In humans, hair follicles first begin to develop in the eyebrows, chin and lip areas when a fetus is between 9 and 12 weeks gestation.1 In the “pre-germ” stage of development, epithelial cells crowd in the basal layer of the epidermis. Mesenchymal cells (undifferentiated stem cells that differentiate into a variety of cell types) cluster beneath the epithelial cells, which then elongate to form the hair germ. As the hair germ enlarges, it grows downward and becomes asymmetrical. The solid column of cells is known as the hair peg.
The hair peg contains a broad, concave tip with mesenchymal cells that eventually form the dermal papilla and dermal sheath. The lower end of the hair peg eventually becomes bulbous, and the tip’s cavity deepens further to enclose the dermal papilla. The bulbous hair germ will go on to form the hair bulb matrix, while the mesenchymal cells surrounding the bulb will form the
dermal sheath. The entire development of the hair follicle is completed by about 22 weeks of gestation.
Two enlarged areas then appear at the bottom of the follicle: the top enlargement is the precursor of the sebaceous gland and the lower bulge is the future site of the arrector muscle attachment. In some follicles (in the areas including the groin, areolae and face), a third bulge appears above the sebaceous gland enlargement to form gland.
Above the hair bulb matrix, a cone of cells differentiates from the matrix and goes on to form the cortex and cuticle of the hair.
1 Dawber, Rodney, Diseases of the Hair and Scalp, p. 3.
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