The little one asked me a question this morning that I had no idea how to answer. Simply put, it was: why is the hair on our head long but the hair on our eyebrows is short?
I had literally no clue. As adults, we have hair over pretty much our entire bodies and it all seems to just ‘know’ how long it’s supposed to be. A bit of digging revealed the answer. As you probably know, the hair that we can see growing out of our skin is dead, it’s just a big string of keratin. The living part is the follicle down in the hypodermis layer of the skin. This is comprised of a blob of stem cells.
Nearly all of the cells in your body are differentiated, meaning that they are a cell of a certain type, i.e. heart cell or liver cell or a neuron. Stem cells have retained the genetic ability to become one of many different types of cell; they are pluripotent. It is the genetics of these bunches of stem cells that control the type of hair that grows (colour, curly or straight) and the length.
It turns out that there are three stages to the growth of a hair: anagen (growth phase), catagen (an intermediate phase where the hair is turned into what’s known as a club hair by laying down a keratinised layer around the root bulb) and telogen (a resting stage that precedes the hair naturally falling out).
The length of each of these phases varies in different parts of the body and gets us towards an answer for my daughter. The catagen phase generally lasts a few weeks whilst the telogen phase would be several months. Crucially, the growth phase varies most of all between the different regions with eyebrows growing for about a month whilst head hairs can be anything from 3-7 years. Those people you see who have a wonderful long mane of hair going down the whole length of their back have the longest growth cycles, not everyone could do that no matter how long they went between haircuts.
The genetics of each individual hair follicle is also why you can’t just transplant hair from one part of your body to another and expect it to grow like hair normally would in the destination. If you transplant hair from your thigh to your head it will still grow like thigh hair because, genetically, it’s still a thigh hair. There are plenty of bad hair transplants out there to testify to that.
That’s about it, then. The different populations of hair on our bodies grow to a different length because of the length of the growth phase in those particular hairs. You’ll know what to say now the next time a 6 year old asks you.