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B.E. Reese, M.A. Raven, K.J. Parker, S.B. Stagg, R. Farajian; Plasticity of Horizontal Cells Following Elimination of Cone Photoreceptors During Development . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1652.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose. To investigate the development of horizontal cells in the absence of cone photoreceptor innervation. Methods. Transgenic mice expressing an attenuated diphtheria toxin gene driven by a human L cone opsin promoter sequence were reared to different ages, when their retinas were immunostained for neurofilaments, calbindin, UV and M cone opsin, cytochrome oxidase, SNAP-25 or caspase-3, as wholemounts or in sectioned tissue. Prior to M cone opsin expression, wildtype and transgenic littermates were genotyped using PCR. Normal and degenerate cone photoreceptors were quantified during development. Horizontal cell density was determined in adulthood, while horizontal cell plasticity was quantified throughout development. Results. In wildtype retinas, M cones were not detected until P-13, while UV cones could be detected from birth. In transgenic mice, the reduction in M cones achieved by maturity was already complete by P-13. UV cones had a normal appearance and density at P-1, but by P-5 and thereafter, transgenic retinas were reliably discriminated on the basis of their reduced UV cone densities. The severity of UV cone loss at P-5 was variable, being markedly advanced in some animals relative to others, and averaging 54% of UV cone density in wildtype littermates at this age. Coincident with this UV cone loss, immunoreactive UV cone photoreceptors became detached from the OLM and migrated into the developing INL, appearing as round, undifferentiated somas, commonly lacking any processes. Similar profiles were detected with antibodies to SNAP-25, presumed to include the M as well as UV cones that were dying. These profiles were most frequent at P-5, when horizontal cells transdifferentiate from their transient stellate morphology into their mature horizontal morphology. Horizontal cell morphology was indistinguishable from wildtype littermates until P-17. Thereafter, horizontal cells showed sprouting of processes that passed radially through the ONL and OLM, adopting a scrambled trajectory amidst the photoreceptor outer segments. The distribution of horizontal cell processes within the OPL appeared otherwise normal, and horizontal cell density was unaffected. Conclusions. Horizontal cells exhibit a degree of plasticity induced by the loss of cone photoreceptors. This plasticity is due to a sprouting of aberrant processes rather than the retention of a developmentally-transient morphological feature. It is to be contrasted with the vitreally-directed horizontal cell sprouting that occurs following rod degeneration in the rd mouse.
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