Lineage tracing during renal repair. There are two primary obstacles to studying stem cells. First, no matter how many markers are used, it is not possible to obtain a pure population of cells. Second, any in vitro assay used to quantitate and characterize stem cell number will necessarily be imperfect because it cannot faithfully recapitulate the in vivo stem cell environment, or niche (Jones, Simons and Watt, 2007). These challenges are particularly applicable to kidney, because there are few if any validated stem cell markers to begin with, and the cellular complexity of the kidney makes any attempt to recapitulate the cellular environment in vitro more difficult. Therefore our laboratory utilizes in vivo lineage tracing to reveal the fate of putative kidney stem cells as well as epithelial progenitors, during homeostasis and repair. Lineage analysis is a powerful tool that is made possible by Cre/Lox recombination technologies and it allows visualization of cellular function in a normal context without disrupting the cellular environment as would be required for in vitro culture or transplantation assays
