ORIGINS AND FORMATION OF MICROVASCULATURE IN THE DEVELOPING KIDNEY

Regulation of microvessel assembly in the developing kidney is not kno wn and may occur through vasculogenic, angiogenic, or both processes. To examine this question, we grafted rat and mice embryonic (E) day 12 (E12) kidneys, which have only a rudimentary vasculature, into anteri or eye chambers of mouse and rat hosts. Species-specific, monoclonal a nti-basement membrane antibodies showed that glomerular basement membr anes, mesangial matrices; and microvessel basement membranes were alwa ys derived from the graft. When wild-type E12 mouse kidneys were graft ed into anterior chambers of ROSA26 mice, in which the beta-galactosid ase transgene is expressed ubiquitously, glomerular and microvascular endothelial cells stemmed from the graft, even after maintenance of ki dneys in organ culture for 6 days before grafting. Immunolabeling with antibodies against the vascular endothelial growth factor (VEGF) rece ptor, Flk1, the EphB1 receptor, and its ligand, ephrin-B1, labeled dis crete mesenchymal cells in embryonic and newborn kidney cortex: as wel l as developing microvessel and glomerular endothelium. In adult kidne ys, Flk1 labeled glomeruli weakly, other vascular structures were unla beled. When wild-type E12 kidneys were grafted under renal capsules of adult ROSA26 hosts, endothelium developing within the graft again cam e from the implanted kidney. In contrast, when E12 kidneys were grafte d into renal cortices of newborns, glomeruli within grafts now contain ed host-derived endothelium. Similarly, when ROSA26 E12 kidneys were i mplanted into newborn wild-type hosts, chimeric vessels containing gra ft- and host-derived endothelium were seen in nearby host tissue. Our results indicate that cells capable of forming the entire microvascula r tree of grafted metanephroi are already present in the E12 kidney. W e hypothesize that Flk1/VEGF and EphB1/ephrin-B1 mediate renal endothe lial mitosis-motility and cell guidance-aggregation behavior, respecti vely.