secretion of sebum is also influenced by genetic factors; not surprisingly, often and willingly, oily skin is a widespread problem among members of the same family.
The most important regulatory factor on sebum secretion is the concentration of androgens, typical of men but present in very small concentrations also in women. In particular, at the level of the sebaceous glands, there is an enzyme, called 5-alpha reductase, which converts delta 4-androstenedione into dihydrotestosterone, a metabolite capable of significantly increasing sebaceous secretion.
Functions of the sebum
The sebum enters the composition of the hydrolipidic film, the functions of which are extensively discussed in the following article. This oily mass also contributes to giving a characteristic and personal smell to the body, so much so that its lipid composition differs slightly from individual to individual.
Sebaceous fats are intermediate compounds of cholesterol synthesis (squalene, farnesol).Squalene is so named because it was first identified in the shark's liver; in man it represents the most immediate precursor of cholesterol, for this reason it is present only in the sebaceous secretion, but not in the rest of the body, where it is immediately converted into cholesterol.
In the sebum there are also waxes made up of particular fatty acids which, in addition to presenting a high degree of unsaturation, make it take on a fluid consistency.
The composition of the sebum also includes, and above all, triglycerides, which alone represent about 60% of the lipid fraction. Unlike food lipids, these lipids are mainly made up of fatty acids with an odd number of carbon atoms, with double bonds in unusual positions and with particularly branched and long carbon chains (up to 30 carbon atoms).
Seborrhea
Seborrhea is a dysfunction of the sebaceous glands, which involves an excessive production of sebum. Probably due to an overexpression of the 5-alpha reductase enzyme, it gives the skin and skin appendages a shiny and oily appearance. Seborrhea is often linked to hereditary factors.
The excess of sebum can cause the appearance of comedones, commonly called blackheads. If the sebum is produced in large quantities, it ends up excessively dilating the walls of the hair follicle into which it is poured.
Acne
Seborrhea is often the antechamber of acne, although these two events are not necessarily related.
Acne is an inflammatory process of the sebaceous gland and the surrounding dermis. It can appear at any age, but prefers adolescent age, probably due to the sudden and abrupt increase in the synthesis of sex hormones.
On the other hand, a direct relationship between acne and bad eating habits has not been demonstrated. This is equivalent to saying that an unregulated diet is not the primary cause of acne, but can indirectly contribute to accentuate the disorder.
Acne can also occur in adulthood, due to various predisposing factors, including: intake of certain drugs (cortisone); hormonal alterations (ovarian cysts, pregnancy, menopause); use of poor or unsuitable cosmetics of skin; work in contact with lubricating oils and hydrocarbons.
The inflammatory process of acne depends on the accumulation of sebum and debris from the sebaceous glands in the hair follicles. These substances represent a good breeding ground for bacteria, especially for the Propionibacterium acnes. Gradually these microorganisms proliferate and produce enzymes, including a lipase that hydrolyzes the triglycerides present in the sebum. The hydrolysis of these molecules produces free fatty acids, which exert an inflammatory action at a local level. bacteria also produce proteolytic enzymes, capable of damaging the wall of the hair follicle. In this way, free fatty acids can diffuse into the surrounding dermis and extend the inflammatory process outside the follicle.
Comedones, or blackheads, appear before this reaction begins. Only when the inflammation becomes important do the so-called papules arise, that is small red patches in relief. The papules represent the first evident sign of local inflammation. When these become infected, pus forms and from papules they evolve into pustules (the classic boils with a yellow pin).
Both papules and pustules, when they regress, do not leave permanent marks on the skin. However, the pustules can form deeper cysts and fill with purolent material. In turn, these cysts can degenerate into a fibrotic process, forming hard cords, called nodules. When they heal, cysts and nodules often result in scarring.
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