The survival and functioning of a bone biomaterial upon implantation requires a rapidly forming and stably functioning vascularization that connects the implant to the recipient. We have previously shown that human microcapillary endothelial cells (HDMEC) and primary human osteoblast cells (HOS) in coculture on various 3-D bone biomaterial scaffolds rapidly distribute and self-assemble into a morphological structure resembling bone tissue. Endothelial cells form microcapillary-like structures containing a lumen and these were intertwined between the osteoblast cells and the biomaterial. This tissue-like self-assembly occurred in the absence of exogenously added angiogenic stimuli or artificial matrices. The purpose of this study was to determine whether this in vitro pre-formed microvasculature persists and functions in vivo and to determine how the host responds to the cell-containing scaffolds. The scaffolds with cocultures were implanted into immune-deficient mice and compared to scaffolds without cells or with HDMEC alone. Histological evaluation and immunohistochemical staining with human-specific antibodies of materials removed 14 days after implantation demonstrated that the in vitro pre-formed microcapillary structures were present on the silk fibroin scaffolds and showed a perfused lumen that contained red blood cells. This proved anastomosis with the host vasculature. Chimeric vessels in which HDMEC were integrated with the host's ingrowing (murine) capillaries were also observed. No HDMEC-derived microvessel structures or chimeric vessels were observed on implanted silk fibroin when precultured with HDMEC alone. In addition, there was migration of the host (murine) vasculature into the silk fibroin scaffolds implanted with cocultures, whereas silk fibroin alone or silk fibroin precultured only with HDMEC were nearly devoid of ingrowing host microcapillaries. Therefore, not only do the in vitro pre-formed microcapillaries in a coculture survive and anastomose with the host vasculature to become functioning microcapillaries after implantation, the coculture also stimulates the host capillaries to rapidly grow into the scaffold to vascularize the implanted material. Thus, this coculture-based pre-vascularization of a biomaterial implant may have great potential in the clinical setting to treat large bone defects.
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