Effects of oxygen on vascular patterning in Tie1/LacZ metanephric kidneys in vitro

Tie1 is an endothelial lineage-specific receptor. Using Tie1/LacZ mice we previously demonstrated in situ differentiation of glomerular capillaries after transplantation of renal precursors into the neonatal nephrogenic kidney cortex. We now report studies with Tie1/LacZ metanephric kidneys explanted in vitro at a stage when Tie1/LacZ-expressing cells surround nephron precursors but glomeruli are unformed. After 4 days of serum-free organ culture in 21% O2, transgene-expressing vessels regressed. In contrast, in 3% O2, transgene was expressed between epithelial tubules by cellular masses containing poorly defined lumens. The normal branching of Tie1/LacZ-expressing vessels which occurred in vivo was absent in vitro and glomeruli forming in culture lacked capillaries. Similar observations were made in wild-type metanephroi using vascular endothelial growth factor receptor 2 (Flk1) as an endothelial marker. We speculate that the metanephros is hypoxic in vivo to permit endothelial growth but other cues must be required for construction of the microcirculation since hypoxia failed to elicit normal patterning in vitro.