The ability to functionally integrate transplanted cells into damaged organs is also a major challenge. For some cell types, such as pancreatic beta cells and retinal pigment epithelial cells, it appears that transplantation of the cells alone will be sufficient to ameliorate symptoms or cure the disease (in these examples, diabetes and macular degeneration, respectively). For others, it is likely that complex organ structure creation through in vitro tissue engineering will be required. Here, the challenge is not only to derive the correct organ structure at scale, but also to maintain long-term function following transplantation. To achieve this will require new collaboration between tissue engineers and stem cell and vascular biologist, as well as improved understanding of how stem cells are controlled by their specific environment (niche) within the body.