That seemingly simple strand of hair on your brush is, in fact, a remarkably complex part of your body. Hair development begins surprisingly early, with about five million follicles already present on a fetus by 22 weeks. Remarkably, this is the complete set of hair follicles a person will ever have; no new follicles develop later in life.
Hair Anatomy
Each hair consists of a follicle embedded in the skin and the visible shaft. The follicle is a multi-layered structure, each layer serving a specific function. At the base lies the papilla, connected to capillaries that supply blood to the bulb, the lower portion of the hair strand.
Surrounding the follicle are inner and outer sheaths, protecting the hair shaft and guiding its growth. The inner sheath runs alongside the shaft, ending at the sebaceous gland, while the outer sheath extends to the gland and the erector pili muscle – responsible for “goosebumps.”
The hair shaft comprises three layers of dead protein cells made of keratin. The innermost layer, the medulla, isn’t always present. The cortex, the second layer, makes up the bulk of the hair shaft and contains the pigment that determines hair color. The outermost layer, the cuticle, consists of overlapping cells, providing luster and sheen.
How Hair Grows
Scalp hair grows at approximately six inches per year, or 0.3-0.4 mm per day. Hair growth occurs in three distinct phases:
* Catagen Phase: A transitional phase lasting two to three weeks, affecting about 3% of hair at any time. Hair growth ceases as the outer root sheath shrinks and attaches to the root.
* Telogen Phase: The “resting period,” affecting approximately 15% of hair. It lasts around 100 days for scalp hair and longer for body hair.
* Anagen Phase: An active growth phase where hair cells divide, and new hair growth occurs. This phase can last two to six years. Shorter anagen phases can indicate difficulty in growing hair long.
Hair Growth Disorders
Two notable hair growth disorders include:
* Hirsutism: Primarily affecting women, this condition causes excessive growth of thick, dark hair in areas typically associated with males, such as the face, chest, and areolae.
* Hypertrichosis: This condition causes excessive hair growth in areas where visible hair is not normally present. Both conditions are treatable with a consultation from your doctor.