Growth plate skeletal stem cells and their transition from cartilage to bone.

The growth plate is an essential component of endochondral bone development. Not surprisingly, the growth plate and its surrounding structure, the perichondrium, contain a wealth of skeletal stem cells (SSCs) and progenitor cells that robustly contribute to bone development. Recent in vivo lineage-tracing studies using mouse genetic models provide substantial insight into the diversity and versatility of these skeletal stem and progenitor cell populations, particularly shedding light on the importance of the transition from cartilage to bone. Chondrocytes and perichondrial cells are inseparable twins that develop from condensing undifferentiated mesenchymal cells during the fetal stage; although morphologically and functionally distinct, these cells ultimately serve for the same goal, that is, to make bone bigger and stronger. Even in the postnatal stage, a small subset of growth plate chondrocytes can transform into osteoblasts and marrow stromal cells; this is in part fueled by a unique type of SSCs maintained in the resting zone of the growth plate, which continue to self-renew for the long term. Here, we discuss diverse skeletal stem and progenitor cell populations in the growth plate and the perichondrium and their transition from cartilage to bone.