Retrograde tracing with fluorescent microspheres reveals bifurcating projections from central retina to tectum and thalamus in chicks

The goal of this study is to demonstrate the dual-projection pattern of retinal ganglion cells (RGCs) projecting to the tectum and visual thalamus in chick using retrograde fluorescent tracers and also to define the morphological properties of these RGCs with dual projections by intracellular injection of Lucifer Yellow (LY) combined with immunohistochemistry. Thirty-two chicks received double injections of green and red fluorescent microspheres into their thalamus and tectum in the same side. In the central retina, most of the labelled RGCs were tec-RGCs (RGCs projecting to the tectum), a quarter was tha-RGCs (RGCs projecting to the thalamus), and almost all of the tha-RGCs were double-labelled RGCs. An intracellular injection of LY into the double-labelled RGCs showed all six groups of RGCs without specific populations in each group (J. Comp. Neurol., 2004, 469: 360). These dendritic patterns were mostly mono- and bistrata, which extended horizontally in the deeper part of the inner plexiform layer.