Changes in hair fiber growth with increasing age

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Hair fiber growth changes with age

Lanugo hair is the first hair produced by the fetal hair follicles. it is usually shed between the 32nd and 36th week of gestation. In up to one third of babies, however, lanugo hair growth, to some extent, is retained until birth. So the babies that are born with lanugo hair over the entire body are considered premature.

During the fetal period, all scalp hair follicles go through the same hair cycle phase. Within a few weeks after birth, regression of the hair follicles to catagen starts. It occurs in two waves starting at the frontal zone, eventually migrating over the scalp from the forehead to the nape of the neck. There is an area over the occiput in which the primary hairs do not enter telogen until after birth. These hairs remain in the scalp for eight to twelve weeks and then fall. As telogen hairs predominate on the occiput, their fall commonly produces an area of temporary, localized alopecia.

In the subsequent hair cycle, intermediate hair fibers form the first scalp hair and an asynchronous hair cycle is established following a mosaic pattern. There is considerable variation in the age at which the mosaic pattern is fully developed. In most of the cases infants have only sparse hair growth in the initial months of their lives and this continues until the first postnatal telogen phase kicks off. This is followed by growth of the hair that is much stronger and more consistent in density. On the scalp, the fully developed terminal hair follicle is detectable around 12-16 months of age.

After the baby loses the fine lanugo hair, vellus hair follicles cover most of our body surface as the most prominent hair follicle type in the postnatal period. Initially, only the hair of the eyebrows, eyelashes and scalp contain terminal highly pigmented hairs. Over the time, vellus hair follicles start getting transformed and begin to produce terminal hair fiber. This process is conditioned by age of the individual, gender, and body region. The vellus hair in some body areas starts to get replaced by longer, coarser, pigmented terminal hair at puberty. This transformation first sets in the pubic region moving to other areas making the eyelashes and eyebrows thicker as compared to prepuberty.

Axillary hair and male facial hair begin to appear about two years after the growth of pubic hair begins. Body and beard hair continues to grow long after puberty. This growth is stimulated by male hormones. Again male hormones paradoxically, cause regression of scalp terminal hair follicles into vellus hair-like follicles in androgenetic alopecia. The likelihood of androgen induced terminal hair growth presence on body regions varies with age, sex and ethnicity. In general, Caucasians have a greater frequency of body hair growth as compared to Asian and African peoples. While beard hair growth is almost always present in Caucasians, beard growth can be more limited in Asian ethnic groups and extremely limited in aboriginal peoples.

Post puberty hair growth rates depend on body location. Prepuberty, hair growth is most rapid on the vertex of the scalp, but post puberty the hair growth rate on the vertex slows and becomes less rapid as compared to the hair growth rate on the occiput in both males and females. The rate of terminal hair growth on the scalp occiput, thigh, and eyebrow remains relatively consistent throughout adult life, but the growth rate of more androgen responsive hair follicles on the chin and axillary hair change with advancing age. Hair follicles on the chin in men are the fastest growing hair follicles producing 0.38 mm of fiber per day in adulthood, but this rate slows somewhat with age. The reduction of the hair growth rate and atrophy of axillary hair follicles in old age is caused by Androgens.

Each human being is born with approximately two million and five million vellus and terminal hair follicles. Among them, 100.000 – 150.000 hair follicles are located on the scalp and around 20,000 on the face. Again the number of scalp hair follicles may vary with the individual’s skin, hair colour and ethnicity. Dark haired individuals generally have fewer scalp hair follicles than fair haired individuals. Overall, scalp hair density is slightly greater in boys than in girls and all individuals have a density gradient of hair follicles from the vertex to the occiput.

Hair follicle density in different body regions change with age. The formation of hair follicles gets completed during embryonic development and no additional follicles are naturally formed after birth in humans. But this is not always the case with other mammals. As for example, deer antler velvet involves annual formation of new hair follicles. As compared to an adult, the surface area of skin in new born humans is relatively small and so the hair follicles appear to be much denser. As the humans grow, the same numbers of hair follicles are spread over a progressively larger surface area of skin. Different regions of the body grow at different rates and subsequently, hair follicles in some regions are spread further apart than in others. While our heads grow somewhat after birth, the greatest increase in growth occurs in our limbs. Consequently, hair follicle density on the head remains relatively high while density on the lower arms and legs is much reduced in adults.

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