Quantitative proliferation dynamics and random chromosome segregation of hair follicle stem cells

Abstract

Regulation of stem cell (SC) proliferation is central to tissue homoeostasis, injury repair, and cancer development. Accumulation of replication errors in SCs is limited by either infrequent division and/or by chromosome sorting to retain preferentially the oldest 'immortal' DNA strand. The frequency of SC divisions and the chromosome-sorting phenomenon are difficult to examine accurately with existing methods. To address this question, we developed a strategy to count divisions of hair follicle (HF) SCs over time, and provide the first quantitative proliferation history of a tissue SC during its normal homoeostasis. We uncovered an unexpectedly high cellular turnover in the SC compartment in one round of activation. Our study provides quantitative data in support of the long-standing infrequent SC division model, and shows that HF SCs do not retain the older DNA strands or sort their chromosome. This new ability to count divisions in vivo has relevance for obtaining basic knowledge of tissue kinetics.

Runx1 and p21 synergistically limit the extent of hair follicle stem cell quiescence in vivo

“…2F) (SI Materials and Methods) and indicated approximately three divisions per bulge cell in one hair cycle, as we previously reported (10). Intriguingly, this model did not fit the KO experimental data indicating that the bulge cells do not have a homogeneous response to p21 KO ( Fig.…”

“…Intriguingly, p57 and p27 involved in HSC quiescence (4,5), were up-regulated in the bulge relative to nonbulge (CD34 − /α6 + ) at all hair cycle stages (Fig. 1B), suggesting that they may be responsible for the overall low rates of bulge cell division (10). Moreover, p57 and p15 mRNAs were found at higher levels in the catagen and telogen bulge than in the anagen.…”

“…To understand whether p21 loss affects bulge cell proliferation and skin homeostasis in vivo we used our previously developed H2B-GFP tet-repressible pulse-chase system (10,23). Doxycycline (doxy) shuts down H2B-GFP expression and results in dilution of the label at each division by twofold.…”

“…Here we use the mouse HF as a classical quiescent SC mammalian system (10,11) to examine transcriptional regulation and the role of CDKis, in an attempt to decipher the robustness of hair follicle stem cell (HFSC) cell cycle control in normal and stress conditions. HF adult homeostasis (hair cycle) is composed of distinct and synchronous phases of SC self-renewal, differentiation, and quiescence ( Fig.…”

Development and Homeostasis of the Skin Epidermis

“…Bromodeoxyuridine pulse during the early stage of HF regeneration indicates that the first cells to incorporate bromodeoxyuridine are the cells of the hair germ (HG), followed by bulge SCs, suggesting a two-step mechanism of HF regeneration in which the first cells to be activated are the HG cells, followed by bulge SCs (Greco et al 2009). After an initial burst of proliferation that accompanies the early stage of HF regeneration, bulge SCs continue to divide at a lower rate during all of the growing stages of the hair cycle and then stop dividing during catagen (Sotiropoulou et al 2008;Waghmare et al 2008). Lineage tracing of single bulge SCs shows their ability to migrate from the bulge to the HG and their multipotency at a clonal level (Zhang et al 2009b).…”