Facts About Hair
The Hair Follicle
Your hair is made in hair follicles which are located deep in the dermis. The dermis is located in your skin, between the top layer or epidermis and the bottom, or subcutaneous layers.
How Hair Grows
The colour, size, depth and angles at which your hair grows all vary depending on where the hair is on your body, your ethnicity and some hormonal factors. That said, most hair grows in cycles and the cycles, in turn, have 4 phases, namely anagen (growth), catagen (regression), telogen (resting) and exogen (shedding). This cycle is controlled by an internal "hair cycle clock" and is influenced in varying degrees by endocrine, vascular and genetic functions. Your immune system also has an influence on your hair growth cycle.
At any given point in time 85%-90% of your hair will be in the anagen phase, 10%-15% in the telogen phase and less than 1% will be in the catagen stage. The anagen phase usually lasts for 2-6 years, the catagen stage for 2-3 weeks and the telogen phase for roughly 3 months.
However, the length of the anagen cycle will depend on where the hair follicles are located. For example, the moustache anagen cycle lasts between 1 and 3 months whereas your arm hair anagen cycle usually lasts between 6 and12 weeks. The anagen phase for leg hair is typically between 20 and 26 weeks.
While you can induce new hair growth by plucking the existing hair, shaving or cutting will not have an effect on growth.
Estrogen prolongs the anagen growth phase and thereby reduces the rate at which the hair grows, and drugs such as corticosteroids slow down the onset of the anagen phase.
By contrast, the male sex hormone androgen and the steroid hormone testosterone which it produces, increase the rate at which hair grows. Androgens also increase the diameters of the hair, particularly in the beard area. Thyroxin, a hormone produced by the thyroid, also increases hair growth.
Types of Hair
The shape of the hair shaft varies by ethnicity. For example, Asian hair tends to be circular whereas Caucasian hair is elliptical. The oval shaped hair tends to become curlier over time.
Vellus hair is very short, fine hair that covers your body to help you maintain a steady temperature. It does not have any pigment and is not attached to muscle, although vellus hair follicles can be attached to large oil glands.
Vellus hair undergoes a full hair cycle which is shorter than the full cycle seen in terminal hair.
During puberty, naturally occurring androgens, such as testosterone, initiate the conversion of vellus hair to terminal hair in certain parts of your body including the pubic area and armpits. Terminal hair is typically thick, pigmented and mature and also includes your eyebrow, eyelashes and scalp hair. In men it includes the beard area.
Lanugo hair refers to the very fine hair found on newborns in the first few weeks of life.
Facts About Hair: