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Hair can be curly, straight, thick, thin, brown, black, blonde, or auburn. It can be long or short, frizzy or lush. While the huge diversity in our genes and environments makes it a no-brainer that hair varies so much, it makes less sense that all these hair types can be present on one single body. What determines all this variety in our hair?
We have two types of hair, says dermatologist Elizabeth Houshmand. Vellus hairs, or “peach fuzz,” cover virtually our entire body but aren’t easy to see. Our head, chest, armpit, and pubic hair consists of terminal hairs. These are thicker and darker.
But not all terminal hairs are alike. For example, the hair on our head can grow far longer than that on the rest of our body. To understand why, we have to dive deep into our skin.
Cell growth phases and hair
Each hair on our body sprouts from a follicle nestled in our skin’s dermis layer. Each follicle grows in a regular cycle, explains Houshmand. This consists of a growth phase called anagen, a regression phase (catagen), a rest phase (telogen), and shedding (exogen). During anagen, the hair follicle will produce a new shaft–the part of the follicle that pokes above the skin. “The anagen phase, where hair actively grows, is significantly longer for head hair,” says Houshmand. These cells can remain in anagen for several years.
Ross Radusky, a dermatologist, principal investigator, and medical director at the Dermatology Treatment & Research Center in Dallas, Texas, compares anagen to human adolescence. “If we don’t maximize our growth by the time we get to be an adult, the height that we have is the height that we have for the rest of our life,” he says. Body hair cells have anagen phases lasting just a few weeks or months, giving them less time to grow.
Why does our body hair look different?
But our terminal body hair isn’t just short head hair. It differs in texture and often in color as well. Radusky explains that variation in texture is due to our follicles reacting differently to hormonal influences, as well as environmental contributions from our clothing and lifestyle. Color variation is due to a hormone called melanin. This is released from melanocytes, the same cell type that creates skin pigment. This melanin also colors our hair. There are two types of hair melanin: pheomelanin, which produces blond and red hair, and eumelanin, which makes darker hair.
The balance between these types of melanin is regulated by more than 100 genes. How hair color varies in different body regions will depend on how these genes are expressed. The activity of melanocytes also tends to increase with age. This is why babies and toddlers with a shock of blond hair can end up a mousy brown color in adulthood.
Body hair and baldness
Age doesn’t just change hair color. As many men (and some women) will be acutely aware, hair thickness can wane with age.
But bald men can still retain thick body hair. Radusky, who has worked on clinical trials for hair loss conditions, explains this is due to the conversion of testosterone as we get older. An enzyme called 5-alpha reductase changes the hormone into dihydrotestosterone. This hormone shrinks hair follicles and causes them to produce thinner, finer hairs. The distinctive hairline created by male pattern baldness is due to the position of dihydrotestosterone receptors.
“They’re typically found in temples along the front and all the way back to what’s called the vertex or the crown of the scalp,” says Radusky. In women, the receptors are found in a more centralized arrangement that explains why female-pattern baldness thins hair across the head. The lack of dihydrotestosterone receptors across the rest of our bodies explains why these areas can remain hairy even after our hairlines have lost their fight against these hormonal changes.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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