Bone deficiency following trauma, resection of tumour, periodontal disease or congenital malformation can be associated with functional and aesthetic problems. To address these issues and to improve patients' well-being bone tissue engineering has been proposed [1–4]. Tissue engineering techniques have mainly been applied on avascular tissue or on other tissue that can grow without an additional vascular supply, such as epithelia, cartilage or large vessel substitutes [5, 6]. However, one of the major challenges in regeneration of bone tissue is its vascularization because the center necrosis of the engineered bone tissue will occur if blood supply (nutrition and oxygen) cannot be established quickly . Since diffusion of oxygen in the active tissue is limited about 150 μm from capillary (mean of intercapillary distance (ICD) was 304 ± 30 μm.), vascularization becomes crucial in larger volume of tissue-engineered construct. Growth factors, such as vascular endothelial growth factor (VEGF), collagen type II, myometrial prostaglandin E2, epithelial growth factor and basic fibroblast growth factor (bFGF), have been widely used to accelerate neovascularization in order to regenerate damaged tissues [9, 10]. Previously, in vivo secondary vascularization of engineered tissue was attempted with partial success . Alternatively, in vitro construction of vascular stroma could serve as a scaffold for soft or hard-tissue transplant.
Reciprocal regulation and functional interaction between endothelial and osteoblast-like cells during osteogenesis has been reported [1–4, 11, 12]. Villars et al suggested that membrane proteins as well as systemic hormones and growth factors have an active role in this process . Therefore, to transplant large volume of engineered bone tissue successfully, vascularized bone tissue with the endothelial cells in three-dimensional scaffold in vitro could be used . This may not only solve the nutrition and oxygen diffusion to the middle of the bone tissue , but also stimulate osteogenesis by the endothelial cells.
Although some of previous studies showed that vascular endothelial cells and growth factors of vascular endothelial cells could play a role in osteogenesis, it still didn't document well if the direct contact or interaction could be the best way to stimulate osteogenesis, especially in vivo. We hypothesized that the direct contact or interaction between vascular endothelial cells and bone marrow mesenchymal stem cells could be an optimal way to stimulate osteogenesis in vitro and in vivo. Therefore, our objective of present studies was to know what kind of interaction the vascular endothelial cells could efficiently stimulate osteogenesis in vitro and in vivo. To achieve our objective, rat kidney vascular endothelial cells (VEC) and bone marrow mesenchymal stem cells (MSC) were cultured together or alone on PLGA scaffold. The in vitro effect of endothelial cells on osteogenesis by MSC was evaluated. Additionally, MSC-plated PLGA or MSC and VEC-plated PLGA were implanted into the rat's thigh and bone formation was evaluated by soft X-ray analysis and histologically. Our results demonstrated the dramatically effects on osteogenesis in vitro and in vivo while the vascular endothelial cells directly contacted or interacted with the bone marrow mesenchymal stem cells on PLGA scaffold.