Articular and epiphyseal cartilage : its formation, maintenance and repair

Abstract

Long bones develop via a series of ordered processes, initiated from mesenchymal stem cell condensation, cartilage anlagen formation, followed by central cell hypertrophy and vascular invasion and the finalized by the formation of primary and secondary ossification centers (SOC), where the latter is also called endochondral ossification. As long bone develops, it can be distinguished in to several morphologically distinct parts: the main shaft of a long bone, called diaphysis; a narrow disc of growth plate, providing a continuous supply of chondrocytes for longitudinal growth; a thin layer of articular cartilage at the ends of epiphysis, supporting joint movement, and a SOC, sandwiched between the two pieces of cartilage.

Evolutionary analysis revealed that growth plate first appeared as an individual organ in amniotes due to the formation of SOCs, therefore we hypothesized SOCs might be evolved to meet the mechanical demands faced by bones growing under weight-bearing conditions. Combination of mathematical modelling and physical and biological validations demonstrated that SOC significantly improved the stiffness of the epiphyseal structure; meanwhile it decreased normal shear and stresses within the growth plate, allowing chondrocytes of the growth plate to stand a six-fold higher load before undergoing apoptosis. In addition, hypertrophic cells were more sensitive to loadings than cells from proliferating zone right above them (Paper I). Growth plates provide a continuous supply of cells for childhood longitudinal growth; however, its growth mechanism is still unclear. In the second paper, we aimed to understand the growth model of the growth plate and its maintenance. We demonstrate that a depletion manner of chondro-progenitor occurs during the fetal and neonatal stages; whereas after the formation of SOC, the chondro-progenitors obtain the capacity for self-renewal, generating large and stable monoclonal columns. The hedgehog and mammalian target of rapamycin complex 1 (mTORC1) signaling pathways regulate this stem cell pool (Paper II).

Articular cartilage has a poor capacity to self-repair due to its particular structure. Therefore, the existence of chondro-progenitors in the articular cartilage superficial zone has been attracted more attentions. Here, we further characterized these superficial cells in vivo (Paper III) and explored their capacity to form hyaline cartilage in vitro (Paper IV). We showed that superficial cells proliferate more slowly than the underlying chondrocytes. Moreover, they divide symmetrically to self-renew and differentiate symmetrically and asymmetrically into underlying chondrocytes. Furthermore, the progenies of superficial cells fully substitute fetal chondrocytes during early postnatal life (Paper III). In monolayer and 3D in vitro culture, we found that exogenous Jagged1 (a Notch signaling against) had the most capacity to facilitate cell expansion while sacrificing their chondrogenic potential. Conversely, XAV (a Notch signaling antagonist) preserved the chondrogenic potential. In addition, the dedifferentiation might be via Jagged1/Notch3 signaling pathway (Paper IV).

Collectively, we first show that the evolution of epiphyseal cartilage into a separate organ allows epiphyseal chondrocytes to withstand the high mechanical stress placed on them by the terrestrial environment. Secondly, the stem cell niche forms coinciding with the formation of the secondary ossification center, which provides a continuous supply of chondrocytes for postnatal bone growth. Finally, superficial cells are progenitors of articular cartilage whose progenies fully replace the fetal chondrocytes. Furthermore, the inhibition of Notch signaling preserves the chondrogenic potential of articular cartilage progenitors during monolayer expansion.