Vascular Patterning affects Intramembranous Ossification through HIF1α-Vegf Signaling.

Organs and tissues develop in close association with 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 the Mediator, a multiprotein complex, which regulates transcription, was essential for proper vascular development. Here, we investigated the specific role of the Mediator tail subunit Med23 in endothelial cells. Endothelial specific knockout of Med23 in mouse embryos using Tek-Cre results in vascular anomalies, including edema, hemorrhage, and mispatterned vasculature, alongside craniofacial defects such as micrognathia and cleft palate. Neural crest cell formation and migration were normal, however, osteogenic differentiation of neural crest cells was severely impaired in the craniofacial region in Med23 mutants. Spatial transcriptomics revealed downregulated expression of key vascular and osteogenic genes, including Vegfr1 and Col1a1, with altered signaling dynamics between endothelial and osteoblast populations. Elevated HIF1α expression and reduced VEGF signaling were observed in Med23 mutants, suggesting a hypoxia-driven suppression of osteoblast maturation. Consistent with this model, pharmacological 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, play an instructive role in neural crest cell differentiation during craniofacial development.