Parallel processing begins in the retina, where input from photoreceptors is transmitted to 12 types of bipolar cell. Bipolar cells are interneurons that propagate visual signals to over 17 types of ganglion cell, which are output neurons of the retina. In this way various vertical pathways are formed that deliver different sensory signals to the brain. This thesis comprises a detailed map of the cell types that contribute to parallel processing in primate retina.
Chapter 1 introduces the structure of the primate retina and describes the morphology of retinal cells and their contribution to visual processing.
Chapter 2 provides a survey of ganglion cell types in marmoset retina. Ganglion cells were transfected with a plasmid for the expression of a synaptic marker conjugated to green fluorescent protein. At least 17 morphological types of ganglion cell were identified. The contribution of widefield ganglion cells is greater to peripheral than to foveal vision, whereas the fovea is dominated by midget and parasol cells. Outside the fovea ganglion cell diversity in marmoset retina is likely as great as that reported for non-primates.
In Chapter 3 particle-mediated gene transfection was applied to post mortem human retina. Human retinas maintained their morphology and immunohistochemical properties for at least 3 days in culture. This study showed that gene transfection can be used to target cells in the human retina, with the potential to study their connectivity and structural changes in diseases.
Chapter 4 provides a quantitative analysis of the major cell populations in the inner nuclear layer (INL) of normal human retina. Immunohistochemical markers were applied to vertical sections to label and quantify horizontal, bipolar, amacrine and Müller cells across the retina. Cone photoreceptors and ganglion cells were also counted. With the exception of the fovea, the proportion of different cell populations in the INL is comparable across all eccentricities and comparable to non-human primates and other mammals. The cone to cone bipolar cell ratio was constant across the retina suggesting that convergence and divergence do not change with eccentricity.
The data provided in this thesis will serve as a reference for the interpretation of abnormalities in disease, and the informed targeting of treatments in human retinas.