Hair and hair follicles are far more complex than they appear on the surface. In this lesson, we tried to impart biological insights into the secret lives of hair follicles. These structures exhibit remarkable regenerative capacity by constantly cycling through phases of growth, regression and rest. So perhaps this leaves you wondering: what is the source of new cells that rebuild the follicle after the resting stage? And what signals are involved in mediating this process? The answer is that multiple adult stem cells engage in a collective process of rebuilding the hair structure and the hair fiber. They accomplish this by giving rise to differentiated cells following the resting phase. These cells then go on to build the distinct structures found within the hair follicle in the growth phase.More specifically, two prominent stem cell populations that regulate this cycle are epithelial stem cells and mesenchymal stem cells. To watch more about stem cell interactions within hair follicle, click here . Epithelial stem cells live in an area known as the ‘bulge’ whereas mesenchymal stem cells reside in the dermal sheath. To initiate the growth stage, signals from mesenchymal cells instruct the epithelial cells to divide and form new hair. Although scientists are starting to appreciate the function of each stem cell and the interactions that happen during this process, the identity of these signals is not completely understood. Scientists are now trying to elucidate the identity and the temporal pattern of these signals with the hope that sending these signals alone can reinvigorate stem cells to build new hair. To read about new bioengineering approaches involving the use of adult stem cells to rebuild functional hair, click here .Finally, since hair follicle cycling is also an example of a stem cell dependent process, these self-renewing structures offer an accessible model to study the behavior and the interactions between adult stem cells. To read about a recent study using hair follicle cycling to explaining how stem cells are activated, become quiescent and ultimately die from programmed cell death, click here