Hair Growth Biology

Generality

The hair originates from the hair follicle, which is located in the dermis. Each follicle is associated with a sebum-producing gland ( sebaceous ); follicle and gland form the pilosebaceous unit . Connected to the hair follicle there is then the erector muscle of the hair, whose contraction allows the piloerezione.

At the level of the pilosebaceous unit, there is a high expression of a particular type of enzyme: type II 5-alpha reductase. This enzyme converts testosterone into its active form, dihydrotestosterone, the main cause of hair loss in patients with androgenetic alopecia.

Hair Life Cycle

The extent and type of hair growth is genetically predetermined.

The growth of the hair is not continuous but cyclical: in fact the growth phase, called anagena, is followed by a rapid involution ( catagena phase), which is in turn followed by a period of quiescence ( telogen phase).

Hair length is mainly determined by the duration of the growth phase. The impression of continuous growth or periodic fall that the hairs give in some body areas, is due to the degree of asynchronism in the growth existing between neighboring hair follicles. Thus on the scalp, where growth is asynchronous, there is the impression of a continuous growth of hair, while in other areas, due to the synchrony between the different hair follicles, the hairs simultaneously reach the resting phase, giving the impression of falling.

Hair growth

The factors that control hair growth are governed by the dermal papilla, whose disappearance is a crucial factor in hair loss. Factors able to influence hair growth can be substantially divided into two groups: hormonal and non-hormonal.

Hormonal factors

Among the various hairs, there are many that respond to the stimulation of steroid hormones (derived from cholesterol), including above all sexual ones; these are called sexual hairs . They are located on the face, on the lower part of the abdomen, on the front face of the thighs, on the chest, on the breasts, on the pubic area and on the armpits.

The pilosebaceous unit is regulated by the maturative cyclic nature of the hair follicle and its sensitivity to the action of androgens, due to the presence of specific receptors for these hormones.

The pilosebaceous apparatus is particularly affected by androgen activity in the skin because it has the enzyme 5-alpha-reductase type II, which has the ability to convert testosterone into dihydrotestosterone. In the shaggy woman, this conversion activity seems to be accelerated.

Testosterone is the most powerful androgen and, in most hirsute women, the production rate of this hormone is exaggeratedly high, both due to an increase in its ovarian production, and due to an increase in ovarian and adrenal production of androstenedione (a precursor of testosterone, which is then converted into dihydrotestosterone by the 5-alpha reductase of the hair follicle).

Under normal conditions, in women, the structures responsible for the production of androgenic steroid hormones are the adrenal gland and the ovary. A further, but smaller, share of androgens is also produced at the level of the pilosebaceous unit.

However, in most hirsutisms, the excess production of androgens is of ovarian origin.

As for female hormones, however, the subject changes. In fact, estrogens have an action completely opposite to that of androgens on the hair follicle, because they delay the onset and extent of hair growth. Progestins, on the other hand, have a completely negligible effect on the follicle.

Pregnancy, characterized by high levels of estrogen and progesterone, can increase the synchrony of hair growth, leading to periods of increases and others of hair loss.

Numerous hormonal pathologies can affect the growth of sexual hair: the insufficiency of the pituitary gland (reduction of piliferous growth), acromegaly (increase), and hyperthyroidism (increased growth of axillary, pubic hair and the outer part of the eyebrow) .