To date, there have been no effective means to treat large regions of bone defects. Bone tissue engineering brings new hope to solve this problem. Bone tissue engineering is an approach that uses proper materials as the scaffold for cell growing, in vitro co-cultures materials and cells for a period of time to prepare tissue-engineered bone, and finally implants the tissue-engineered bone into the body for repair of defects[21-22] . nHAC/PLA composite prepared based on the concept of bionics mimics the component and structure of natural skeleton and can be used as scaffold material for bone tissue engineering. Seed cells are the source of osteoblasts and BMSCs are the best[23] . Among adult stem cells, BMSCs are a kind of multipotent stem cells that can be isolated from bone marrow and can be in vivo and in vitro induce-differentiated into various mesenchymal tissue cells, involving bone, cartilage, tendon, fat and bone marrow stroma[24] .
Bone marrow stromal cells have been found to be a kind of mesenchymal stem cells that exhibit not only the feature of stem cells, i.e., self-renewal and multi-lineage differentiation potentials, but also plasticity, i.e., under certain condition, specified cells can differentiate and dedifferentiate towards various directions[25] . Tissue-engineered bone requires a considerable number of functional cells inoculated onto scaffold material, generally 106-107 order of magnitude[26] . BMSCs easily survive and proliferate within a short period of time, which meet the requirement of tissue-engineered bone. A previous study demonstrated that integrin family members CD29, adhesion molecule CD44 and CD105 are the important markers of BMSCs[27] . BMSCs do not express hematopoietic cell surface antigens, such as hematopoietic progenitor cell antigen CD34, leukocyte common antigen CD45, and leukocyte activation antigen CD11b. Results from this study were in accordance with above findings, indicating that cells cultured are mesenchymal stem cells. Results from this study suggest that BMSCs are a kind of satisfactory functional cells for repair of bone defects owing to its great amplification, easy harvesting, and no immunological rejection in autologous application.
Substrate materials are the basic framework for cell attachment and the place for cell metabolism, and their morphology and function directly influence the morphology and function of constitutive tissue. The topological structure of polymer scaffold surface yields important influences on cell morphology, adhesion, spreading, directed-growth, and biological activity[28-30] . Ripamonti et al[31] reported that an ideal substrate material for bone tissue engineering should correspond to the following requirements: (1) enable rapid invasion of vessels and mesenchymal stem cells and help to contact with inducing factors adhering on material; (2) can be gradually degraded and absorbed and finally disappear from implantation region; (3) without immunological rejection after implantation in vivo; (4) easy to trim its outline to match defects of different shapes; (5) able to provide mechanical support if necessary; (6) able to produce best osteogenesis at the lowest dose of inducing factors.
nHAC material used in this study shows similar structure to natural skeleton and benefits cell adhesion, growth, collagen secretion, and calcification. The nano-sized crystal on nHAC surface is easily phagocytized and degraded by cells, which meet the requirement that new bone formation matches to material reduction in speed and enables normal reconstruction of natural bone. Scanning electron microscope results demonstrated that nHAC possesses large pore diameter and high porosity which favor cell proliferation. Immonological rejection has not been found in the clinical application. nHAC material used in this study can be made into proper three-dimensional shapes according to defects shape provided by preoperative three-dimensional reconstruction, which shortens intraoperative re-shaping time and operation time, and reduces hemorrhage volume. The mechanical strength of nHAC meets the requirement of bone grafting. So nHAC is an ideal bone substitute material.
In the process of in vitro construction of tissue-engineered bone, functional cells and substrate materials are always the research hotspots. Nano-sized bioactive material used in this study is a developing direction. BMSCs as functional cells show a wide range of application prospects. Clinical application of tissue-engineered bone is in its infancy and cannot completely meet various clinical requirements. Clinical patients have great individual difference, show complicated disease conditions, and suffer from various degrees of bone defects. Through the cooperation of multidisciplinary specialists, bone tissue engineering will acquire more achievements and better serve human being.
