Vascular Patterning Shapes Intramembranous Ossification via the HIF1α-VEGF Axis.

Organs and tissues develop in close association with the vasculature, which transports blood and nutrients and helps to remove waste. The vasculature is composed primarily of endothelial cells, which provide structure, form barriers, and are a source of developmental signals. We recently found that Mediator, a multiprotein complex that regulates transcription, was essential for proper vascular development. Here, we investigated the specific role of the Mediator tail subunit Med23 in endothelial cells. Endothelial cell-specific knockout of Med23 in mouse embryos using Tek-Cre resulted in vascular anomalies, including edema, hemorrhage, and mispatterned vasculature, alongside craniofacial defects such as micrognathia and cleft palate. Spatial transcriptomics revealed the downregulated expression of key vascular and osteogenic genes in Med23 mutants, including Vegfr1 and Col1a1, with altered signaling dynamics between endothelial and osteoblast populations. Elevated hypoxia-inducible factor 1-alpha (HIF1α) expression and reduced vascular endothelial growth factor (VEGF) signaling were observed in Med23 mutants, suggesting a hypoxia-driven suppression of neural crest cell-derived osteoblast maturation. Consistent with this model, the pharmacologic inhibition of HIF1α, combined with VEGFA supplementation, rescued craniofacial ossification and extended embryonic viability. These findings reveal a critical role for Med23 in coordinating vascular patterning and intramembranous ossification and highlight distinct hypoxic and angiogenic requirements in craniofacial dermal bone versus axial and appendicular endochondral bone development. Thus, the cranial vasculature, and more specifically endothelial cells, plays an instructive role in neural crest cell and osteogenic differentiation during cranioskeletal development.