The mosaic of alpha cells in the cat retina is not dependent on axon terminal interactions during development

Alpha ganglion cells in the cat retina are distributed in a regular array. It has been proposed that the development of this mosaic pattern is achieved by class-specific interactions between the dendrites and/or axon terminals of neighbouring alpha cells, but the relative contributions that are made by each of these factors to the regularity of the mosaic remain unclear. An opportunity to address this question is provided by a comparative study of the distribution of alpha cells across the nasotemporal division of normally pigmented and Siamese cat retinae. In this strip of retina the alpha cell population divides its axon terminals between the two sides of the brain. Hence, the potential for interactions between axon terminals of alpha cells whose somata are adjacent within this region of the retina is undermined. This situation is exacerbated in Siamese cats because they have a congenital abnormality in this region, which results in an abnormally wide nasotemporal division. In this study the regularity of the alpha cell mosaic has been analysed at a wide range of eccentricities, including the nasotemporal division. In normally pigmented cats the nasotemporal division was visualised directly following unilateral horseradish peroxidase injections into the thalamus. In Siamese cats, the location of the abnormally wide nasotemporal division was inferred from electrophysiological recordings undertaken in their striate cortices. In both strains of cat the regularity of the alpha cell mosaic was shown to be largely independent of their density or retinal location. Furthermore, in the normals, the separate mosaics for ipsilaterally or contralaterally projecting alpha cells within the nasotemporal division were found to be no more or less regular than that for the combined population. This finding implies that there is no disturbance in their order across the discontinuity formed by their separate chiasmatic routes. Similar results were obtained from within the abnormal nasotemporal division of the Siamese cat. It is concluded that terminal interactions play an insignificant role in the formation of the adult alpha cell mosaic, the regularity of which is probably established by class-specific intraretinal interactions.