Why does hair on our head keep growing but hair on eyebrows and eyelashes grow to a certain length and then stop?
Hair is a filamentous biomaterial containing keratin and is one of the defining characteristics of mammals. Hair plays a role in protection, Thermoregulation, sensation and in gender identity.
Full grown hair is divided into two parts:
1. Hair Root is the part located below the surface of the skin
2. Hair Shaft is the portion that projects above the skin. The lowermost portion of the hair shaft called the Hair bulb has the living cells which divide and grow to build the hair shaft. Hormones act on this hair bulb to influence the rate of growth and the structure.
Hair growth occurs in cycles consisting of three phases:
Anagen is the active growth phase of the hair. The cells in the root of the hair are dividing rapidly.
Catagen is the transitional phase lasting for 2- 3 weeks in which hair growth slows and the hair follicle shrinks.
Telogen is the resting phase in which the hair growth stops and the old hair detaches from the hair follicle. A new hair begins the growth phase, pushing the old hair out. The telogen phase lasts for 3-4 months.
The length of the hair depends up on the time the hair follicle spends on the anagen phase. The Scalp hair stays in this active phase of growth for 2-7 years.
The differential growth of the hair in the eyelids and the eyebrows is due to their short anagen phase of 4 - 6 weeks. Hence they will not grow beyond a certain length.
Moreover the rate of growth is also less in these areas compared to scalp hair which are determined genetically. The hairs in arms and legs are also shorter for the same reasons.
Apart from this, the difference in length of hair between individuals is because of genetic, hormonal, nutritional and gender influences.
Dr. SELVA SEETHA RAMAN. S
Consultant Plastic surgeon
SRM Institutes for Medical Science