Retinal connections and their remodeling. (a) Simplified diagram of a retinal circuit, illustrating the organization of inputs from photoreceptor to bipolar to retinal ganglion cells (RGCs). Photoreceptors (yellow) transfer visual information to bipolar cells (blue) that in turn contact dendrites of RGCs (red). Amacrine cells (gray) also provide synaptic input to RGCs. (b) Schematic representation of the relationship between dendritic architecture and synaptic connectivity of a developing RGC. Excitatory synaptic sites on RGC dendrites can be visualized by the punctate distribution of PSD95-YFP (green) on neurons expressing a red fluorescent protein (red). During development, the dendritic arbor of an RGC extends its synaptic territory (oval) through dynamic remodeling of its branches. The number of synaptic contacts per unit area of the bipolar cell surface (represented by the hexagon) remains constant as dendrites remodel and exuberant branches are pruned back. Synapse density is maintained by an increase in the number and/or density of synapses on those branches that are retained. For visual clarity, synaptic sites are illustrated only in a portion of the dendritic arbor. GCL, ganglion cell layer; IPL, inner plexiform layer.