SUMMARY During endochondral bone development, the first osteoblasts differentiate in the perichondrium surrounding avascular cartilaginous rudiments; the source of trabecular osteoblasts inside the later bone is, however, unknown. Here, we generated tamoxifen-inducible transgenic mice bred to Rosa26R-LacZ reporter mice to follow the fates of stage-selective subsets of osteoblast lineage cells. Pulse-chase studies showed that osterix-expressing osteoblast precursors, labeled in the perichondrium prior to vascular invasion of the cartilage, give rise to trabecular osteoblasts, osteocytes, and stromal cells inside the developing bone. Throughout the translocation, some precursors were found to intimately associate with invading blood vessels, in pericyte-like fashion. A similar coinvasion occurs during endochondral healing of bone fractures. In contrast, perichondrial mature osteoblasts did not exhibit perivascular localization and remained in the outer cortex of developing bones. These findings reveal the specific involvement of immature osteoblast precursors in the coupled vascular and osteogenic transformation essential to endochondral bone development and repair.

Vascular endothelial growth factor (VEGF)-mediated angiogenesis is an important part of bone formation. To clarify the role of VEGF isoforms in endochondral bone formation, we examined long bone development in mice expressing exclusively the VEGF120 isoform (VEGF120/120 mice). Neonatal VEGF120/120 long bones showed a completely disturbed vascular pattern, concomitant with a 35% decrease in trabecular bone volume, reduced bone growth and a 34% enlargement of the hypertrophic chondrocyte zone of the growth plate. Surprisingly, embryonic hindlimbs at a stage preceding capillary invasion exhibited a delay in bone collar formation and hypertrophic cartilage calcification. Expression levels of marker genes of osteoblast and hypertrophic chondrocyte differentiation were significantly decreased in VEGF120/120 bones. Furthermore, inhibition of all VEGF isoforms in cultures of embryonic cartilaginous metatarsals, through the administration of a soluble receptor chimeric protein (mFlt-1/Fc), retarded the onset and progression of ossification, suggesting that osteoblast and/or hypertrophic chondrocyte development were impaired. The initial invasion by osteoclasts and endothelial cells into VEGF120/120 bones was retarded, associated with decreased expression of matrix metalloproteinase-9. Our findings indicate that expression of VEGF164 and/or VEGF188 is important for normal endochondral bone development, not only to mediate bone vascularization but also to allow normal differentiation of hypertrophic chondrocytes, osteoblasts, endothelial cells and osteoclasts.

Bone is a highly vascularized tissue, but the function of angiogenesis in bone modeling and remodeling is still poorly defined, and the molecular mechanisms that regulate angiogenesis in bone are only partially elucidated. Genetic manipulations in mice have recently highlighted the critical role of the hypoxia-inducible-factor/vascular endothelial growth factor pathway in coupling angiogenesis and osteogenesis. In this brief perspective, we review the current understanding of the mechanisms responsible for this coupling. Elucidation of such mechanisms will expand our knowledge of bone development and homeostasis, and it may aid in the design of new therapies for accelerating bone regeneration and repair.