Most of us have had to deal with a broken bone or two in our day. Usually these breaks are minor, requiring a cast and a little time to let the body heal itself. But when damaged bone cannot be repaired naturally, bone grafts from either the patient or a donor, or man-made metal or ceramic composite implants become necessary. This is often the case for patients with age-related degenerative bone diseases such as arthritis or osteoporosis. Grafts and implants do, however, present challenges, especially when it comes to the potential for a negative immune response.
“The original inspiration is the very large need for bone grafts to treat various bone deficiencies,” said Marolt. Although this team previously created bone constructs from human embryonic stem (ES) cells (2), use of those cells raises serious ethical concerns as well as questions of clinical safety—primarily due to the risk that implanted cells will not maintain the desired phenotype.
To start their work, Marolt’s team grew 3-D bone substitutes measuring 4–5mm by culturing human iPS cells in a bioreactor with decellularized bovine bone as the solid substrate. Several iPS cell lines and reprogramming methods were tested, demonstrating that multiple tissue sources could be used in future studies and enhancing the opportunity to identify cell sources with the best osteogenic potential.
When Marolt’s lab implanted those bone substitutes subcutaneously into immunodeficient mice, the team observed bone growth through 12 weeks with no indication of implanted cells adopting harmful phenotypes. Importantly, the constructs showed tissue mineralization and recruitment of osteoclastic cells to construct edges; microvascular ingrowth of the human tissue constructs after implantation suggests these bone substitutes could be well-suited to implantation and integration into host tissue.
In the long term, Marolt expects such integration to happen, with these the implanted tissue slowly remodeling and new healthy bone tissue forming in and around the defect.
While still in the initial phases of development, Marolt’s engineered bone constructs would likely be used to repair bone defects rather than replacing missing or diseased bone at first. But she has an ambitious vision for the future of therapeutic tissue engineering. “You could have cell banks storing these cells, and if there is a need, then you would create a tissue substitute.” Of course, the dimensions and shape of a bone graft would vary from patient to patient, but approaches such as the one Marolt has developed will make it easier to create such customized human tissue replacements in the future.
1. de Peppo, G. M., I. Marcos-Campos, D. J. Kahler, D. Alsalman, L. Shang, G. Vunjak-Novakovic, and D. Marolt. 2013. Engineering bone tissue substitutes from human induced pluripotent stem cells. Proceedings of the National Academy of Sciences (May).
2. Marolt, D., I. M. Campos, S. Bhumiratana, A. Koren, P. Petridis, G. Zhang, P. F. Spitalnik, W. L. Grayson, and G. Vunjak-Novakovic. 2012. Engineering bone tissue from human embryonic stem cells. Proceedings of the National Academy of Sciences 109(22):8705-8709.