The Human Body: What makes hair curl?

The secret to tousled tresses lies under the skin in the shape of the hair follicles

Hairs might look similar from the outside, but put them under the microscope and you’ll notice tiny differences in shape. Straight hair is perfectly round in cross section, while curly hair is oval or D-shaped. The strands flex more easily in one direction than the other. If the shape varies along the length of the hair it will tend to twist. If it flips back and forth the hair will crimp or kink. And if the turns are regular it will make waves.

These patterns are determined by the hair follicle itself. A look beneath the skin reveals that straight hair follicles are straight and curly hair follicles are S-shaped. In the straight follicles, all the cells work together to make a symmetrical hair, but in curly follicles everything is asymmetrical.

Straight hair: Straight hair is circular in cross section and the follicles are straight.

Wavy hair: Wavy hair is slightly flattened and the follicles aren’t perfectly straight.

Curly hair: Curly hair is oval or D-shaped and the follicles bend into an S.

This article was originally published in How It Works issue 112, written by Laura Mears

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