Engineering a Replacement Bone Tissue with Adult Adipose-Derived Stem Cells

The current gold standard for bone repair, autologous bone graft, is costly and destructive to the harvest site. Moreover, the amount of available donor tissue is limited, so it is not always a viable option. The aim of this work was to investigate tissue engineering as an alternative to autograft-based bone repair through the combination of donor cells and three-dimensional cell carriers. The results presented herein describe the progress made toward that goal.

To evaluate the intramembranous bone forming capacity of human adipose-derived stem cells, novel collagen scaffolds with or without hydroxyapatite reinforcement were loaded with cells and treated with growth or an osteogenic differentiation medium. Upon implantation, the cell-scaffold constructs produced varying amounts of immature bone characterized by highly vascularized osteocalcin and osteopontin-positive matrix and an increase in mineralized tissue volume. Hydroxyapatite reinforcement and the particular pretreatment of the cells were identified as key factors influencing tissue formation in vivo. In order to determine important characteristics for engineered constructs capable of inducing an endochondral response in vivo, an ectopic model of endochondral ossification was developed with pre-chondrogenic murine cell line ATDC5. When coupled with an osteoinductive scaffold in vivo, pellets been pre-treated in an optimized chondrogenic differentiation medium underwent chondrogenic maturation and partial ossification over the course of 8 weeks. This effect was dependent on the treatment medium applied and a three-dimensional culture environment.

With these findings in mind, chondrogenic treatment was applied to human adipose-derived stem cells seeded in high density pellet culture or encapsulated in alginate beads to investigate the potential for endochondral bone formation in a clinically relevant cell type. Despite identical pre-treatment, the culture systems induced significantly different volumes of mineralized tissue formation in vivo. Cell pellets resulted in a greater amount of mineralized tissue which accumulated around a large volume exterior to the implants, while mineralized tissue deposits were confined to a smaller volume in cell seeded alginate beads. These findings further highlighted the importance of pretreatment and culture methods with respect to the in vivo response induced by tissue engineered constructs.