Dermatology and aesthetics
HairWhen hair is lost: problems, pathologies, types Alopecia: classification according to the cause Diagnosis Treatment
- When hair is lost: problems, pathologies, types
- Anatomy and physiology of the pilosebaceous follicle
- Alopecia: classification according to the cause
When hair is lost: problems, pathologies, types
Before discussing the physiology and pathology of hair, it is perhaps appropriate to reflect on the function and meaning that these skin appendages have in Western and European society today.
In the animal world, the function of the fur is primarily thermoregulatory, protective and mimetic (but in some species it can also be ornamental and sexual), while in the human race the maintenance of thermal homeostasis is entrusted to mechanisms that substantially prescind from the presence of a piliferous cover., so much so that the bald man does not present substantial physiological alterations; mankind today could survive safely even if its components were completely hairless.
The interest in one's own hair is, therefore, more anthropological and psychological than physiological, but it is still so strong as to push men and women to face often difficult, long and expensive therapies.
The distribution, color and length of the coat, in many animal breeds, constitutes an external morphological character that distinguishes the male subject from the female one and also serves as a reference for the other sex. In the human species, for example, the length of the hair represented for a long time a distinctive sign between the two sexes, since the growth phase (anagen) lasts in the male about 3 years, determining an average length of about 30-35 cm, while in the female it lasts between 6 and 10 years with an average length that can reach 100-120 cm. From a psychological point of view, hair loss causes several rather unpleasant sensations, for example the sensation of regressing to neonatal age, losing one's virility or being castrated (in men), losing one's femininity (in women).
Throughout history, civilizations from all over the world have attributed different meanings to hair, making it a sign of strength and energy (for example, the story of Samson or the noble wigs), of fertility (tonsure of children up to their maturity), virility, belonging (for example religious, where the tonsure expresses the choice of chastity), royalty (just remember the wonderful wig of ringed curls of Louis XIV or the etymological meaning of the titles of "Cesare", "Kaiser", " Tsar ", which refer to" long hair to be cut "), mysticism (the Native Americans thought that Manitù grabbed the warriors killed by the hair to bring them to heaven, hence the meaning of the scalp, which would have prevented the ascent to heaven of the enemy).
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Anatomy and physiology of the pilosebaceous follicle
The hair is a roughly cylindrical structure with a diameter of about 65-78? M, derived from the hair follicle, which is divided into a stable portion and a cyclic (caducus) separated from the collar. In the upper portion of the follicle, attached to the hair, there are the sebaceous gland and the hair erector muscle, responsible, through an adrenergic stimulation (consequent, for example, to the reaction of escape or attack in the face of a danger), of the phenomenon called horripilation (goosebumps). Under these structures there is a bulge, called bulge, in which the stem cells are contained which will serve, for each cycle, for the formation of a new hair. The molecule that attributes the compactness, resistance, elasticity and hardness to the hair is the so-called sclerokeratin (or hard keratin); keratin is a complex molecule made up of two proteins of different composition (18 different amino acids contribute to the structure). The hair shaft consists of the cuticle, formed by a series of keratinized cells without pigment; inside the cuticle there is the bark or cortical, in which melanin is found and, inside the bark, the marrow, composed of empty spaces ("bubbles" of air) and filaments of keratin. The terminal part of the hair, the one that sinks into the dermis, is called the bulb and envelops a portion of the connective tissue called the dermal papilla. Hair follicles are distributed throughout the body except on the palmoplantar region, distal phalanges, semi-mucous membranes and on the skin of the penis. Two categories of hair are commonly distinguished, the fluff or vellus, formed by small, thin, non-pigmented, almost invisible hairs (they are found on the ears, forehead, trunk and, in the female sex, cheeks), and the terminal hairs, large and pigmented (present in the remaining body sites).
The hair is a terminal hair in which three parts are distinguished:
- the stem, which constitutes its external and visible part;
- the root, or the internal part invisible because it is immersed in the skin;
- the bulb, which constitutes the deepest and terminal portion of the root.
Melanins are responsible for hair color, in particular eumelanin gives them the black and dark brown color, pheomelanin the blond and red colors.
The average hair present in a young adult is about 100, 000-150, 000 (between 160 and 240 per cm2), but this number decreases with age, reducing, even in the absence of pathologies, to about 2/3 of the initial number in the elder. The diameter of the hair varies between 65 and 78? M, and it too is often subject to a decrease with age (it can be less than 50? M).
Only one out of three follicles hosts a true terminal hair (the other two host a vellus hair). In the follicles alternate periods of growth (anagen) and rest (telogen) interspersed with a period of progressive arrest of vital functions (catagen); the first period is the most lasting and reaches 2-4 years in men and 3-7 years in women (this explains why the hair reaches a greater length in the female sex), while the telogen phase, or functional rest, it lasts on average 90-100 days and ends with hair loss. If subjected to traction, in the latter period the hair can fall easily without experiencing pain.
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