| Abstract: |
The cyclic renewal of hair follicles offers an exclusive opportunity to study stem cells behaviors, that is their contributions and mode of maintenance. The hair follicle renewal comprises a growth phase of morphogenesis and elongation that is interrupted by a phase of massive apoptosis followed by a resting phase. The hair follicle contains two types of stem cells: stem cells that function during the growth phase and ensure the continuous elongation of the hair follicle and stem cells that maintain the hair follicle over cycles. Lineage-tracing analyses demonstrated a so far unexpected heterogeneity of contribution to the diverse hair follicle lineages among the long-term stem cells. This heterogeneity however does not reflect the heterogeneity of the stem cell markers identified to date and the contribution of a cell likely depends on the position adopted by its descendants. Moreover, recent studies demonstrated that the pool of long-term stem cells was highly dynamic, undergoing rounds of depletion and replenishment by symmetrical division and contribution of cells from the previously called non-permanent differentiated part of the hair follicle. In contrast, during the elongation of the hair follicle, asymmetrically dividing cells with stereotypical behaviors sustain hair follicle growth. Their functioning is interrupted by the intervention of apoptosis. Therefore, the hair follicle utilizes two strategies to maintain itself. This illustrates the notion that stemness is not so much the properties of a cell but the properties of a system that can adopt various strategies to achieve its function, the long-term maintenance of an organ. © Springer Science+Business Media B.V. 2012. |
