Structure and Growth
Hair is composed primarily from a long protein polymer called keratin, which is found in two forms alpha keratin (which has a helical structure) and amorphous keratin. This protein is secreted in the hair follicle from cortical cells in a structure known as the bulb. Within this, amorphous keratin is gradually organised into the helical alpha keratin from the bottom to the top of the bulb.
By the time the growing hair emerges from the bulb, the keratin has been organised into filaments, embedded in another type of keratin, so called matrix keratin. Five of these keratin molecules twist together to form a protofibril and again five of these protofibrils are joined to form a microfibril, which are again in turn bound together to form cables.
In turn the cables bind together to form the hair strand, which are covered by cuticle cells. These are bound at their base to the strand, and overlap each other to form a protective coat for the cortex of the hair strand. The cuticle cells are transparent, which helps diffuse and reflect light, to give hair a glossy appearance. Damage to the cuticle, perhaps by to much heat, or to much brushing can cause the hair to lose its sheen. If the cuticle cells are pushed back from the hair strand itself, they can tangle with other hairs making the hair as a whole matted.
As the hair strand moves out of the follicle cells in the papilla called melanocytes deposit grains
pigment into the spaces between the fibrils and cables. The amount of pigment
grains, and the density of melanin within the grains determines how dark your hair is, from blonde
through brown and onto black. Red hair
is caused by the addition of an iron containing pigment.
In curly hair the disulphide bonds between amines (i.e. cysteine) in the matrix keratin are much more plentiful; the amount of these bonds present can be enhanced by the process of perming which can turn straight hair into curly hair. Or by breaking the disulphide bonds and re-forming them, turn you from curly to straight. Also water can disrupt the hydrogen bonding present in the proteins in hair, if you wrap the hair around a roller, as the hair dries the disrupted bonds will tend to reform in their new positions, making it curly. This is temporary, as getting the hair wet again will again disrupt the bonding that gave it its nice wavy shape.
(There are actually many different types of hair, which can vary from species to species, in this write-up, I'm only going to
talk about the human ones...)
Hair can be catergorised
in many different ways, but one of the most common is used by Dermatologists and
pediatritions which distinguishes types according to the hairs's length
and thickness; forming three classifications. The amount of each type a person has is usually (although illness or disease can also have their effects)
determined by their genetic make-up.
1.) Lanugo or primary hair. This is the first hair to grow from a
follicle and happens during the growth of the foetus in the womb. The
hair is very long and fine and is unpigmented; this hair is
replaced during the eighth month of pregnancy by the next stage of hair
2.) Vellus or secondary hair. This hair is again unpigmented and fine, but is
short. On children and adults it's the fuzzy hair on their nose and
cheeks. Follicles producing vellus hair can produce the next
stage, terminal hair, and revert back to vellus again at different
3.) Terminal hair is the coarsest of all hair, and comes from large
follicles which often contain a medulla. During puberty hormones
trigger the conversion of vellus follicles to terminal follicles, the
reverese process androgenetic alopecia sees the terminal hair go back
to vellus hairs.
Terminal hair sub-categories
Again terminal hair can be sub-categorised according to the hairs
location and appearance.
hair, this hair has a very slow growth rate, reaching an
average length of 10 mm at about 0.16mm per day! Also plucking eyebrows
can damage the follicle, which can cause complete loss of hair
hair is very similar to eyebrow hair aside from being a
bit shorter at 7.5mm.
hair grows in a clockwise spiral
pattern from the top of the
head at about 0.36mm a day for women
and 0.34mm for men
hair is considered to be another type of hair, originating
from the action of hormones
on the follicle. You won't be surprised
to hear it's one of the fastest growing types at 0.38mm per day!
5.) Body hair. Some of the body
hairs are slightly responsive to the sex
hormones, whilst other neighbouring ones are not; the ones that are
develop into terminal hairs. This helps explain why
some people are more hairy I suppose, their follicles just respond differently...
6.) (Chin) whisker
hair. These hairs are very susceptable to sex
hormones in both men and women, explaining why an imbalance in
levels in women can trigger beard growth....
hair is very large in diameter, pigmented and kinky. Again
sex hormone levels can determine it's appearance. Abnormal
growth can be a sign of a genetic condition that cause abnormal
hairs. Yes, the hairs around your bum-hole are of a
distinct type, different from other hairs near there by virtue of their unusually large
size and sebaceous glands
. Perhaps once-upon-a-time
produced helped us mark territory, or attract a mate from miles away, something that our
sense of smell is now 'blind' to!
Factors affecting Growth
As hair is often affected by androgen hormones, another classification
system exists based on the response of a hair follicle to the different
1.) No effect, hairs on the eyelashes, brows and extremities of the
hands and feet aren't really affected by any of the androgens.
2.) Female hormone responsive follicles. Generally pubic and armpit
hairs (and some limb and torso hairs) are responsive to the low female androgen
3.) Male androgen level responsive follicles. The beard, mustache,
nasal, chest upper back and scalp vertex hair. (The last one, scalp
vertex explains baldness I suppose!)
As I said, the exact number and appearance of each type of hair is
genetically determined, giving rise to the many different appearances of
individuals throughout the world. Also conditions such as cancer can affect both the pattern and growth of hair.