The patterning of hair follicles is called trichoglyphics. The hair shaft directional slope is secondary to the plane of the stretch exerted on the skin when the hair follicles are forming. During the first two months of gestation, the embryo is completely hairless. Then the primary hair, called lanugo, starts to grow. The origin of the hair directional patterning is the original sloping angulation of the hair follicle. The hair follicles start to push downward into the underlying mesenchyme at 10-12 weeks of gestation (Fig. 12.9). On the scalp, the angle of the hair shaft reflects the prior plane of growth of the skin (e.g., the plane of growth present at the time when the follicle was forming). The plane of growth of the scalp skin is usually determined by the growth of the underlying brain.
The posterior parietal hair whorl is considered to be the focal point from which the growth stretch is exerted by the dome-like outgrowth of the brain during the time of hair follicle development. Central nervous system malformations that antedate hair follicle development, such as
i Posterior view
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