CXCR4 and osteoporosis: By taking advantage of the homing and engraftment capacity induced by the SDF-1/CXCR4 axis [26–28], we coated MSC-Sec NPs with human microvascular endothelial cell (HMEC) membranes, which have high levels of CXCR4 (MSC-Sec/CXCR4 NPs) for the treatment of osteoporosis.