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M.A. Raven, A. Scott, A. Swaroop, B.E. Reese; Afferent Specification of Horizontal Cell Morphology: A Comparison of Cone–Less, Cone–Full and Wild–Type Mouse Retinas . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2782.
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© ARVO (1962-2015); The Authors (2016-present)
The first inhibitory interneurons of the retina, the horizontal cells, stratify within the outer plexiform layer of the retina, extending dendritic terminals that contact the pedicles of cone photoreceptors, and an axon terminal system contacting the spherules of rod photoreceptors. How these horizontal cells acquire the characteristic morphologies associated with their dendritic and axon terminal fields is unknown, but instructive interactions with the afferents may play a role
Immuno– and DiI–labeling techniques were used on retinal sections and wholemounts to examine the architecture of the outer plexiform layer and the morphology of single horizontal cells, respectively, in cone–less mice (in which a human L–opsin promoter sequence drives expression of an attenuated diphtheria toxin gene), in cone–full mice (in which the transcription factor, nrl, is knocked out, leading to the re–specification of all rods as cones), and in wild–type C57BL/6 mice (being the genetic background of the other mice).
Horizontal cells in both the cone–less and cone–full retinas had clearly discriminable dendritic and axon terminal domains. The morphology of the axon terminal system and the dendritic field were found to be selectively regulated by innervation from their respective afferents: Early elimination of the cones produced an atrophic dendritic field, yet left the axon terminal system largely intact. By contrast, in retinas of nrl knockout mice containing no rods and an excessive number of cones, the dendritic field was hypertrophic, while the axon terminal system had severely atrophied.
While cell–intrinsic mechanisms most likely account for the formation of independent dendritic versus axonal domains, the afferents play a selectively instructive role in defining the respective morphologies of the dendritic and axon terminal fields.
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