Development of Retinal Ganglion Cell Dendritic Structure and Synaptic Connections

The neuronal information of the visual scene that is processed by the retina is conducted to the brain by a set of separate spatio-temporal synaptic pathways. The morphological basis for the formation of these parallel synaptic pathways is the laminar-specific structure of the retina, in which specific subtypes of retinal neurons form synapses only with highly selective presynaptic and postsynaptic cells (1-3). Retinal ganglion cells (RGCs) are the output neurons of the retina. In the retina, RGCs synapse with bipolar and amacrine cells in the inner plexiform layer (IPL) to receive excitatory and inhibitory synaptic inputs respectively. The axons of RGCs travel through the optic nerve to retinorecipient structures in the brain, where they transfer their specific aspects of visual information to the higher centers (3). Because different subtypes of bipolar cells (Fig 1) (4) and amacrine cells (Fig. 2) (5) have their axonal/dendritic terminals in the specific sublaminae of the IPL, it is crucial that dendrites of individual RGCs are also confined to specific strata in order to synapses with them. Thus, the synaptic circuitries processing distinct visual features, the so called “parallel pathways” (1, 2, 6-10), start in the retina. In most mammals, RGCs can be divided into about 20 morphological subtypes based on their distinctive dendritic structure and synaptic connections (11-19). The wholemount drawings of mouse RGCs (Fig. 3) illustrate the diversity of morphologies present in mammalian RGCs ((19). See also RGCs of human, cat and rabbit retinas in the ganglion cell chapter in Webvision). Most of these RGCs have specific dendritic distribution in the IPL in adult retina as exemplified by the schematic (Fig. 4) showing the branching patterns of mouse RGCs. In most mammals, these lamina-restricted distributions of RGC dendrites and synaptic connections are formed during pre- and post-natal development. The question is how this lamination arises.