Effects of pre-osteogenic differentiation on the bone regeneration potentiality of marrow mesenchymal stem cells/poly(ethylene glycol)-diacrylate hydrogel using a rat cranial defect model

Transplanting cell/hydrogel constructs into a bone defect site is an effective strategy to repair the damaged tissues. However, before transplantation, there are various methods to culture cell/hydrogel constructs. Especially, the preferred pre-osteogenic differentiation period to achieve satisfied bone regeneration should be determined. To this end, Bone marrow mesenchymal stem cells (BMSCs) were firstly photo-encapsulated into poly(ethylene glycol)-diacrylate (PEGDA) hydrogel. Then the constructs were implanted in rat calvarial defects after being osteogenically induced for 0, 7, 14, and 21 days. In vitro experiments demonstrated that the proliferation of BMSCs in the hydrogels deceased significantly from 0 day to 7 days. The activity and the gene expression of alkaline phosphatase, besides the gene expression of bone morphogenetic protein-2 peaked at day 14, whereas the gene expression of osteocalcin and the formation of calcium nodules increased with the prolongation of differentiation time. In vivo results showed that limited areas of newly formed bone were found in the day0 and day21 groups. In the day7 group, obvious new bone with bone marrow space was found, while the day14 group nearly achieved complete bone healing. Our data suggested that the period of in vitro pre-osteogenic differentiation played a crucial role for the osteogenesis of BMSCs/PEGDA hydrogels. Furthermore, we found that a pre-differentiation for 14 days is preferable for bone regeneration in the rat cranial defects.