Retinal degeneration in the nervous mutant mouse. I. Light microscopic cytopathology and changes in the interphotoreceptor matrix

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Abstract
Nervous is an autosomal recessive mutation in mice (gene symbol, nr) that produces a progressive cerebellar and retinal degeneration. We have examined various cytopathological features of the photoreceptor degeneration by light microscopy. An increase in the number of pyknotic photoreceptor nuclei in the outer nuclear layer (ONL) is first seen at postnatal day (P) 11. Between P13 and P19 there is a rapid loss of photoreceptors, with the ONL about 60% the thickness of littermate controls at P19. Between P19 and 2.5 months of age, photoreceptor cell loss is minimal, and there is a relatively slow loss of these cells between 3 and 7.5 months of age. At 7.5 months, the ONL consists of single row of nuclei, most of which are lost over the ensuing months, although a few photoreceptor nuclei persist at 17 months of age and older. Both rods and cones are lost at comparable rates for the first 2 months of life, but rods are somewhat preferentially lost at later ages. A very slight central-to-peripheral gradient of photoreceptor degeneration exists in the nr/nr retina, but no superior-inferior hemispheric differences are evident. The rate, spatiotemporal gradient, and hemispheric similarity in photoreceptor degeneration are the same in albino nr/nr mice reared either in cyclic light or in the dark, and in pigmented nr/nr mice. Autoradiographic analysis of rod outer segment renewal shows that outer segment membranes are synthesized in nervous homozygotes. Rhythmic outer segment disc shedding and phagocytosis by the retinal pigment epithelium occur at approximately normal rates in nr/nr mice. Histochemical and immunocytochemical study of the interphotoreceptor matrix (IPM) reveals the exclusion of stainable IPM from the outer segment zone by lamellar whorls of outer segment membrane, accumulation of stainable IPM in the basal region of the outer segment zone, and the absence of an intense band of stainable IPM at the apical surface of the retinal pigment epithelium. These changes in the IPM are similar to those seen in the Royal College of Surgeons rat. However, comparison of cytopathological changes in these two mutants reveals that the IPM defect probably is not the primary cause of photoreceptor cell death in nr/nr mice, and that similar phenotypic appearance does not necessarily signify similar pathological processes.