Technical as well as economic hurdles must be overcome before therapies based on tissue engineering will be able to reach the millions of patients who might benefit from them. One long-recognized challenge is the development of methods to enable engineering of tissues with complex three-dimensional architecture. A particular aspect of this problem is to overcome the mass transport limit by enabling provision of sufficient oxygen and nutrients to engineered tissue prior to vascularization and enhancing the formation of new blood vessels after implantation. The use of angiogenic factors, improved scaffold materials, printing technologies, and accelerated in vitro maturation of engineered tissues in bioreactors may help to address this problem. Of particular interest is the invention of novel scaffold materials designed to serve an instructive role in the development of engineered tissues. Methods to prepare improved cell-scaffold constructs by growth in bioreactors before implantation will serve a complementary role in generating more robust clinical products.
A second key challenge centers on a fundamental dichotomy in strategies for sourcing of cells for engineered tissues — the use of autologous cells versus allogeneic or even xenogeneic cells. On the one hand, it appears most cost effective and efficient for manufacturing, regulatory approval, and wide delivery to end users to employ a minimal number of cell donors, unrelated to recipient patients, to generate an off-the-shelf product. On the other hand, grafts can be generated from autologous cells obtained from a biopsy of each individual patient. Such grafts present no risk of immune rejection because of genetic mismatches, thereby avoiding the need for immunosuppressive drug therapy. Thus, the autologous approach, though likely more laborious and costly, appears to have a major advantage. Nonetheless, there are many tissue-engineering applications for which appropriate autologous donor cells may not be available. Therefore, new sources of cells for regenerative medicine are being sought and assessed, mainly from among progenitor and stem cell populations.
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