Why do we have more hair on our head than any other part of the body?
Human body has different types of hairs in different parts. Vellus hairs, which are tiny and virtually invisible, intermediate hairs, which are shorter and less pigmented and terminal hairs which are larger, longer and pigmented are the three major types. From childhood onward, vellus hair covers the entire body regardless of sex or race except in the following locations: the lips, the nipples, the palms of hands, the soles of feet, certain external genital areas and scar tissue.
Increase in the level of androgens, the male hormones during puberty causes a transformation of vellus hair into intermediate and terminal hairs on several parts of the body in both male and female. The hair follicles in these areas respond to androgens, especially testosterone and its derivatives to convert the vellus hair into terminal hairs. The hairs in these locations are termed as androgenic hairs.
Sush areas are the underarms and the pubic area. In contrast, normally only men grow androgenic hair in other areas. So there is a sexual dimorphism in the amount and distribution of androgenic hair, with males having more intermediate hairs and females having more vellus hair, which is less visible. In men, approximately 5 per cent of testosterone undergoes 5á-reduction to form a potent androgen derivative called dihydrotestosterone (DHT). The enzyme 5á-reductase synthesises DHT in the prostate gland, testes, hair follicles and adrenal glands. When the percentage of DHT increases more than this, the male-pattern baldness occurs in men since their hair follicles, usually on the top and front of the head, are sensitive to the hormone DHT.
In women the DHT, which is synthesized in the hair follicles and adrenal glands and can prevent hair growth especially in the scalp is counteracted by estrogen. Hence for women, hair loss usually begins at menopause when estrogen levels begin to drop. The baldness in males due to inhibition of scalp hair growth is medically termed as androgenic alopecia. The response to androgen and its derivatives varies with the body site, and is the cause for different growth pattern of hairs in human body.
Editor, Research Journal of Biological Sciences
J.J. College of Arts and Science
Pudukkottai, Tamil Nadu