The Biology of Hair
The hair follicles start to develop in the third month after conception. We are born with on average 100,000 hair follicles on our head. On the head of a newborn, there is an average of 1,000 hair follicles per cm² .
At the age of 25, this number has diminished to about 500 hair follicles per cm². Between the ages of 30 to 50 years, the hair density decreases to about 300 hair follicles per cm². If there is no question of a balding process, the hair density will decrease mildly as you age.
The hair follicles grow in arbitrary groups, divided over the scalp. These groups are called " follicular units ". These follicular units exist of between 1 and 5 hairs. The average number of hairs per follicular unit is 2 to 3 hairs
The anatomy of the hair
A hair consists of keratin, a heavily structured protein. It is the same kind of protein which is responsible for the epidermis and the composition of our nails. Keratin is a molecule that is structured from smaller units, namely amino acids. The structure of these amino acids is comparable to the structure of a chain. The diameter of a single string of hair varies from person to person. The average diameter is between 0.05mm and 0.09mm.
The hair root is the part of the hair that finds itself in the follicle or hair sac under the skin, and is the living part of the hair. The outer part of the hair follicle, which is called the hair bulb, is the part that is nestled in the skin. In the basis of each hair bulb is the dermal papilla, which contains capillaries, or tiny blood vessels. The hair follicles are covered in the skin with inner and outer sheaths. The outer sheath of a hair follicle goes all the way up to the epidermis or the layer of overskin.
The hair shaft
The hair shaft is the part of the hair that is visible above the skin
and is technically dead, with no blood vessels or nerves. This also
explains why we feel no pain when we get our hair cut.
The hair shaft consists of 3 layers:
1. The inner layer or the medulla (marrow) consists of a soft keratin, which is only present in thick, broad hairs.
2. The middle layer, better known as the cortex, is responsible for the strength, elasticity and texture of the hair. The cortex also produces the substance of melanin, which determines the natural hair color.
3. The outer layer is called the cuticle, also known as layer of scales. The cuticle is thin and colorless and is composed of 6 to 10 overlapping separate layers. They serve to protect the cortex.
The hair growth cycle
Human hairs do not grow evenly and constant. They grow in phases, in always returning cycles. A single hair cycle consists of 3 phases:
The anagen phase The catagen phase The telogen phase
The anagen phase or the growing phase
The first phase, or growing phase, is also known as the active phase, because the cells in the roots of the hair only produce hair in this phase. In this phase, the hair bulb finds itself within the skin, and it will remain there until the resting phase. Around 85% of all hairs are in the growing phase at the same time. The anagen phase or growing phase can last for 2 to 6 years. On average, the hair will grow around 10 cm per year, and an individual hair can become longer than 1 meter.
The catagen phase or the transitional phase
At the end of the growing phase, the hairs are in the transitional phase, which lasts for around 1 to 2 weeks. During this phase, the hair follicles prepare themselves for the resting period. Around 2% of all hairs are in the transitional phase simultaneously. During this phase, the production of hair stops and the hair follicle shrinks to around 1/6th of its normal length.
The telogen phase or resting phase
The resting phase follows the transitional phase and lasts for on average 5 to 6 weeks. During this period, the hairs do not grow but they do stay attached to the follicle, while the dermal papilla below is in a phase of resting. Around 13% of all hairs are in the resting phase simultaneously – however, this percentage may vary from 4% up to 24%. At the end of this phase, the aged hair will fall out and the newly formed hair will find its way to the outside of the skin, and will remain there for several years. The hair bulb will return to its place and the hair will grow further in a healthy manner.
During a human life, this hair growth cycle will repeat itself on average 20 times from birth on.