Orchestration of Endothelial and Osteogenic Marker Expression During Osteogenesis.

Vascular endothelial cells (ECs) coordinate with osteogenic processes to establish the specialized vasculature of bone tissue, where endothelial cells and bone cells interact, and bone cells regulate EC proliferation and differentiation. However, it remains unclear how ECs and bone cells are coordinated during early bone formation and whether these interactions differ between endochondral ossification (e.g., femur) and intramembranous ossification (e.g., skull). To address this question, we analyzed endothelial and osteogenic marker expression in the femur and skull between postnatal days 3 and 39. We identified distinct expression patterns of endothelial markers (Endomucin, VE-cadherin and CD31) and osteogenic markers (Osterix, Cbfa1 and BGLP) during osteogenesis in these tissues. In the femurs, endothelial marker expression alternated with the expression of osteogenic markers, suggesting potential reciprocal regulation. In contrast, in the skull, endothelial and osteogenic markers exhibited similar temporal expression patterns without alternation. We also analyzed the expression of VEGF and its receptor FLK1. In the femur, VEGF expression paralleled osteogenic marker expression, whereas in the skull VEGF expression differed from both osteogenic and endothelial marker patterns. Together, these results demonstrate that the coordination of endothelial and osteogenic marker expression, as well as VEGF signaling, differs between endochondral and intramembranous ossification, suggesting distinct modes of interaction between endothelial and bone cells during the formation of long and flat bones.