Re: rod and cone cells located in the retina

Dear Joseph -

Thank you for your interesting question.  You are correct, vertebrate rod 
and cone photoreceptors can become damaged in several ways (light damage is 
one common way), and if the cell is not killed it can recover.  

Rods and cones have two major compartments, and outer segment where one 
finds stacks of photosensitive membrane (this is the part of the cell that 
sees the light), and an inner segment which contains the nucleus, and all 
the protein synthetic machinery.  All proteins are made in the inner 
segment, and those that are involved in detecting and responding to light 
are subsequently shipped selectively to the outer segment. The details of 
how the cell shorts and delivers the correct proteins to the outer segment 
is not yet worked out. 

Every day throughout the life of the vertebrate photoreceptor it sheds 
packets of membranes at the distal tip of its outer segment and these shed 
membranes are “eaten” by an adjacent cell called the pigmented epithelium. 
 New membranes and proteins made in the inner segment are inserted at the 
base of the outer segment usually at about the same rate as membrane is 
shed, so normally the outer segment stays at about the same length.  

But sometimes things do wrong and the outer segments become damaged.  If 
this happens, it appears that the damaged membranes are shed and “eaten” by 
the pigmented epithelium, and that the cell makes more new membrane. So 
damaged cells can repair themselves to some extent by making new membrane.

However if a photoreceptor in the mammalian retina is killed, it cannot be 
replaced.  The current evidence indicated that no new photoreceptors can be 
produced in the adult mammalian retina, so if a photoreceptor dies, it is 
not replaced.  The situation is different in fish retinas.  Fish retinas do 
have the ability to generate new photoreceptors.  This may be because fish 
eyes continue to grow throughout the life of the fish, and new 
photoreceptors are normally generated.  So if we could figure out how fish 
retinas are able to generate new photoreceptors through out the life of the 
animal, perhaps we would know better how to repair a damaged mammalian 
retina.