While in the womb, follicles begin to form on the scalp then move downward to the remainder of the body. Short, fine, and soft hairs known as lanugo hairs are the first to develop. These hairs disappear by the 32nd to 36th week of pregnancy but can continue to present in up to one-third of newborns. Usually thought, the lanugo hairs change over to vellus hairs causing 90% of the scalp hairs to become larger and thicker terminal hairs. Upon birth, new follicles will no longer develop on the scalp.
FORMATION OF THE EPITHELIAL PLACODE AND PRIMARY HAIR GERM
The hair follicle first develops in the human fetus at about 10 weeks into pregnancy. The hair follicle develops from a small collection of cells called the epithelial placode. These cells then begin to proliferate to form the primary hair germ that then will develop into the epithelial portion of the hair follicle. The formation of hair follicles is established by multiple interactions between the epithelial cells and the mesenchyme (primitive dermis).
THE HAIR FOLLICLE PEG
In the next stage, the primary hair germ extends into a follicle peg. The mesenchymal cells that line the peg develop into a fibrous sheath. with the tip of the peg developing into the dermal papilla. Melanocytes responsible for the hair shaft pigment begin to migrate from the neural crest down to the region above the somite then through the dermis to the epidermis.
THE HAIR BULBOUS PEG
The bulbous peg arises from the elongation of the follicle peg. The epithelial cells within the bulbous base portion of the bulbous peg become the matrix cells, which will, in turn, give rise to the hair shaft and inner root sheath. The outer root sheath forms two bulges on the posterior portion of the hair follicle: the superficial bulge develops into the sebaceous gland, whereas the deeper bulge becomes the mature bulge area into which the arrector pili muscle inserts.
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