An ex vivo appositional growth model to study periosteal activity and associated BMP signaling.

The periosteum is an osteogenic tissue containing periosteal stem and progenitor cells (PSPCs) that form and repair bones. Much is known about how the periosteum regulates fracture repair, but its key role in bone accrual during postnatal growth and adult load-induced formation remains a significant knowledge gap. The periosteal activity that drives bone formation and repair can be studied using existing mouse models, but traditional in vivo techniques are not feasible for investigating appositional growth, or widening of the bones, in young mouse pups. We therefore developed an ex vivo appositional growth model that combines existing in vitro fluid flow approaches and a custom support to secure mouse femur explants in a culture system. We observed that femurs widen after daily exposure to fluid flow, and this coincides with expansion of the periosteum and activation of BMP signaling, which is essential for periosteal osteogenesis. We further validated our system by recapitulating an in vivo thin-bone phenotype in mice that exhibit a lack of periosteal activity due to disrupted BMP signaling. Using our device, we were able to partially recover the thin-bone phenotype by supplementing culture media with recombinant BMP2. This ex vivo appositional growth model is an important step in obtaining a more holistic understanding of how the periosteum and its resident PSPCs drive bone formation and repair. Furthermore, we anticipate this apparatus can be used to interrogate the periosteum's osteogenic capabilities and motivate future bone anabolism treatments for skeletal diseases and the prevention of fractures or enhancement of repair.