In the retina of mice homozygous for the retinal degeneration slow (rds) gene, receptor outer segments failed to develop and typical disc structures were never observed. However, cilia surrounded by a plasma membrane were regularly present. At the time when outer segments grew in the normal retina, the optic ventricle in the mutant showed an accumulation of membrane-bound vesicles of varying size and density. The vesicles declined in frequency at later stages of degeneration. After initial growth, the inner segments in the mutant retina remained stunted but maintained their morphological differentiation, including the ciliary structures. Their number declined with loss of visual cells.
In the mutant retina development of synapses, between the receptor cells and the processes of the horizontal cells and the bipolar cell dendrites, followed in normal sequence. Formation of spherule terminals of the rods, with one triad synapse, and of pedicle terminals of the cones, with multiple triad synapses, was recorded in the same way as in the normal retina. With loss of visual cells in the mutant retina, some of the profiles of the surviving spherule terminals showed an increased occurrence of two, three, or more synaptic sites. This growth resulted from enlargement and branching of the ribbons and sprouting from the postsynaptic elements. Similar changes were not observed in the pedicle terminals.
The pigment epithelial cells in the mutant mice appeared initially normal, but phagolysosomal structures were absent. However, various inclusion bodies appeared within the pigment epithelial cells following degenerative changes in the retina. In some places, the basal infoldings deepened their furrows while thinning the cytoplasmic part of the epithelium.
Failure to form the outer segments and the subsequent lysis of the visual cells appear to be the primary effects of the rds gene, whereas the synaptic growth in the sperule terminals and the changes in the pigment epithelium appear to be consequential to those defects.