Mouse embryonic kidney transplantation identifies maturation defects in the medulla
Kidney organoids are connected to the host circulation and mature after transplantation. However, they are still immature compared to the adult kidneys, and their precise maturation stages remain unclear. By transplanting the mouse embryonic kidney as a model system for organoid transplantation, we report here the maturation defects of the graft, especially in the medulla. Single cell profiling of the developing kidneys in vivo identified gene sets associated with the maturation of the collecting duct epithelium and medullary stroma. These data revealed an upregulation of genes associated with channel/transporter functions and immune defense, as well as a downregulation of neuronal genes. Using these marker genes, we found that the maturation of the collecting duct and medullary stroma in the grafts barely corresponds to the perinatal stage, which was confirmed histologically by using representative genes. Thus, the gene sets obtained serve as maturation coordinates for the renal medulla and will be helpful in analyzing its maturation defects after transplantation. They will also provide a useful basis for further maturation of transplanted kidney organoids. Overall design: Kidney mRNA profiles of embryonic day  E15.5, E17.5, postnatal day 0 and Day7 at single cell resolution