Menu

Development And Distribution Of Hair Follicles


Body hair grows everywhere except for palms, soles, ventral sides of fingers, inner sides of prepisium, glands penis and inside of female outer genitals.

Depending on the age and effects of hormones different types of body hair are produced from different follicles. In humans in the prenatal stage body surface is covered with thin, soft body hair (lanuga hairs) without medulla and usually unpigmented. Normally in the intrauterine life in the 8th or 9th months of pregnancy lanuga body hair is lost except for hair, eyebrows and eyelashes. Then again, lanuga body hair may continue to survive for life in a rare hereditary syndrome called hipertrichosis lanuginose.

 

Congenital body hair is in two types:

1.- Vellus hair: soft, without medulla, sometimes pigmented, rarely longer than 2 cm.
2.- Terminal hair: longer, thick, with medulla and pigmented hair.

Besides there might be intermediate types between these two types. Normally while terminal body hair before puberty is only located in hair, eyebrows and eyelashes, following puberty terminal body hair is formed of vellus hair as a secondary sexual characteristic due to the effect of androgens.

Average body hair density in forehead and cheeks is 800/cm2, in thighs and legs this number is 50/cm2. Vellus body hair is mostly dense in forehead. In both genders the density here is 400-450/cm2. Density of vellus body hair is lower for women (50-100/cm2).

Body hair follicles of hairy skin in the head undergo a significant decrease as people are aged. In adults between ages 20-30 it is 615/cm2, between the ages 30-50 density becomes 485/cm2 average. In bald headed individuals a long term decrease is seen in the number of follicles. When bald headed are compared with hairy individuals between the ages 30 to 90 density of hair is 306/cm2 in bald headed, 459/cm2 in others.

In puberty vellus hair are replaced by terminal hair starting from the pubic region. Initial pubic hair will be rare, long, slightly pigmented and smooth in both genders.

Later its color will become darker, will be curlier and will follow an inverse triangle distribution. The base of this triangle will be straight in female and male gender it will be more convex and extend to the belly. Axillary hair starts to be seen 2 years after pubic hair. Their numbers increase starting from puberty to the ends of twenties and later it decreases gradually. In boys facial hair appears together with axillary hair. Pilosism starts in the corners of upper lips and then it spreads to cheeks and chin and mustache area.

Terminal hair start to become evident in legs, thighs, forearms, hips, back, arms and shoulders in this order.

Hair distribution in the nape, chest, back and extremities varies among individuals. Terminal hair distribution will increase with sexual maturity. Adult hair distribution sample only emerges in forties. Ear hairs become evident in middle age. Density increase in sternal region in males demonstrates that hair grow starting from puberty to 5-6.