Bone is a highly vascularized tissue reliant on the close spatial and temporal association between blood vessels and bone cells. Therefore, cells that participate in vasculogenesis and osteogenesis play a pivotal role in bone formation during prenatal and postnatal periods. Nevertheless, spontaneous healing of bone fracture is occasionally impaired due to insufficient blood and cellular supply to the site of injury. In these cases, bone regeneration process is interrupted, which might result in delayed union or even nonunion of the fracture. Nonunion fracture is difficult to treat and have a high financial impact. In the last decade, numerous technological advancements in bone tissue engineering and cell-therapy opened new horizon in the field of bone regeneration. This review starts with presentation of the biological processes involved in bone development, bone remodeling, fracture healing process and the microenvironment at bone healing sites. Then, we discuss the rationale for using adult stem cells and listed the characteristics of the available cells for bone regeneration. The mechanism of action and epigenetic regulations for osteogenic differentiation are also described. Finally, we review the literature for translational and clinical trials that investigated the use of adult stem cells (mesenchymal stem cells, endothelial progenitor cells and CD34(+) blood progenitors) for bone regeneration.

Medication-related osteonecrosis of the jaw (MRONJ) is a serious adverse effect of antiresorptive and antiangiogenic therapies. MRONJ is identified by chronic wounds in the oral mucosa associated with exposed necrotic bone. We hypothesized that zoledronic acid (ZOL) impairs keratinocyte and fibroblast function and reduces soft tissue vascularization; therefore, treating MRONJ with proangiogenic cells may benefit MRONJ patients. The effect of ZOL and dexamethasone (DEX) on gingival fibroblasts and keratinocytes was investigated.In-vitro, ZOL inhibited fibroblast and keratinocyte proliferation, delaying scratch healing.In-vivo, exposed bone was detected at tooth extraction sites, mainly in ZOL(+)/DEX(+) rats; and was associated with significantly decreased soft tissue vascularization, serum-VEGF, and tissue-VEGF. Local injection of early and late endothelial progenitor cells (EPCs) healed 13 of 14 MRONJ lesions compared with 2/7 lesions in the mesenchymal stem cells, and 2/6, in culture-medium group. The EPCs reduced necrotic bone area, increased serum and tissue VEGF levels. EPCs engraftment was minimal, suggesting their paracrine role in MRONJ healing. The EPC-conditioned medium improved scratch healing of keratinocytes and fibroblasts via VEGF pathway and elevated mRNA of VEGFA and collagen1A1. In conclusion, a novel MRONJ treatment with EPCs, increased vascularization and improved epithelial and fibroblast functions as well as cured the lesion.

Vascularization is a prerequisite for bone formation. Endothelial progenitor cells (EPCs) stimulate bone formation by creating a vascular network. Moreover, EPCs secrete various bioactive molecules that may regulate bone formation. The aim of this research was to shed light on the pathways of EPCs in bone formation. In a subcutaneous nude mouse ectopic bone model, the transplantation of human EPCs onto β-TCP scaffold increased angiogenesis (p < 0.001) and mineralization (p < 0.01), compared to human neonatal dermal fibroblasts (HNDF group) and a-cellular scaffold transplantation (β-TCP group). Human EPCs were lining blood vessels lumen; however, the majority of the vessels originated from endogenous mouse endothelial cells at a higher level in the EPC group (p < 01). Ectopic mineralization was mostly found in the EPCs group, and can be attributed to the recruitment of endogenous mesenchymal cells ten days after transplantation (p < 0.0001). Stromal derived factor-1 gene was expressed at high levels in EPCs and controlled the migration of mesenchymal and endothelial cells towards EPC conditioned medium in vitro. Blocking SDF-1 receptors on both cells abolished cell migration. In conclusion, EPCs contribute to osteogenesis mainly by the secretion of SDF-1, that stimulates homing of endothelial and mesenchymal cells. This data may be used to accelerate bone formation in the future.

Background: Alveolar bone deficiency is a major clinical problem in maxillofacial reconstructive surgery. The available surgical techniques to enhance extracortical bone augmentation are generally unpredictable and not satisfying. The aim of the present study is to quantify extracortical bone augmentation and tissue mineral density (TMD) after cotransplantation of peripheral blood‐derived endothelial progenitor cells (EPCs) and bone marrow‐derived mesenchymal stem cells (MSCs) by microcomputed tomography (micro‐CT).Methods: Bone regeneration was tested in the guided bone regeneration rat calvaria model. Gold domes filled with beta tricalcium phosphate (β‐TCP; control [CNT]) or β‐TCP mixed with 5 × 10⁵ rat EPCs and 5 × 10⁵ rat osteogenic transformed MSCs (EPC/otMSCs) were fixed to the exposed calvaria. Rats were sacrificed after 3 months. Bone volume fraction (BV/TV) and TMD were analyzed using micro‐CT. In the middle of the dome, a cylindrical region of interest was defined (it represents the area in which implants are placed) and subdivided into bottom, middle, and top to analyze the effect of the distance from the calvaria on bone formation.Results: In the whole cylinder, BV/TV and TMD were higher in the EPC/otMSC group compared with CNT (BV/TV: 22.9% ± 4.4% versus 29.1 ± 2.2%, P = 0.02; TMD: 937.79 ± 18.68 versus 960.78 ± 5.8 mgHA/ccm, P = 0.03; CNT versus EPC/otMSC, respectively). In each of the three subregions, BV/TV was higher in the EPC/otMSC group compared with CNT (top: 20.25% ± 2.4% versus 23.74% ± 1.5%, P = 0.007; middle: 23.2% ± 4.8% versus 28% ± 2.2%, P = 0.05; bottom: 25.3% ± 7.6% versus 35.7% ± 4.9%, P = 0.02; CNT versus EPC/otMSC, respectively).Conclusion: Three‐dimensional quantification by micro‐CT demonstrated that cotransplantation of EPC/otMSCs significantly improved bone formation and mineral density.