39.13
Unrenewable Cells
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 replace…
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.
View the full transcript and gain access to JoVE Core videos
Q1: Why can't photoreceptors in the eye be replaced when damaged?
Photoreceptors lack stem cells, making them unrenewable. When rods and cones degenerate from age or high-intensity light exposure, they cannot be regenerated. This permanent loss of photoreceptors leads to partial or complete blindness, as the eye cannot replace these specialized light-sensitive cells.
Q2: What happens to hearing when sensory hair cells are damaged?
Sensory hair cells in the ear lack stem cells and cannot regenerate when damaged or destroyed. Loud noise or age-related deterioration ruptures these cells permanently, leading to partial or complete hearing loss. Since the organ of Corti contains no stem cells to replace damaged hair cells, hearing loss becomes irreversible.
Q3: How do photoreceptors differ in structure between rods and cones?
Rods and cones are two types of photoreceptors that differ in their outer segment shape. Rod photoreceptors contain rod-shaped outer segments with stacked membrane-bound discs holding the photosensitive pigment rhodopsin. Cone photoreceptors have cone-shaped outer segments with photosensitive pigments in membrane infoldings rather than discrete discs.
Q4: What is Usher's syndrome and how does it relate to unrenewable cells?
Usher's syndrome causes both blindness and deafness due to loss of photoreceptors and sensory hair cells. Since these cells lack stem cells, they cannot be regenerated once damaged. The combined loss of vision and hearing results from the permanent degeneration of two unrenewable sensory cell types.
Q5: How do sensory hair cells respond to sound vibrations?
Hair cells in the organ of Corti detect sound through stereocilia, hair-like projections on their apical surfaces. When pressure waves move the basilar membrane, the tectorial membrane slides across stereocilia, bending them toward or away from the tallest member. This bending opens or closes ion channels, depolarizing the hair cell membrane and triggering nerve impulses that transmit sound signals.
Q6: What maintains the resting membrane potential of hair cells?
Hair cells maintain a slightly depolarized resting membrane potential through tension on protein tethers connected to stereocilia. Even when no sound is present and stereocilia stand straight, this baseline tension keeps ion channels partially open. The tethers control ion channel activity, allowing hair cells to respond sensitively to even small vibrations from sound waves.
Q7: How do high-intensity light and loud noise damage unrenewable sensory cells?
High-intensity light exposure damages photoreceptors in the retina, while loud noise ruptures sensory hair cells in the ear. Because both cell types lack stem cells, damage is permanent and cannot be compensated through regeneration. These external stressors cause irreversible loss of vision or hearing depending on which sensory cells are affected.