The maintenance of the interfollicular epidermis (IFE) remains a controversial subject. Conflicting studies support the existence a stem cell hierarchy in the IFE or a committed progenitor model of random fate choice between self renewal and differentiation without hierarchy. To allow fate tracking of rare stem cell populations, we performed a high density multicolour labelling of all basal keratinocytes at P21 and P42 using rainbow technology and followed their fate over time.
Epidermal clones within the IFE of dorsal skin grew in size from 3 weeks to 6 months after labelling (p<0.001) in accordance with the committed progenitor in the group injected at P21. However, when considering the anatomical distribution of clones, those that were connected to hair follicles were significantly larger (p<0.05) and grew more steadily over time (p<0.0001).
Further investigations showed that clones in areas of active hair cycling were larger compared to clones not attached to HF (p<0.05) and that proliferation in the HF infundibulum occurred mainly in anagen (p<0.0001) suggesting that this hair cycle phase promoted the growth of IFE clones via a HF contribution. A mathematical model where stem cells in HF generated CPs that further behaved with random fate decision was tested against a CP model with no stem cell in hair follicles. Only computer simulations of the model with stem cells, integrating 6 colour staining and animal growth over time, predicted clone sizes that closely matched our observations.
In conclusions, our results support a hierarchical contribution of upper hair follicle stem cells to dorsal IFE by generating committed progenitors. Our findings and our proposed model reconcile recent and previous studies showing the importance of the upper portion of the HFs for IFE homeostasis in murine dorsal skin.