Twenty guinea pigs were systemically deafened and allowed to recuperate for 2 weeks. Four days after the deafening surgery, undifferentiated MESCs underwent a 9-day induction protocol to sequentially generate embryoid bodies (EBs) and neurectoderm (early neuronal tissue), as described in methods. Two weeks post-deafening, 15 guinea pigs received a 2μL suspension of MESCs, delivered into the left cochlea (Figure 1). A further 5 animals received a transplant of DMEM alone into the left cochlea. The right cochleae of all animals served as a deafened, untreated control. Animals receiving a transplant of MESCs (n=15) were placed randomly into 3 groups (5 animals per group) and perfused 1, 2 or 4 weeks post-stem cell transplantation (group 1, 2 or 3 respectively). Animals receiving a transplant of DMEM (n=5) were perfused 2 weeks post-delivery.