Scientists from L'Oreal have identified two stem cell reservoirs in the human hair follicle.
Recently, scientists from L'Oreal Research have published in the dedicated American scientific review "differentiation" (ref. Differentiation (2000) 66:157-164) the existence of stem cells located in two different zones on the human hair follicle: one in the upper part of the follicle and the other in the deep part of the follicle.
The stem cells are undifferentiated cells, which may engender different cell-lines. Those stem cells may generate skin cells as hair cells.
Besides, researchers from L'Oreal Research have been able to produce human epidermis from hair cells (keratinocytes).
Many explanations still remain to be found, but it is clear now that those stem cells are playing a key role in the hair growth.