Human hair shape is programmed from the bulb

Summary Background Few biological data on curly hair follicles have been reported in the literature. Objectives To investigate the growth and morphology of curly hair follicles. Methods Follicles were dissected from scalp skin samples from African, Guyanese and caucasian volunteers and were observed macroscopically, in culture in William's E medium, and by immunohistochemistry. Results Macroscopic study of scalp biopsies obtained from African volunteers showed that the dermal implantation of follicles was curved with a retrocurvature at the level of the bulb, as opposed to a straight shape in caucasian follicles. The bulb itself was bent, in the shape of a golf club, while both the outer root sheath (ORS) and the connective tissue sheath were dissymmetrical along the follicle. In vitro growth of curly hair follicles was slightly slower than that of caucasian follicles but, more importantly, the curvature was maintained in the hair shaft produced in vitro. As shown by immunohistochemistry, the proliferative matrix compartment of curly hair follicles was asymmetrical, Ki‐67‐labelled cells being more numerous on the convex side and extending above the Auber line. On the convex part of the follicle, the ORS was thinner and the differentiation programmes of the inner root sheath and hair shaft were delayed. Furthermore, some ORS cells expressed α‐smooth muscle actin protein on the concave side of the curvature, reflecting a mechanical stress. Conclusions Hair curliness is programmed from the bulb and is linked to asymmetry in differentiation programmes.