hair
Eva Rudolf-Müller is a freelance writer in the medical team. She studied human medicine and newspaper sciences and has repeatedly worked in both areas - as a doctor in the clinic, as a reviewer, and as a medical journalist for various specialist journals. She is currently working in online journalism, where a wide range of medicine is offered to everyone.
More about the experts All content is checked by medical journalists.The hair is a structure made of keratin, which in humans and mammals covers the whole body more or less densely. They protect against the weather and increase the sense of touch. In humans, they also have the function of belonging to a group and are an expression of attitudes towards life and values. Read everything you need to know about: What is hair made of? How are they developing? What health problems can hair affect?
What is the hair
Hair is long horn threads made of keratin. As so-called skin appendages, they form in the epidermis from the third embryonic month.
There are three types of hair in humans:
- Lanugo hair (downy hair): fine, short, thin and unpigmented hair that occurs during the embryonic period and is shed no later than the 4th month of life.
- Vellus hair (wool hair): these short, fine, slightly pigmented hairs initially replace the lanugo hair. They form body hair in children, but also in some women.
- Terminal hair (permanent hair): mostly long, thick and more or less pigmented hair that has formed the scalp hair, eyelashes and eyebrows since birth. During puberty, the vellus hairs in the armpits and genital area develop into such terminal hairs. The same goes for most of the male body hair.
Hair: build-up
Hair emerges from cone-shaped systems in the depths of the epidermis, which grow into the embryonic connective tissue. From this the hair papilla develops, a cone of connective tissue that is supplied with blood. The hair bulb sits around this, the thickened end of the hair root that extends diagonally into the subcutaneous tissue.
Only the hair shaft protruding from the skin is externally visible. The hair is surrounded on the outside by a simple, keratinized squamous epithelium (cuticle), which has jagged edges pointing upwards. These edges overlap like roof tiles. When they are tightly intertwined, the hair looks smooth and shiny. But when they are apart, it looks brittle and lackluster.
Since the hair is at an angle in the skin, a direction, a "line", can be recognized. This is particularly noticeable in the vertebrae that form the hair.
A hair follicle muscle runs between the hair follicle and the skin surface, which can contract when excited, causing the hair to stand up and making the skin surface look like "goose bumps".
Whether hair is straight or curly depends on the cross-section of the hair shaft. If the cross-section is round, they are usually very smooth. With a round to oval cross-section, they are smooth or can form curls. With a strongly elliptical cross-section, they usually form very strong, small curls.
The development of a hair is cyclical, and each hair follicle or hair follicle has its own cycle that is independent of other hair follicles. The cycle can be divided into three parts: anagen, catagen and telogen phase.
Hair development: anagen phase
The hair bulb takes on different shapes as the hair shaft develops:
In the growth phase (anagen phase), in which a new hair is formed, a new bulb is also formed in the hair root, which is layered in several layers due to the constant formation of new cells. There is a high metabolic activity, but also a high sensitivity to pollutants of all kinds.
The anagen phase lasts two to six years and depends on age, gender and where the hair grows. About 90 percent of the scalp hair is in the anagen phase.
Hair development: catagen phase
In the transition phase (catagen phase), the metabolic activity and thus the cell production of the hair bulb end - it is closed and keratinized (storage of keratin). The hair is rounded at the bottom and enclosed by the outer sheath of the hair root and slowly moves upwards.
The catagen phase lasts one to two weeks. About one percent of the scalp hair is in it.
Hair development: telogen phase
In the final or resting phase (telogen phase), the onion is displaced, the inner hair root sheath disappears and the newly created matrix renews the hair papilla and cell division begins again. A new "anagen hair" forms, which the piston hair then expels in its telogen phase.
At this stage, around 18 percent of the hair is on the head. The telogen phase lasts two to four months.
How much hair does a person have?
The number of hairs on the scalp is around 90,000 to 100,000. There are, however, clear differences among people with different hair colors: On average, blonde people have the most hair, around 140,000. Then follow brunette people with around 100,000 hairs on their heads. At the bottom of the list are redheads with only around 85,000 hairs.
Despite differences in the total number, the number of hairs that fall out every day is the same for everyone: natural hair loss is around 70 to 100 hairs per day.
The hair grows by about 0.3 millimeters daily, i.e. about one centimeter per month. The hair thickness (diameter / hair) is 0.04 millimeters for vellus hairs and 0.12 millimeters for terminal hairs. The density is around 200 hairs per square centimeter.
The haircolour
The color of the hair is created by pigments that are produced by certain cells called melanocytes. These cells are found in abundance in the area of the hair bulb. When air enters the hair pulp, it leads to graying. The initial mixture of colorless and natural-colored hair gives the impression of "gray". When all hair is pigment-free, it looks white.
What is the function of the hair?
In many animals, the hair is important for thermal insulation, as protection against external influences and as an organ of orientation and touch. These hair functions no longer play a major role in humans. Only special hair has a protective function. This protects the scalp hair against cold and UV radiation and the hair in the nose and ear canal against the penetration of dust particles.
In addition, human hair (such as animal) can pass on sensations of touch, pressure and tactile stimuli - thanks to the many nerve endings at the hair root.
Last but not least, hair has an important function as jewelry in all cultures.
Where is the hair?
They are found on the entire surface of the body with a few exceptions - palms and soles, inner fingers, nipples and lips are hairless. Mucous membranes are also basically hairless.
What problems can hair cause?
A purulent inflammation of the hair follicle gland is called a boil. Their most severe form is known as carbuncle. Several neighboring hair follicles are inflamed (with tissue melting).
Pollutants
Toxins damage hair, especially in the anagen phase. The strength and duration of exposure to a pollutant and the sensitivity of the individual follicle play a role in the intensity of the damage.
With lighter pollutants, anagen hairs prematurely convert into telogen hair, which leads to hair loss after two to four months (depending on the duration of the telogen phase).
In the case of stronger pollutants, only part of the anagen hair is converted into telogen hair. The majority of sensitive anagen hairs become dystrophic and break off at the narrowest point, which leads to rapid onset of hair loss.
In the case of very strong pollutants, the transformation and the onset of hair loss occur within hours to days.
Extremely strong or suddenly acting pollutants lead to the destruction of the entire hair matrix within hours: The hair breaks and falls out.
Hair loss and lack of hair
The formation of baldness in men is based on a hereditary predisposition. It can start shortly after puberty.
Androgens (male sex hormones) like testosterone have different effects on hair growth. The development of the scalp hair is inhibited, hair loss and baldness occur (whereby heredity and age also play a role here). The development of body hair in men, on the other hand, is promoted by androgens.
Some people suddenly experience circular hair loss (alopecia areata). For example, it can affect the hair on the head, beard, under the armpits, in the genital region or the eyebrows.
Hair deficiency (hypotrichosis) can have various causes, for example metabolic disorders (such as iron deficiency or malnutrition), hormonal changes (such as during pregnancy, breastfeeding and menopause) or infectious diseases (such as typhoid, late stage syphilis, flu). However, hypotrichosis can also be hereditary and congenital.
Excessive hair
In women, an excess of androgens leads to a male hair type (hirsutism). The cause of this remains unclear in some cases. In others, the reason is an overproduction of testosterone (for example in ovarian tumors, Cushing's disease, obesity or polycystic ovarian syndrome).
People with so-called hypertrichosis congenita wear long, thick, silky hair on their bodies: the innate lanugo hairs persist and are not replaced by vellus hairs. Those affected are often colloquially called "ape people".
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