Method: A classical lineage tracing experiment was performed using an αSMACreERT2 transgenic mice crossed with fluorescent cre-reporter Ai9, which generates a strong red fluorescent signal upon tamoxifen exposure. Animals received two doses of tamoxifen (75 mg/kg separated by 24 hours) and were subsequently euthanized at 27 hours, 33 hours, 2 days, 17days, 30days, 45days, 70 days and 90 days. The samples were fixed in formalin and examined as non-decalcified frozen sections
Result: Ai9 labeling of the cells within the superficial layer was completed by 2 days and by day 17 the majority of the cells expressed Ai9. Between day 30 and 90 the label was gradually lost with the exception of distinct vertical tracts of cells extending from the superficial zone into the hypertrophic zone. These residual islands are reminiscent of the superficial islands of Dkk3-GFP cells that can be appreciated by 2 photon optics
Conclusion: The transgenic reporter technology suggests the MCC maintains its cellular composition from progenitors within focal islands in the superficial zone that proliferate and expand toward the hypertrophic zone.