Traditional Theories About The Mechanism of Disc Shedding
One grey area in the entire mechanism of outer segment disc shedding is in what exactly triggers the detachment of the discs and how they are transported out of the OS and phagocytosed by the RPE cells.
Dr. Young and his team, among others, observed the disc detachment from the rod OS and through morphological studies, suggested that disc detachment preceded engulfment and that an active process in the ROS distal tip delineates the site of attachment.
However, in a 1986 paper, an Emory professor Dr. Besharse and his team, suggested that the distinction between the processes of disc detachment and phagocytosis was made ambiguous by the observation of pigment epithelial processes intruding into the OS during disc detachment. They documented the ultrastructural changes that occur within the photoreceptor OS and the RPE during photosensitive membrane turnover. They induced shedding in Xenopus laevis by adding the excitatory amino acid L-aspartate. They found that during L-aspartate-induced shedding, the RPE cells formed, on their apical domains, previously undescribed processes that were directly involved in disc phagocytosis. These processes were structurally similar to processes formed by macrophages during phagocytosis and were accordingly referred to as pseudopodia. While pseudopodial formation also occurred during a normal light-initiated shedding event, the low frequency of shedding, the asynchrony of individual shedding events and the transient appearance of the pseudopodia prevented a full appreciation of their role during normal disc schedding. The team stated that these pseudopodia were the organelles of phagocytosis and that they may play a role in disc detachment as well.
In addition, a photoreceptor-RPE interaction was proposed to play a role in determining the domains that would detach from the OS.
Interestingly, another early theory proposed by Dr. Young was that, unlike rods, mature cones neither assemble new discs nor shed old ones, replacing instead only some of their molecular constituents. This idea arose from the observation that the band of radioactive protein that they injected in the two photoreceptor cells appeared at the base of the rods within hours but slowly diffused through throughout the OS. This theory, in turn, led to a proposed distinction between rods and cones based upon whether the outer segments were renewed by membrane replacement or by molecular replacement. It was supported by some findings that showed an absence of phagosomes within the RPE of several cone-dominant species. However, teams of researchers, including that of Dr. Steinberg, soon brought evidence to the table that at least some mammalian cones, like their rod counterparts, continue to assemble as well as shed their discs as a normal ongoing process. The cone visual pigment is apparently based on an apoprotein component similar to rod opsin which turns over as part of the OS membrane system.
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