Our results suggest that optimizing injectable scaffolds dually delivering FTY720 and BMP-2 could potentially result in a cell free therapy for CSDs in cranial bone. The results of FTY20 delivery in chitosan microgels are consistent with previously reported in vivo findings. Delivery of FTY720 promotes the development of a vascular network in the vicinity of a cranial defect that may enable the recruitment and differentiation of bone progenitor cells due to increased blood flow and growth factor/nutrient delivery. Locally delivered FTY720 has been shown to result in an increase in vascular length and density. Microfil enhanced microCT in our CSD study similarily showed that local FTY720 treatment resulted in enhanced vascularization of the defect region, a phenomenon that persisted 9 weeks post-injury. Though approaches involving local delivery of VEGF to increase blood supply around a bone fracture site have been shown to increase healing, signaling through S1P receptors not only stimulates endothelial morphogenic processes, such as lumen formation and branching, but also promotes mural cell recruitment, resulting in more stable vasculature at the defect site. We report for the first time how potential signaling crosstalk between S1P receptors, chemokines, and growth factors that are active during bone wound healing may substantially enhance bone progenitor recruitment. It is well known that SDF-1 production increases at the site of injury and SDF-1/CXCR4 signaling plays a significant role in the initiation of MSC differentiation and declines once the cells choose a differentiation pathway. Thus, we were particularly interested in whether crosstalk between S1P receptor activation via FTY720 and the endogenously active SDF-1/CXCR4 axis in the healing microenvironment may enhance migration of progenitor cells toward injected chitosan microgels. Our results provide evidence that bone marrow derived cells exhibit enhanced chemotaxis towards SDF-1 after pre-treatment with FTY720. Though the dual treatment of BMP-2 and FTY720 did not yield substantially better results than treatment with FTY720 alone, many factors must be considered. It is SCH772984 possible that a longer study is required to tease out the impact of the dual treatment compared to each factor alone, as FTY720 and BMP-2 may act in bone wound healing on different time scales. The second issue is that of dosage. Higher doses of BMP-2 have been shown to have substantial impact on bone regeneration, though too high a dose has led to negative consequences.