Background: Studies have clearly established the existence and activity of epidermal stem cells (SCs) in the hair follicle (HF), the sebaceous gland (SG) and the interfollicular epidermis but the respective hierarchy has not been studied.
Hypothesis: We hypothesized that HF bulge SCs are heterogeneous in self-renewal with few cells required to maintain the bulge over long term. Our aim is therefore to clearly evaluate the number of clonally related SCs in the bulge.
Methods: To address this question we generated transgenic mice containing a Brainbow-1.0 cassette under the control of a ubiquitous promoter allowing the random expression of dTomato (red), mCerulean (blue) or eYFP (yellow) upon cre recombination. When crossed with K14-Cre/ER or K15-Cre/PR transgenic mice, this will allow us to induce recombination respectively in all basal keratinocytes regardless of their anatomical location or in bulge stem cells and to follow their fate over time. Injection of Tamoxifen or Mifepriston in the K14-Cre/ER or K15-Cre/PR mice respectively, occurred at either 3 or 6 weeks post-natal. Tissue was collected at 1, 3, 5, 12, and 24weeks post-injection
Results: Our preliminary data showed that 75.15% of clones within the bulge were small (0-3 cells in size) in 3wk mice, reduced to 62.85% in 12wk old mice, indicating an increase in clone size over time. This trend was stronger in mice injected at 3 weeks, particularly in K15-Cre/PR mice. An average of 3 clones per bulge was observed, suggesting that only few SCs are required to maintain the bulge.
Conclusions: Rainbow technology allows the long term tracking of clones derived from individual stem cells. Our results clearly indicate that few SCs are mandatory to support the bulge, however does not support the existence of a single epidermal SC giving rise to all epidermal lineages.