Abstract
Purpose: :
To describe changes induced by retinal detachment in the morphology of rod terminals and their connections to the horizontal cell (HC) axon terminals and rod bipolar cell (RB) dendrites with which they normally synapse.
Methods: :
Confocal microscopy: To observe macroscopic changes in the pattern of rod cell innervation by HC and RB processes, sections of feline retina (control and 28 day detached) were immunolabeled with markers for photoreceptor terminals and the second order neurons that innervate them. Electron microscopy (EM): Over 40 spherules were studied in serial sections to detail changes in rod ultrastructure and innervation.
Results: :
Following retinal detachment many rod terminals retract varying distances towards their respective cell bodies in the outer nuclear layer (ONL). Presynaptic structures are found in all retracted terminals whether completely withdrawn into the basal perinuclear cytoplasm or only partially retracted. By EM, less than half of the retracted terminals examined to date remain contacted by RB dendrites. In contrast, almost every surviving spherule is contacted by neurite outgrowths from the axon terminals of the B–type HC. Although retracted spherules have similar presynaptic structures as controls, numerous charges occur in their synaptic architecture. Most HC contacts with retracted terminals occur in pairs and lack typical RB dendrites. In control retina each HC "lobe" comes from a different axon terminal system (Kolb, 1974, J. Comp. Neurol.); after detachment many originate from branches of the same axon terminal. These post–synaptic HC processes appear very swollen in comparison with controls and are generally devoid of organelles except for occasional vesicles. RB dendrites are thinner than normal, unbranched, often electron dense, and lack organelles.
Conclusions: :
Following detachment, neurites have been shown to arise de novo from RB dendrites as well as from B–type HC axon terminals. By immunocytochemistry both can appear post–synaptic to retracted rod terminals. However, at the ultrastructural level, HCs seem to more consistently achieve connection with the retracted spherules than the RBs. It would seem that the lack of RB central elements as well as the drastic alterations in the architecture at most retracted terminals would necessarily alter the function of this complex synapse. Insofar as a population of rods dies after detachment, perhaps contact with either HC or RB processes plays a role in the survival of those that remain.
Keywords: retinal detachment • retina: distal (photoreceptors, horizontal cells, bipolar cells) • retinal connections, networks, circuitry