Hair biology
Hair follicles start developing in the third month of pregnancy. By the time a child is born, on average 100,000 hair follicles have developed on its head, equivalent to 1,000 follicles per square centimetre of scalp. Follicle density declines from birth onwards, in line with the growth of a child's scull. As an adult, the total number of follicles declines to some 500 follicles per square centimetre by the time a person reaches the age of 25, to 150-250 between 30 and 50, further declining as we get even older.
"Follicular units"
Hair follicles grow in random groups over the whole scalp. These groups are known as "follicular units" and consist of 1-5 hairs. On average, each follicular unit has 2-3 hairs.
3 types of human hair
Lanugo hair is the fine, soft woolly kind of hair that grows all over the human foetus. In most cases, this is replaced shortly before birth by vellus hair, fine “peach fuzz” without pigment. During puberty, the increase in androgenic hormone levels causes the thinner and shorter vellus hair to be replaced with terminal hair in certain parts of the human body. Terminal hair is found not just on the head - beards, nostril and ear hairs, eyelashes, eyebrows, armpits and pubic hair as well the hair on our arms and legs all consist of terminal hair.
Hair anatomy
Hair - a mixed bag
A single hair contains a number of different substances, including carbon, oxygen, nitrogen, hydrogen and sulphur - and on average about 10 per cent water. The main constituent of any hair - some 90 % - is the so-called keratin, a highly structured protein also found in our upper layer of skin and in our nails.
Hair - constituents like beads on a string
As a molecule, keratin is made up of smaller units, amino acids, resembling beads on a string. The diameter of an individual hair varies from one person to the next, with the average somewhere between 0.05 and 0.09 mm. Keratin is a very durable substance, as witnessed by the virtually intact hairs found in Egyptian graves dating back centuries.
The hair shaft
The hair shaft is the visible part of any hair, i.e. outside the skin. From a technical perspective it is dead material, containing no blood vessels or nerves. This explains why having our hair cut doesn't hurt. The hair shaft consists of 3 layers:
1. The inner layer, the medulla, consisting of soft keratin and only found in thick, strong hair.
2. The middle layer, better known as the cortex and responsible for giving hair its strength, elasticity and texture. The cortex also produces melanin, the substance responsible for hair colour.
3. The outer layer is the cuticle layer. This is thin and transparent, consisting of several layers of flat, thin cells laid out overlapping one another as roof shingles. The purpose of this layer is to protect the cortex.
The hair follicle
The hair root is te part of the hair located in the follicle beneath the skin. It is the living part of any hair. The end of the hair follicle, also known as the hair bulb, is the part of the hair implanted in the skin. Each hair bulb contains a dermal papilla, consisting of a number of small blood vessels. Inside the skin, a hair follicle is covered by internal and external root sheaths, with the external sheath reaching up to the epidermis or upper skin layer.
Hair growth cycle
Human hairs do not grow at the same speed and intensity throughout one's life. Growth takes place in phases, in constantly repeating cycles, which are not always of the same intensity and regularity. A single hair cycle consists of 3 phases:
1. The anagen or active growth phase
The first phase is also known as the active phase, as it is the only phase during which the hair root cells actually produce hair. In this phase the hair bulb remains deep in the skin, where it remains until the resting phase. Some 85 % of all hair on the head will be in the active phase at any one time. The anagen phase can last 2 - 6 years. Hair grows on average 10 cm per year, and any single hair can grow to be over one metre long. A scalp contains about 1 million hair follicles / hair roots, though generally speaking only 100,000 - 150,000 hairs are actually visible. This gives us an indication of how many hair follicles are not active.
2. The catagen or transition phase
At the end of the growth phase, hair moves into a short transition phase lasting 1 - 2 weeks. In this period the hair follicles get ready for the resting period. At any one time, some 2 % of all hairs will be in this transition period. During this phase, hair growth stops, with hair follicles shrinking to about 1/6 of their normal length.
3. The telogen or resting phase
Coming at the end of the transition phase, the resting phase generally lasts 5 - 6 weeks. During this period a hair does not grow at all. It nevertheless remains connected to the follicle, while the dermal papilla sinks down to the bottom, going into a resting phase. Some 13 % of all hair will be in the resting phase at any one time, though this percentage can vary between 4 % and 24 %. At the end of this phase the older hair falls out, with a new hair finding its way through the skin to the surface, where it will remain for the next few years. The hair bulb returns to its usual place and the hair re-continues its healthy growth. In the course of a person's life this cycle will be repeated 20 times on average.