The biology and genetics of curly hair

Abstract

Hair fibres show wide diversity across and within all human populations, suggesting that hair fibre form and colour have been subject to much adaptive pressure over thousands of years. All human hair fibres typically have the same basic structure.However, the three-dimensional shape of the entire fibre varies considerably depending on ethnicity and geography, with examples from very straight hair with no rotational turn about the long axis, to the tightly sprung coils of African races. The creation of the highly complex biomaterials in hair follicle and how these confer mechanical functions on the fibre so formed is a topic that remains relatively unexplained thus far.We review the current understanding on how hair fibres are formed into a nonlinear coiled form and which genetic and biological factors are thought to be responsible for hair shape. We report on a new GWAS comparing low and high curl individuals in South Africa, revealing strong links to polymorphic variation in trichohyalin, a copper transporter protein CUTC and the inner root sheath component keratin 74. This builds onto the growing knowledge base describing the control of curly hair formation. K E Y W O R D Scurl, hair follicle, hair trait genetics, human hair | INTRODUCTIONThe basic structure of hair-a cuticle, cortex and medulla (in some)-does not reveal how these structures are built by the hair follicle or shaped into the curly fibre, suggesting there is a level of "fine control" on the process of hair fibre formation by the hair follicle.The distribution of forms of curly hair is shown in Figure 1 and a closer inspection reveals that curly hair fibres are rarely a true coil but exhibit heterogeneity in the direction of the curl in all but the mildest cases. Curly hairs have an elliptical or "D" shape in cross section.This enables bidirectional bending stiffness, bending most easily in the direction of the flattened axis. The relationship of the long and short diameter to the direction of hair growth also changes (unlike the eyelash where this relationship is maintained [1] ). Therefore, we need to understand how the arrangement of cells results in a fibre that is elliptical with the orientation of the ellipse changing with time during hair growth to form a coil.Hair fibres across all races and geographies show degrees of curl that are readily measurable.[2-4] Both Hrdy and de la Mettrie [2,5] studied various hair types sampled from countries and cultures across the world. The degree of curvature of a fibre in its natural state appears to account for most of the variation (87% [5] ), which is as expected. The presence of a medulla is chiefly correlated with hair diameter. The major curl forms are "twist" (with irregular natural constrictions in the fibre producing a discontinuity in curvature), "crimp" (change in direction of curvature), "wave" (number of oscillations/coils per unit length) and "kink" (sharp twist or bend).Hrdy 1973 [2] showed that kinking and crimp were not always correlated with curvature and irregular curvature ca...

Why is hair curly?—Deductions from the structure and the biomechanics of the mature hair shaft

“…The final shape of a mature hair shaft (diameter, ellipticity, curl) is prepared and predetermined in the follicle zones of the immature and premature hair (LH stage). Here, a wide variety of factors is active, which determine the commitment of a cell towards a specific pathway and the subsequent regulation of cell development and segregation …”

“…Though graphical presentations of the hair follicles tend to emphasise the straight form, there are numerous observations that wool and hair follicles show a pronounced tendency to be curved . The assumption that such a curvature is linked to an eventually curved hair appears plausible . It may even be considered as obvious, if also laterally differentiated rates of cell proliferation or cell shape changes are assumed …”

“…We consider the well‐established observation that wool as well as hair may actually show “retro‐curvature” as an even more stringent argument against a primary role of follicle form. In this case, the hair is bent into a direction opposite to the follicle curvature.…”

“…The processes in the follicle are complex, involving a large number of different factors . However, all of the processes which lead to cell differentiation, specific shape formation, and segregation in the cross section occur before the hair reaches the transition zone.…”

“…Against this background, we specifically consider the two major and general models for crimp and curl induction in the HS and the LH range, respectively [HS: cell‐separation model and LH: cell proliferation model]. We will also assess, though with lesser focus, the alternative LH‐based view that follicle shape imposes hair shape (LH: follicle‐shape model) …”