Let it grow
Dear HIW, I am an avid HIW reader. I love the variety of topics you cover! I have a question that has been bugging me for a while. Some hairs on the human body can grow infinitely, like those on the head, while others don’t grow beyond a certain length, like eyebrows. How does the body distinguish between those? Where does this ‘hair-intelligence’ reside in our body? Prachi Miittal
Thank you for your letter, Prachi. Hair is a pretty impressive addition to the body of modern humans. While our ancestors would have had thicker coverings of it, we have selective patches with an incredible range of lengths. Each hair follicle has its own cycle, depending on where it is on the body. They produce new cells for a period of time – causing hair to grow – before resting, shedding and restarting. It is the duration of this growth phase which determines each hair’s length. The length of your hair in different areas is determined by your genetics, and we as a species have evolved to sustain hair properties suitable for their purposes. Scientists believe chemical signals from stem cells provide follicles with area-specific instructions.