Retraction and remodeling of rod spherules are early events following experimental retinal detachment: an ultrastructural study using serial sections

PURPOSE: To describe changes induced by retinal detachment in the ultrastructure and organization of rod terminals and their connections with B-type horizontal cell (HC) axon terminals and rod bipolar cell (RB) dendrites. METHODS: Sections from control, 3 day, 7 day, and 28 day detached feline retinas were prepared for confocal immunofluorescence, light microscopy, and electron microscopy (EM). In addition, 100 μm-thick vibratome sections were immunolabeled with markers for photoreceptor terminals, HCs, and RBs. More than 40 rod spherules were studied in 90 nm-thick serial sections by transmission EM to greater detail changes in their ultrastructure and innervation. RESULTS: Following retinal detachment, many rod terminals retracted varying distances toward their respective cell bodies in the outer nuclear layer (ONL). In retinas detached for 1 to 4 weeks, an altered synaptic vesicle population and associated ribbons were found in all retracting terminals. Many rod somata in the distal ONL seemed to lack synaptic terminal structures altogether. In a retina detached for 1 week, EM showed that less than half of the retracted terminals remain in contact with RB dendrites. In contrast, almost every surviving spherule was contacted by neurite outgrowths from the axon terminals of the B-type HC. Although retracted spherules had several presynaptic structures similar to those in normal retina, numerous changes occurred in their overall synaptic architecture. The spherule’s invagination was shallower, contained fewer postsynaptic processes, and often had “opened,” allowing swollen HC processes apposing the synaptic ribbon to directly contact other processes of the outer plexiform layer (OPL) neuropil. Whereas in normal cat retina each HC “lobe” comes from a different axon terminal system, after detachment, the opposing lateral elements can stem from the same terminal. The innervating RB dendrites that branched off stout RB dendritic trunks that extended up into the ONL were thinner than normal, unbranched, often electron dense, and lacked organelles. When present, most merely lay adjacent to retracting spherules rather than enter any synaptic invagination that might still occur. CONCLUSIONS: Immunocytochemistry enabled RB and HC neurites to appear postsynaptic to retracted rod terminals. However, at the ultrastructural level, HCs seemed to more consistently retain connection with the retracted spherules than the RBs. The highly conserved architecture of the rod spherule was lost as the invagination opened and postsynaptic contacts became fewer. It would seem that the lack of RB central elements as well as the drastic alterations in the architecture of most retracted terminals would necessarily alter the physiology of this complex synapse.