Medullary cavity expansion is mediated by distinct cell populations during fetal bone development

Bone marrow (BM) is the primary site of postnatal hematopoiesis, yet how it is established during embryogenesis remains poorly understood. BM formation is initiated by the invasion of vascular and mesenchymal cells into a preformed cartilage template, generating the primary ossification center (POC). Here, we identify CD55⁺CD90⁺ mesenchymal cells as early regulators of osteoclastogenesis within the POC. These cells share a matrix-degrading transcriptional program with FABP5⁺ septoclasts that localize to the chondro-osseous interface of developing bone. Fate-mapping analyses reveal that CD55⁺CD90⁺ cells transiently give rise to septoclasts during perinatal development, whereas osteoclast-supporting activity is subsequently assumed by LepR⁺ bone marrow stromal cells (BMSCs). During fracture repair, FABP5⁺ septoclasts and LepR⁺ BMSCs are redeployed within the callus, indicating reactivation of a developmental osteogenic program. Together, our findings uncover a conserved mechanism that links skeletal remodeling to the establishment of the BM microenvironment required for hematopoiesis.