• Fetal thymus;
  • Lgr5;
  • Lineage tracing;
  • Thymic epithelial cells

Leucine-rich repeat-containing G protein-coupled receptor (Lgr)5 is a marker for epithelial stem cells in the adult intestine of mice. Lgr5 transcripts have also been detected in the developing murine thymus, leading to speculation that Lgr5 is a marker for the long-sought stem cell of the thymus. To address the nature of the Lgr5-expressing thymic epithelial cells (TECs), we used Lgr5-GFP reporter mice. We show that epithelial cells expressing Lgr5 protein are present in the fetal thymus during a specific developmental window yet are no longer detectable at birth. To analyze the function of the Lgr5 protein during thymus development, we generated Lgr5−/− mice. These experiments unequivocally show that thymus development is not perturbed in the absence of Lgr5, that all TEC subsets develop in Lgr5−/− mice and that T cells are produced in the expected ratios. Finally, by using an inducible lineage tracing system to track the progeny of Lgr5+ fetal TECs in vivo, we demonstrated that Lgr5+ fetal TECs have no detectable progeny in the later fetal thymus. In sum, we show that presence of the Lgr5 protein is not a prerequisite for proper thymus organogenesis.