Cellular Requirements for Building a Retinal Neuropil

SummaryHow synaptic neuropil is formed within the CNS is poorly understood. The retinal inner plexiform layer (IPL) is positioned between the cell bodies of amacrine cells (ACs) and retinal ganglion cells (RGCs). It consists of bipolar cell (BC) axon terminals that synapse on the dendrites of ACs and RGCs intermingled with projections from Müller glia (MG). We examined whether any of these cellular processes are specifically required for the formation of the IPL. Using genetic and pharmacological strategies, we eliminated RGCs, ACs, and MG individually or in combination. Even in the absence of all of these partner cells, an IPL-like neuropil consisting of only BC axon terminals still forms, complete with presynaptic specializations and sublaminar organization. Previous studies have shown that an IPL can form in the complete absence of BCs; therefore, we conclude that neither presynaptic nor postsynaptic processes are individually essential for the formation of this synaptic neuropil.

Graphical AbstractFigure optionsDownload as PowerPoint slideHighlights
► Bipolar cells are among the first cell types to colonize the nascent IPL
► Presynaptic BCs can autonomously build an IPL-like neuropil
► The BC-only neuropil exhibits sublaminar structure and presynaptic specializations
► No single contributing cell type is essential for the formation of the IPL