Normal hair growth patterns and how androgens change them

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Normal Androgen Hair Growth

Development of human hair is a biological process that begins in the mother’s womb. A number of hormones determine generalized human hair growth. Out of these hormones, the androgen group is the major factor that determines hair distribution and development in our body. Progesterone and estrogens are the two other hormones that have a lesser effect on hair biology.

To understand normal androgen hair growth better, you must first know about the three types of human hair and their appearance.

The three general kinds of human hair

  • Lanugo hair: The first manifestation of hair is seen in the human fetus and is known as lanugo. Lanugo is dense, soft, unmedullated and is distributed over the surface of the fetus. It disappears either late in gestation or early postpartum.
  • Vellus hair: This is non-pigmented, unmedullated, soft, short (approximately less than 2 mm in length), and covers the seemingly hairless human body parts.
  • Terminal hair: This kind of hair is pigmented, medullated, dense, coarse and longer than vellus hair and makes the eyebrows, eyelashes, scalp hair, the pubic hair and axillary hair, etc.

The physiology of normal development of hair

Humans have approximately 5 million (20% of which is in the scalp) hair follicles and this amount is genetically predetermined. They form after birth, and their amount normally declines with the process of ageing. The soles of feet, the palms of the hands and the lips are the few areas that do not have any hair follicles.

The normal growth and life cycle of hair consist of three alternating stages:

  • Anagen is the active, growth phase. The duration of this phase determines the general length of hair.
  • Catagen is the involution stage wherein the hair stops growing and the hair bud shrinks.
  • Telogen is the resting stage during which hair is shed and it completes the growth cycle.

Within this three-stage life cycle of hair, androgens determine the growth rate and the transformation of vellus to terminal hairs. The conversion and development of vellus to terminal hair is permanent, irreversible and a normal biological process caused by androgens (testosterone and dihydrotestosterone) in body parts that are androgen receptive.

Although this process may involve a number of growth cycles, during which the conversion may actually be reversed, it is not reversible once complete. This process normally starts at puberty, continues through adult, reproductive life and slowly lessens with ageing and reproductive status in humans.

Androgen biology and sources

Androgens are steroids and hydrophobic compounds. They are generally protein-bound in the plasma. In women, the ovaries and adrenal glands produce androgens. They contain steroidogenic enzymes, which trigger production of androgens from cholesterol. The peripheral layer of the skin is also a major source of androgen. However, peripheral skin cannot produce androgens from cholesterol rather it can convert weak androgens to more potent androgens to trigger off faster activity.

Androgen action on hair

Testosterone is the main circulating androgen that determines hair growth. 5a-reductase converts testosterone to dihydrotestosterone (DHT) in the hair follicle. The more powerful dihydrotestosterone then drives the dermal papilla to create a terminal medullated hair out of vellus hair. Other less potential androgens like androstenedione and dehydroepiandrosterone (DHEA) can also be transformed in the skin to testosterone and dihydrotestosterone to induce more hair growth.

The variations in androgen receptor and 5a-reductase factor of the skin of a particular body part, determines its androgen-triggered hair growth pattern.

For instance, the skin of the eyelash, eyebrow and lateral and occipital aspects of the scalp (mainly non-sexual skin) are comparatively less sensitive to androgens. On the other hand ambosexual skin (includes the lower pubic triangle and the axillary parts) are more receptive to androgens, where vellus hair is converted to the terminal kind with the lightest androgen trigger. The process of conversion to terminal hair in these body regions starts in early puberty, with only a minimal rise in adrenal androgen levels.

Then there are the skins of sexual areas (chest, lower abdomen, the lower back, the upper thighs, the upper arms, the chin, the face, and the upper pelvic triangle) that are sensitive to only high levels of androgens. The formation of terminal hairs in these parts are male-like and when they occur in women can be apparently diagnosed as hirsutism and its associated causes like Polycystic Ovarian Syndrome (PCOS), hyperandrogenism, hypothyroidism, the hyperandrogenic insulin-resistant acanthosis nigricans syndrome (HAIR-AN), 21-hydroxylase non-classic I adrenal hyperplasia (late-onset CAH), 21-hydroxylase-deficient congenital adrenal hyperplasia, Hyperprolactinemia, Androgenic tumors, Cushing’s syndrome etc.

Hence, though androgen is essential to the proper development of human body hair, any variations in its normal levels can give rise to various medical complications.