In humans, two sensory structures of the body- the retinal epithelium and the auditory epithelium, lack stem cells. In the case of retinal epithelium, if the photoreceptors degenerate with time or are damaged due to exposure to high beam light, the loss cannot be compensated, leading to partial or complete blindness. Similarly, if the sensory hair cells of the ear deteriorate with age or are ruptured due to loud noise, they cannot be regenerated, leading to partial or complete loss of hearing. Moreover, people suffering from Usher's syndrome are both blind and deaf due to the loss of photoreceptors as well as sensory hair cells, and the lack of stem cells to regenerate these cells.