In humans, the photoreceptor cells of the eye and sensory hair cells of the ear lack stem cells. These cells are thus unrenewable and cannot be replaced when they are damaged or destroyed.
Photoreceptors
The retina is composed of several layers and contains specialized cells called photoreceptors. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. Since the eye photoreceptors lack stem cells, the loss of rods and cones is permanent and cannot be regenerated. The loss of photoreceptors or damage can be caused by age-related problems or exposure to high-intensity light.
Sensory Hair Cells
Like photoreceptors, the ear's sensory hair cells also do not contain stem cells. The hair cells are present in the organs of Corti, which are named for the hair-like stereocilia extending from the cell’s apical surfaces. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, the tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens, and the ion channels close. When no sound is present and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized. The loss or damage of hair cells is also permanent and often leads to permanent deafness.
This text is adapted from Openstax Anatomy and Physiology 2e, Section 14.1: Sensory Perception.
