Fig. 5. The stem cell hierarchy in the hair follicle (HF) and interfollicular epidermis. As previously suggested (Cotsarelis et al., 1990), the bulge region of the HF outer root sheath (ORS) harbors a population of stem cells (hatched blue). The proliferation of bulge-derived transient amplifying (TA) cells results in repopulation of the interfollicular epidermis and formation of the ORS (blue arrows), which both represent homogenous, terminally differentiating keratinocyte populations and share many common features. Under the influence of the neighboring mesenchyme (the follicular papilla in HF, and the underlying dermis in the interfollicular epidermis – red arrows), some cells of the bulge region progeny may reacquire their stem cell characteristics and form derivative stem cell populations (EPU progenitors in the interfollicular epidermis, and lateral disc cells in the HF – in blue).