Neuronal structure of the lacertilian parietal eye, I: A retrograde label and electron-microscopic study of the ganglion cells in the photoreceptor layer

Abstract

The cellular connectivity of the lacertilian parietal eye is not well understood. Because the intercellular connections establish the foundation for information processing, we have investigated cellular connectivity of one cell type in this simple vertebrate retina. We also developed an in vitro preparation to study the anatomy of the parietal eye visual system. Horseradish peroxidase transport in the in vitro preparation revealed a class of displaced ganglion cells occupying positions among the photoreceptors, in a location where the presence of interneurons had been suggested. Three-dimensional reconstruction at the electron-microscopic level showed that the morphology and synaptic input of these displaced ganglion cells is different from that of the previously known ganglion cells. The displaced ganglion cells receive an average of about 13 ribbon synapses from photoreceptors. The ribbon input is equally distributed between the soma and dendritic arbor. Junctional membrane measurement and ethanolic phosphotungstic acid-staining provided evidence for the existence of non-ribbon synaptic contacts (synaptoid junctions). Displaced ganglion cells make about 20 synaptoid junctions, 65% of which are on the dendritic arbor. The morphology of the displaced ganglion cell is such that a significant measure of synaptic input to the dendritic arbor will be transmitted to the soma.