Prior to LVAD implantation, participants in Groups 1 and 2 will have a sample of bone marrow removed. Following LVAD implantation, study visits will occur at Days 45 and 90, and then every 60 days thereafter until participants receive a heart transplant. At each visit, participants will undergo a LVAD wean procedure, during which the mechanical pump of the LVAD will be gradually turned off, and the LVAD will be operated with a hand pump. The length of time that a participant tolerates the wean procedure will be evaluated. Heart size and function, blood flow, and nerve function will also be assessed. Some participants will undergo a 6-minute walk test. At the time of heart transplant surgery, the LVAD will be removed, and heart cell regeneration and heart function will be evaluated.