Defining the cellular lineage hierarchy in the interfollicular epidermis of adult skin

Abstract

The inter-follicular epidermis regenerates from heterogeneous basal skin cell populations that divide at different rates. It has previously been presumed that infrequently dividing basal cells, label retaining cells (LRCs), are stem cells, while non-LRCs are short-lived progenitors. Here we employ the H2B-GFP pulse-chase system in adult mouse skin and find that epidermal LRCs and non-LRCs are molecularly distinct and can be differentiated by Dlx1CreER and Slc1a3CreER genetic marking, respectively. Long-term lineage tracing and mathematical modelling of H2B-GFP dilution data show that LRCs and non-LRCs constitute two distinct stem cell populations with different patterns of proliferation, differentiation, and upward cellular transport. During homeostasis, these populations are enriched in spatially distinct skin territories and can preferentially produce unique differentiated lineages. Upon wounding or selective killing, they can temporarily replenish each other’s territory. These two discrete inter-follicular stem cell populations are functionally interchangeable and intrinsically well adapted to thrive in distinct skin environments.

Skin and Its Regenerative Powers: An Alliance between Stem Cells and Their Niche

“…The notion that cycling rates are not a reliable measure of evaluating stemness within the epidermis has also been suggested by Sada et al (2016), who noted that basal layer progeny of even very fast cycling (on average once per 2 days) progenitors can self-renew and persist in back skin for at least a year (Sada et al, 2016). Another key point is that basal location is not synonymous with progenitor status.…”

“…For mouse back skin, the homeostatic flux of epidermal cells in upward vertical columns has been estimated to be considerably faster, typically a week (Potten et al, 1987; Sada et al, 2016). However, turnover rates have been hard to measure accurately at densely hairy body sites, where the epidermis only consists of a few layers.…”

“…When pulse-chase analyses were used to examine radiolabeled nucleotide or H2BGFP fluorescence dilution within the proliferative basal populations of paw, ear, or scale regions of tail skin epidermis, progenitors divided more frequently, with estimates of divisions every 2–5 days (Doupe et al, 2010; Lim et al, 2013; Sada et al, 2016). Interestingly, however, when Roy et al (2016) crossed Krt14-CreER and Rainbow mice to conduct multicolor lineage tracing and track individual back skin epidermal clones, they discovered that the clones nearest to actively cycling HFs were considerably more proliferative than the ones more distant (Roy et al, 2016).…”

“…In reviewing the collective studies on epidermal progenitors, it seems clear that most of the differences in interpretation have arisen from whether progenitor fates were assigned according to molecular differentiation markers (Clayton et al, 2007; Mascre et al, 2012), basal residence status (Roy et al, 2016; Sada et al, 2016), or the ability of two daughters to divide at least once prior to exiting the basal layer (Rompolas et al, 2016). Additional confusion has arisen from whether researchers have assigned stemness according to cell-cycling rates (Lim et al, 2013) or according to clonal longevity (Sada et al, 2016).…”

“…Additional confusion has arisen from whether researchers have assigned stemness according to cell-cycling rates (Lim et al, 2013) or according to clonal longevity (Sada et al, 2016). When considering that (1) epidermal progenitors can give rise to long-lived clones irrespective of cycling rates, (2) cycling rates and clonal expansion can vary markedly depending upon proximity to HFs, and possibly other appendages such as SwGs and scales, and (3) progenitors can acquire molecular signs of commitment prior to exiting the basal layer, the results from these various studies are remarkably in unison (Figure 4).…”