Evolution of the growth plate into a spatially separated structure allows bone growth on land

Abstract Growth plates are narrow discs of cartilage, ultimately required for longitudinal growth of all mammals including humans. However, originally the growth plate and articular cartilage were a single anatomical entity, an epiphyseal cartilage, as appeared in early tetrapods and in mammalian development. The reason, why the growth plates evolved as spatially separate organs, remains unknown. Here, we demonstrate that the epiphyseal growth plate first appeared as an individual organ in amniotes due to the formation of a novel bony structure, the secondary ossification center (SOC), which spatially separates articular cartilage and the growth plate. Since amniotes translocate their entire growth period on land, we next explored the role of mechanical demands faced by bones growing under weight-bearing conditions. Comparison of mammals whose limbs are subjected to greater or lesser mechanical demands (i.e., Chiropterans (bats), Cetaceans (whales) and Dipodidae (jerboa)) revealed that the presence of an SOC is correlated to the extent of these demands. Mathematical modelling in combination with physical and biological experiments showed that the SOC reduces shear and normal stresses within the growth plate, allowing epiphyseal chondrocytes to withstand a six-fold higher load before undergoing caspase-dependent apoptosis via the YAP-p73 pathway. Furthermore, the hypertrophic chondrocytes, the cells primarily responsible for bone elongation were least mechanically stiff and most sensitive to weight bearing. Our results demonstrate that evolution of the epiphyseal cartilage into a separate organ allows epiphyseal chondrocytes to withstand the high mechanical stress placed on them by the terrestrial environment.