Other forms of cell therapies are being developed to treat complex tissues such as the liver, brain, and heart. It is difficult to obtain and culture the main cell types of these complex tissues, and therefore progenitor and stem cell–based approaches are being actively investigated. Clinical trials for each of these tissues have been initiated using both adult mesenchymal stem cells as well as tissue-specific stem cells. The derivation of liver cells from stem cells is of interest both to treat hepatic failure, as well as to produce improved in vitro screens for drugs. Bone marrow and stem cell transplantation into the heart to treat ischemia and prevent the progression to heart failure has shown promise in the clinic, but there are still many important issues that need to be resolved for such procedures to be used widely. Cell transplantation to the brain also has been attempted clinically with mixed results. Recently, Geron Corporation was given clearance to start the first human clinical trial of

                 FIGURE 6.7 Chondrocyte transplantation in the right femoral condyle.

embryonic stem cells to treat acute spinal cord injury. These therapies, while exciting and of high potential, are still at a very early stage of development. From the tissue engineering perspective, there are still many unanswered questions regarding the most appropriate cell sources, mass transfer limitations, delivery methods, control of the microenvironment, integration with the host, and provision of an acceptable functional outcome.