Jonas Frisén and his colleagues at the Karolinska Institute in Stockholm provided evidence that ependymal cells act as reservoir cells in the forebrain, which can be activated after stroke and as in vivo and in vitro stem cells in the spinal cord. However, these cells did not self-renew and were subsequently depleted as they generated new neurons, thus failing to satisfy the requirement for stem cells.[4][5] One study observed that ependymal cells from the lining of the lateral ventricle might be a source for cells which can be transplanted into the cochlea to reverse hearing loss.[6]