Embryology of the hair follicle

A study of hair follicle development has enabled us to differentiate four stages by relating them to corresponding gestation ages. Stage I of the hair bud is characterized by an epiblastic proliferation penetrating the subjacent mesenchyma at the extremity of which mesenchymal cells accumulate. This appears, at the level of the lower lip, before the 11th week of intrauterine life. Stage II of the hair bulb is reached on the 12th-13th week of intrauterine life, as soon as the distal extremity of the hair bud, which has extended, becomes depressed at the mesenchymal papilla level. Stage III is defined by the observation of hair cone and rough sebaceous glands on the 15th week of intrauterine life. Finally, stage IV shows a differentiated sebaceous gland with a hair which comes through the skin surface on the 18th week of intrauterine life.