Date Published: July 18, 2019
Publisher: Public Library of Science
Author(s): Ming-Kai Hsieh, Chia-Jung Wu, Xuan-Chun Su, Yi-Chen Chen, Tsung-Ting Tsai, Chi-Chien Niu, Po-Liang Lai, Shinn-Chih Wu, Gianpaolo Papaccio.
Cells, scaffolds, and factors are the triad of regenerative engineering; however, it is difficult to distinguish whether cells in the regenerative construct are from the seeded cells or host cells via the host blood supply. We performed a novel in vivo study to transplant enhanced green fluorescent pig mesenchymal stem cells (EGFP-pMSCs) into calvarial defect of DsRed pigs. The cell distribution and proportion were distinguished by the different fluorescent colors through the whole regenerative period.
Eight adult domestic Ds-Red pigs were treated with five modalities: empty defects without scaffold (group 1); defects filled only with scaffold (group 2); defects filled with osteoinduction medium-loaded scaffold (group 3); defects filled with 5 x 103 cells/scaffold (group 4); and defects filled with 5 x 104 cells/scaffold (group 5). The in vitro cell distribution, morphology, osteogenic differentiation, and fluorescence images of groups 4 and 5 were analyzed. Two animals were sacrificed at 1, 2, 3, and 4 weeks after transplantation. The in vivo fluorescence imaging and quantification data showed that EGFP-pMSCs were represented in the scaffolds in groups 4 and 5 throughout the whole regenerative period. A higher seeded cell density resulted in more sustained seeded cells in bone regeneration compared to a lower seeded cell density. Host cells were recruited by seeded cells if enough space was available in the scaffold. Host cells in groups 1 to 3 did not change from the 1st week to 4th week, which indicates that the scaffold without seeded cells cannot recruit host cells even when enough space is available for cell ingrowth. The histological and immunohistochemical data showed that more cells were involved in osteogenesis in scaffolds with seeded cells.
Our in vivo results showed that more seeded cells recruit more host cells and that both cell types participate in osteogenesis. These results suggest that scaffolds without seeded cells may not be effective in bone transplantation.
Skeletal defects require surgery using bone grafts. Autografts are the gold standard for bone grafting ; however, donor site morbidity and the limited amount of available donor tissue restrict their application [2, 3]. Regenerative tissue engineering using cells, scaffolds, factors and blood supply  has become an alternative method to treat skeletal bone defects.
Allografts may provide the same osteoconductive conduits for bony fusion as traditional autografts and may have comparable biomechanical properties [5, 6]. Although depleted of osteoprogenitor cells like MSCs, the fusion rate still reaches 73% to 100% in instrumented spinal fusion [7–16], making allografts a clinically feasible alternative for fusion. Cells, scaffolds, and factors are the triad of regenerative engineering, and the lack of one of these three graft properties made us wonder about the role of seeded cells in bone regeneration.