The cellular landscape of the endochondral bone highlights its multipotent and immunosuppressive features during the transition to extrauterine life.

The cellular complexity of the endochondral bone underlies its essential and pleiotropic roles during organismal life. While the adult bone has received significant attention, we still lack a deep understanding of the perinatal bone cellulome. Here, we have profiled the full composition of the murine endochondral bone at the single-cell level during the transition from fetal to newborn life and in comparison to the adult organ, with particular emphasis on the mesenchymal compartment. In contrast to that of adults, the perinatal bone contains several uncommitted fibroblastic clusters with blastema-like characteristics in organizing and supporting skeletogenesis, angiogenesis, peripheral nervous system development and hematopoiesis. Our data also reveals dynamic inter- and intra-compartment interactions and a highly anti-inflammatory bone marrow milieu, promoted by specific fibroblastic progenitor populations and abundant putative Myeloid-Derived Suppressive Cells (MDSC), which may ensure the proper program of lymphopoiesis and the establishment of central and peripheral tolerance in early life. Overall design: Mouse endochondral bone cells at E18.5 and postnatal day 1 were dissociated, labelled with antibodies and isolated by Fluorescence-activated cell sorting (FACS). Gates were defined by the use of a mix of pan-antibodies to mesenchymal (CD140a/PDGFR-a) and endothelial cells (CD31/PECAM), together with CD9, a marker highly expressed in early hematopoietic progenitors and stromal cells. Sorted cells were mixed in different proportions to represent both abundant and minor populations and processed for scRNAseq.