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The Biology of Hair growth
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The hair follicle is the production site for bodily hair and it goes through stages of growth, resting and hair loss followed by regeneration and re-growth as part of a natural cycle. The follicle is fed with nutrients by its blood supply. The growth of the hair of both men and women is regulated by male hormones called androgens. Scalp hair growth is not directly related to androgen levels, but rather is influenced by local amounts of a testosterone derivative, dihydrotestosterone.
Alopecia, and all its varieties, is the condition where the ability of the hair follicle to regenerate is damaged. This means that when hair is lost naturally through shedding, it is not replaced.
There are three types of hair: vellus, terminal, and intermediate. Vellus hairs are short, fine, soft, usually non-pigmented. Baldness is associated with increased numbers of vellus hairs on the scalp. Terminal hairs are large and darkly pigmented and constitute the normal hair on our heads. Ninety percent of the hairs on the chest, trunk, shoulders, legs, and arms of men are terminal hairs. Intermediate hairs occur on the scalp, and they demonstrate a form between the terminal and vellus hairs. Intermediate hairs contain a moderate amount of pigment and represent a form between the intermediate and vellus types of hair.
Development of the hair follicle
The hair follicles begin to form in the embryo about 60 days after fertilisation. Specialised, elongated cells, called the dermal papilla, aggregate in the dermal (skin) layer and start to develop downwards through the epidermis and into the dermis. These cells are loosely associated within the relatively undifferentiated dermis. The dermal papilla becomes the site for the new hair follicle. Follicles are formed over the body except on the soles of the feet and the palms of the hand. They appear on the head first, followed by those on the body, the arms and legs. These hairs and the hairs that form the eyebrows are the first terminal hairs to appear. Some hair follicles develop at later in life puberty and produce pubic and axillary hair.
As the follicle develops the lower part widens out accommodate (from the lowest part upwards) the germinal matrix, the differentiation zone, the hair bulb, the keratinisation zone and the hair shaft itself.
The mature follicle is a highly modified type of skin. It is an invagination of the skin with the main layers of the epidermis folding inwards to form a narrow sack which contains, at its base, the areas which generate the new hair and, above that, the hair shaft itself. Not all hair follicles are productive, going through a natural life cycle.
Hair growth begins in the germinal matrix which is at the bottom of the follicle. This has a good blood supply bringing the nutrients, hormones and oxygen necessary for growth. Cells are formed in the germinal matrix and push upwards towards the hair bulb where they differentiate into hair-like material which, in turn, is keratinised into hair proper. As new cells are formed in the germinal matrix, then differentiated and keratinised, they push the cells formed earlier upwards and so the hair grows at about half an inch per month in a healthy individual. If hair is pulled out or falls out naturally then the hair parts company with its follicle due to a separation between the hair bulb and the keratinisation zone of the follicle. This leaves the whitish hair bulb at the root of the detached hair. It is also the reason why the scalp does not bleed when hair is lost naturally because the blood supply is restricted to the lower, germinal matrix layer. As the hair detaches in this way, it leaves the germinal matrix intact and ready to start producing the next new hair. In some cases the old hair will detach but not leave the follicle so as the new hair begins to grow beneath the old hair it will gradually force the old one out.
The mature hair comprises three layers, the outer cuticle, the cortex and the medulla. Uncut hairs have a pointed tip. The color of the hair comes from melanocytes containing either eumelanin (a black or brown colored protein) or pheomelanin (a red or yellow colored protein). If these colored proteins are reduced or absent then the hair becomes colorless and appears 'gray' if mixed with hair still with some color or completely white when there is no color (as in albinos). Darker colored hair is the result of the melanocytes being more densely packed such as in black, African hair. As you age hair does not 'go gray' it just looses its melanocytes and the color fades.
Hair on the body
The adult body is covered by about 5 million hairs and only about 3% (around 100,000 to 350,000) are on the head. Natural blondes have more scalp hair than brunettes who, in turn, have more hair than redheads. The texture of hair on the head varies between individuals and between races. Caucasian hair is thinner and elliptical. African hair is flatter and more curly. The darkest hairs on the human body are usually the eyelashes.
The main constituent of hair is the dead protein keratin that also makes up the majority of nails and the outer layers of skin.
Hair growth
Hair growth is strongest between the ages of 15 and 30 years. Hair grows faster in the summer. On the scalp, human hair grows at a rate of 0.5 mm/day on the crown and 0.4 mm/day at the temples Hair growth is divided in three stages which are: Anagen (growth) phase > Catagen (transition or regression) phase > Telogen (resting / hair loss) phase.
Anagen (growth) phase
This is the primary, growth phase of the hair follicle. It lasts between 2 and 6 years. Approximately 85% to 90% of scalp hairs are in the anagen phase at any one time. In the early stages the shortened follicle (as it was at the end of the telegen phase) elongates and re-connects with its dermal papilla.
Catagen (transition or regression) phase
This phase follows the anagen phase. About 1% of scalp hair is in the catagen phase at any one time and lasts between 2 and 6 weeks. The hair stops growing and becomes dormant. The dermal papilla shrinks and ceases to produce new hair cells.
Telogen (resting / hair loss) phase
This is the final phase of hair growth and follows the catagen phase. Approximately 10% of hair follicles are in the telogen phase at any one time and this phase lasts about 3 months. The hair follicle becomes detached from the dermal papilla and its blood supply. The follicle becomes shortened. In this condition the hair can be easily be pulled out by brushing or combing. In some cases the follicle will return to the anagen (growing) phase before the old hair has fallen out and as the new hair grows, it will push out the old one.
Types of hair
The most common hair on the body are the vellus hairs. Vellus hairs are fine, soft and colorless generally no longer than 2cm. They are replaced by terminal hair in some parts of the body at puberty. Terminal hairs are the, normally, colored hairs on the scalp and in the areas where secondary sexual characteristic hair develops at puberty. Intermediate hairs have characteristics between those of terminal hairs and vellus hairs.
Change of hair with age
As hair ages it tends to become less pigmented as the melanocytes decline in number. Follicles also tend to reduce in number. These two factors result in a general graying and thinning of scalp hair. Even where there is no significant hair loss most men see some receding of the hair line, especially at the temples. The balding process is a conversion of the follicles so that they produce vellus hairs rather than terminal hairs. Most Caucasian adults have experienced some hair loss by the age of 30 years. Females experience increased hair loss after menopause.
Natural hair loss
The process of natural hair loss, or shedding, is normal in all animals. In humans 50 to 100 hairs are shed each day as follicles go through the telogen stage of growth and the old hairs are lost. This is known as shedding. If the production of new hair is exceeded by this level of natural shedding, then constitutes hair loss. Most natural shedding occurs when combing or brushing. Excessive hair loss is usually noticed by loose hairs appearing on clothes and pillows. Sometimes the hair can be lost in small patches.
In males hair loss begins at the temples and the crown. Hairs at the sides and back of the scalp are much more resistant to loss than other areas. The reason for this is unknown. As it proceeds the hair line recedes and eventually meets with the balding crown which produces the characteristic horseshoe pattern of male baldness. This is usually accompanied by a general thinning of the hair on the scalp. In females general thinning is more common.
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