Figure 2

Cellular organization of epiphyseal and synchondroseal growth zones. (A) Human growth plate chondrocytes transition through resting-, proliferative- and hypertrophic zones (RZ, PZ, and HZ, respectively) before dying or transitioning to an osteoblast fate at the chondro-osseous junction. Cartilage cells stop dividing and enlarge in the hypertrophic zone. The bone collar forms a sheath around hypertrophic chondrocytes; the secondary ossification flanks the growth plate distally. Primary bone trabeculae derived from extracellular matrix channels populate the bone cavity. (B) In unidirectional (epiphyseal) growth zones, the resting zone is distal to the proliferative zone, itself distal to the hypertrophic zone; this layout produces unidirectional growth. In bidirectional (synchondroseal) growth zones, the resting zone is flanked by two proliferative zones and two hypertrophic zones in a mirror image organization; this layout produces bidirectional growth. (C) Stereotypical zebrafish unidirectional growth zone organization: chondrocytes transition through RZ, PZ and HZ, but they do not enlarge in the HZ. At the zebrafish resorption front, chondrocytes die or transition to either an osteocyte or adipocyte fate. A perichondral bone collar sheathes the zebrafish hypertrophic zone, but no secondary ossification is associated with zebrafish epiphyseal growth zones. Trabeculae are not observed in smaller teleosts such as zebrafish. (D) Histological section of zebrafish proximal radial showing unidirectional endochondral growth zone [originally published in (23)]. (E) Time series of maturation at two zebrafish bidirectional growth zones located within the ventral (left) and dorsal (right) ceratohyal synchondroses [originally published in (24)]. (scale bars = 50 µm).