Hair

Where and what kind of hair we have is genetically predetermined and also dependent on sex. The hair and nails are formed from “hard” keratin, the horny layer (stratum corneum) of the epidermis from “soft” keratin. Nearly the whole of the body is covered with hair, which is only absent on the lips, the palms of the hands, the soles of the feet and parts of the external genitals. Hairiness is influenced by sex, age and race. There are also individual differences in the colour, length and diameter of the hairs, and the speed at which they grow.

While still in the womb, short, thin, unpigmented hair begins to develop on the foetus from about the 4th month (lanugo hair), which is replaced about 6 months after birth by fine, soft, usually unpigmented hair, called vellus hair. The so-called terminal hair, which is long, thick and pigmented, starts to grow after birth on the head. Up to puberty, terminal hair is only found on the head, in the eyebrows and eyelashes. At puberty, under the influence of androgens (male hormones) but in both sexes, the vellus hair starts to change into adult hair, first in the pubic region and then followed by the armpits, lower legs, thighs, forearms, belly, buttocks, back, arms and shoulders.

The hairs are normally arranged in groups. The hairs are set at an angle. Changing the angle causes a “vortex”. Each hair grows in a certain rhythm. This growth rhythm does not take place in any one region or in all the regions of the body at the same time. Of the 100,000 – 150,000 hairs of the scalp, about 100 fall out every day. The hairs on the head grow at an average rate of 1 cm a month. The lifetime of a scalp hair is 2 to 6 years, that of the eyelashes 3 to 6 months. This explains the daily loss of hair, which tends to be greater when we wash our hair, and less marked the day after. The total number of hair follicles (out of which the hairs grow) in humans is about 5 million, with 100,000 to 150,000 on the scalp. The male beard has about 25,000 terminal hairs. Women have the same number of hair follicles, but they usually produce only fine vellus hairs. 600 eyebrow hairs and over 400 eyelashes protect our eyes from sweat and dirt. About 65 % of body hair in women consists of vellus hair, compared to only 10 % in men.

Each growth cycle of the hairs on the head consists of three phases. An active growth phase lasting 2 to 6 years (anagen stage) is followed by a transitional phase (catagen stage) of from a few days to 2 weeks, and finally a resting phase (telogen stage) with loss of the hair after 3 to 4 months. When the hair has been shed (club hair), a new one grows. About 13 % of all the hairs on the head are in the resting phase and only 1 % in the transitional phase. Growth of the scalp or beard hairs is not influenced by cutting or shaving.

Aufbau Haare

Hairs are thin, horny structures made of keratin and grow out of a kind of pocket in the epidermis: the hair follicle. The follicles are funnel-shaped and extend down into the dermis and subcutis. A hair consists of a horny hair shaft with an epithelial root sheath (i.e. originating from the epidermis) and the hair root, which is located with the hair bulb on the hair papilla made of connective tissue. The cells divide at the bulb and form the hair shaft, which becomes increasingly horny on its way to the surface of the skin. The hair shaft consists of the cortex (outer wall), which surrounds the medulla (middle or pith) – though this may not be present throughout the length of the hair – and on the outside the scaly cuticle layer. Vellus hairs have no medulla. In the lower cell layers (matrix cells) of the bulb lie the pigment cells (melanocytes), which are responsible for the hair’s colour. The papilla, bulb and surrounding sheath of connective tissue together form the follicle, which is supplied with blood by blood vessels.

Each hair is located in a funnel-shaped root sheath, into which a sebaceous gland opens. Under the sebaceous gland, the hair muscle (musculus arrector pili) is attached, running upwards at an angle to the junction between the dermis and the epidermis. If the muscle is contracted, e.g. through cold, the muscle raises the hair from its normal angled position into an upright position (“your hair stands on end”). As the muscle pulls the epidermis at the same time, a bump occurs (“goose pimples”). The muscle contraction also squeezes the sebaceous gland, which lies between the hair muscle and the root sheath, causing it to discharge sebum to the surface. The excretory duct of the gland runs together with the hair to the surface of the skin.

The root sheath of the hair contains a network of fine nerve fibres. As these are stimulated by the slightest movement of the hair, hairs also serve as organs of touch.

With the exception of red hair, the colour of the hair is determined by its pigment content (melanin). The different shades of hair result from the many colour nuances of eumelanin or phaeomelanin. Grey hair is due to a lack of pigment. If the production of pigment ceases or if bubbles of air become encapsulated in the hair, it appears grey or white. Early greying is usually the result of genetic factors.