Hair Follicles and Their Growth Process

Long or short, gray or brown, thick or thin, hair is a part of us all. Whether it is on our head, legs, chest, genitals or arms, it continuously grows, helping to create gender identity and helping to transmit sensory information.

Not everyone's hair grows the same, and not always on the same area of the body. However the most noticeable hair is on the head or scalp.

There are about 100,000 hair follicles on the average human scalp, and over 5 million on the body. Each of these follicles can grow around 20 individual hair strands in a persons lifetime. There are even hair follicles on a 22 week old developing fetus.

The hair has two structures, there is the follicle in the skin and the hair shaft that we see.

What is a hair follicle? A hair follicle, is part of the skin that uses old cells, packed together to grow hair. It contains several layers with different jobs. At the base of the follicle is the papilla containing tiny blood vessels that feed the cells. Surrounding the papilla is the living part of the hair called the bulb. The cells in the bulb will divide every 23 to 72 hours, faster than any other cell in the body.

Surrounding the follicle are two sheaths, the inner and the otter sheath. The sheaths mold and protect the growing hair shaft. The inner sheath follows the hair shaft just to the opening of the oil gland and sometimes to the scent gland. The outer sheath goes all the way up the gland. A muscle then attaches below the gland around the outer layer. Whenever this muscle contracts, the hair stands up.

The shaft is made up of three layers of dead hard protein called keratin. The middle layer may not be present and is called the medulla. The cortex is the next layer and the third layer, or the outer layer is called the cuticle.

Hair grown in three separate phases, the anagen phase, catagen phase and telogen phase. These are also known as the life cycle of the hair follicle, and each hair goes through these phases.

The anagen phase, is the growing phase of the hair follicles. Dividing rapidly are cells in the root of the hair, and a new hair is formed. During the anagen phase the hair will grow about 1 cm every 28 days, although hair may grow faster in the winter than in the summer.

Scalp hair stays in this phase of growth for two to six years. Also during this phase, pigment or melanin is made in the hair bulb. Older people produce less pigment, this is why gray hairs appear even though the hair is still growing strongly.

When the anagen phase is complete, the follicles go into the catagen phase.

The categen phase is a short resting stage. Three percent of all hairs are in this phase at any given time. This phase lasts between two and three weeks. During this time growth stops, the outer sheath shrinks and attaches itself to the root of the hair. a club hair is formed. No pigment is made during this phase.

The third and final phase is the telogen phase, the resting phase of the hair follicle and also the shedding phase. At any time ten to fifteen percent of all hairs are in this phase and last for about 100 days for the scalp and much longer for the hairs on the leg, arm, eyelash and eyebrow. At this time the follicle is entirely at rest and the club hair is completely formed. A solid, hard, white, dry material would be at the root of a hair, if pulled out.

The telogen phase lasts for three to four months. During this time a new hair will begin to grow upwards pushing the old hair out, causing natural shedding. Between 25 and 100 telogen hairs are shed normally each day. These are the hairs that come out when you brush, or shampoo your hair. The new hair will emerge from the same opening, as the old hair, and the cycle begins again.

sources:

http://dermatology.about.com/cs/hairanatomy/a/hairbiology_2.htm

From Wikipedia, the free encyclopedia

http://www.transgendercare.com/electrolysis/hair/growth_cycle.htm

http://www.pg.com/science/haircare/hair_twh_23.htm