The pilo sebaceous system includes several structures:
- The hair follicle
- The hair
- The sheaths of the hair
- The erector muscle
- The sebaceous gland
- The apocrine sweat glands (in some locations)
- The hair comes out of the hair follicle, an epidermal invagination in the underlying tissue.
Going from bottom to top, it is made up of 3 zones:
a- the lower segment, ie the deep part of the follicle up to the insertion of the erector muscle
b- the isthmus or collar that proceeds from the intersection of the erector muscle to the outlet of the sebaceous duct in the follicle
c- the infundibulum, terminal segment from the outlet of the sebaceous duct in the follicle, to the skin surface
There are 3 parts in the hair:
a- the stem or stem, the longest part, visible, since it is outside the skin; this part of the hair is inert, dead matter
b- the root, not visible, between the follicular ostium and the inferior attachment of the erector muscle
c- the bulb, the terminal part of the root with an enlarged and rounded shape. The lower part is made up of 2-3 rows of overlapping, rapidly reproducing cells that make up the matrix: they represent the only germinative cells of the hair. The cells of the matrix, as they reproduce, push the previously born cells upwards; during the ascent, as happens in the epidermis, the cells process a protein inside, keratin, becoming progressively more and more rigid. Of these cells, some go to make up the sheaths of the hair, others go to make up the 3 structural parts of the same, cuticle, cortex, medulla (section of the hair cut horizontally).
Above the cells of the matrix there are some melanocytes that perform the function of giving color, introducing melanin, cells that will become the cortex of the hair.
The sheaths are structures that surround the hair and are 3:
a- internal epithelial sheath, wraps about 2/3 of the root
b- external epithelial sheath: placed around the internal sheath and at the root
c- connective sheath: surrounds the entire epithelial structure
By cutting a hair horizontally, you can appreciate its structure, divided into 3 parts:
1- medulla: internal portion, is discontinuous. It consists of rounded cells separated from each other by air bubbles.
2- cortex: intermediate portion; is the most abundant (85-90% of the hair). It is made up of cells larger than the medulla, containing melanin, therefore, it represents the "colored" part of the hair.
3- cuticle: outermost portion; it is a chemically resistant structure because it is protective, that is, it is the hardest layer of the hair, but it also represents the part that is damaged and frayed first, if aggressive products rich in surfactants are used. It is made up of flat cells present in several layers (5-10), arranged between them like tiles on a roof; they are keratinized.
The hair follicle is a dynamic structure, therefore it alternates long phases of proliferation, ie growth (anagen) with short phases of involution (catagen), and finally, phases of rest (telogen). Hence, hair grows cyclically with alternating periods of growth and quiescence, i.e. rest.
Most hair is in anagen, a phase in which the matrix cells reproduce by mitosis by developing cells in the medulla, cortex, cuticle and sheaths; once these cells are completely keratinized, they will form the hair proper.
In catagen there is an orderly and gradual arrest of mitotic activity at the level of the matrix; once this process is completed, the Telogen phase is entered.
Rest period during which the hair bulb acquires a club-like shape and approaches the surface of the skin. The hair falls out; before the fall, however, a new hair has formed in the lower part of the follicle that will take the place of the old one.
At the beginning of the anagen the papilla is placed again in contact with the lower end of the secondary epithelial germ; it proliferates forming a new bulb, from which both the new hair and the new internal epithelial sheath will originate.
In histological sections it is sometimes possible to observe this phase in which the old hair plus the newly formed hair is simultaneously present.
Hair is fiber of a protein nature.
The chemical analysis of the elements contained in them reveals the presence of carbon, hydrogen, nitrogen, sulfur and oxygen, in addition, traces of other elements are found that may play a role in the composition of the pigments.
Mostly, proteins (80-90%), water (10-15%), fat (2-3%) and minerals of which the most important is sulfur.
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