The answer seems simple, stem cells.
How are stem cells involved in hair growth?
It might seem like scientists these days are trying to solve all problems with stem cells. However, in the case of hair, the problem appears to have ‘stemmed’ from them, anyways.
Below the root of each hair strand lies a follicle that nourishes the hair cells while they grow. The follicle is also home to a lot of stem cells, which it can quickly direct to carry out certain tasks.
In the human body, stem cells spring into action as a response to an injury; however, research studies have shown that stem cells work inside hair follicles, even in the absence of an injury.
Qixuan Wang, a mathematical biologist at UCR, and his team set out to discover how this works and arrived at a simple answer, TGF-beta.