Physical and chemical niche of human growth plate for polarized bone development

Abstract

Growth plate (GP), a critical cartilaginous structure in amniotes, underpins longitudinal bone growth, yet the intricate mechanisms behind its polarized mineralization during evolution remain unclear. Herein, employing high-resolution analytical techniques, we reveal that the GP-epiphysis interface displays a sharp transition in tissue modulus, acting as a protective shell for the underlying GP, whereas the GP-metaphysis interface exhibits a gradual modulus increase, enabling efficient load redistribution to the metaphysis. This mechanical microenvironment contributes to unique microstructural and compositional transformations from GP to epiphysis and metaphysis. Notably, the GP-epiphysis interface acts as a mineralization inhibition zone while the GP-metaphysis serves as a mineralization promotion zone, orchestrated by a complex network of proteins. Proteins such as secreted phosphoprotein 1 (SPP1) and alpha-2-HS-glycoprotein (AHSG) at the GP-epiphysis interface inhibit mineralization, forming a defense line; while ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and alkaline phosphatase, biomineralization associated (ALPL), coexisting with SPP1 and AHSG, promote a sequential nucleation and assembly of calcium phosphate minerals at the GP-metaphysis. Such polarized mineralization patterns maintain the homeostasis of GPs and drive polarized bone elongation. Replicating this process in vitro, we synthesize stable amorphous calcium phosphate which shows highly controlled transformation into hydroxyapatite. This work provides a more comprehensive view of the structural integrity of human bone in development and offers strategies for controlled biomineralization.