When does body hair start to sprout? What determines the color of our hair? It's all a question of hormones.
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00:00 We might no longer have fur like our ancestors, but our origins are hard to hide.
00:06 We have hair growth in odd places.
00:09 Like our toes, or the face, our ears, and the armpits.
00:18 That's because hair is formed in follicles,
00:22 which are found practically everywhere in the connective tissue layer of the skin.
00:28 They number about 5 million, and are formed before we're born.
00:33 But initially, most follicles produce thin, colourless, soft, downy hairs that are often barely visible.
00:42 What makes them produce thick hairs are hormones.
00:46 Primarily testosterone and dihydrotestosterone derived from it, or DHT for short.
00:53 Here's what happens.
00:56 Hair grows in cycles.
00:58 First, new roots develop from stem cells, and then hair begins to grow.
01:03 The growth phase lasts 1-2 months for body hair, and several years for head hair.
01:09 After that, the follicles take a break.
01:14 Then the hair falls out, and the cycle begins again.
01:18 Hormones can influence this process via receptors,
01:22 which, when formed, enlarge the follicles and extend the growth phase.
01:27 The result is thicker and longer hair.
01:31 Melanocytes, which are responsible for hair colour, are also stimulated by hormones.
01:38 The follicles of the scalp and the body react differently to hormonal stimulation.
01:43 That's why newborn babies might have some hair on their heads, but nowhere else.
01:50 When hormone production kicks in during puberty,
01:53 hair starts to grow in other areas of the body as well.
01:57 Paradoxically, the hormone DHT can also be a cause of hair loss.
02:05 Depending on genetic predisposition,
02:08 it causes a gradual shrinking of the particularly sensitive follicles of the scalp.
02:13 The hair becomes thinner, and at some point, production stops completely.
02:19 And another leftover from our furry ancestors
02:22 are the small muscles attached to some hair follicles.
02:25 Contracting them cause their fur to stand on end,
02:28 providing thermal insulation so their bodies stayed warm,
02:31 and, when facing predators, making them appear larger.
02:36 In us, the reflex merely causes goosebumps.
02:40 (upbeat music)