Neuronal circuitry of the pigeon retina (Columba livia) : the morphological classification and organization of various neuronal types
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2013
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Querubin, Angeliza Marave
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The three studies presented in this thesis were conducted to advance our understanding of the retinal circuitry that contributes to processing high visual acuity in the pigeon. In the first study, the topographic density changes and degree of photoreceptor (PR) to retinal ganglion cell (RGC) convergence in the pigeon retina was determined. DAPI or Propidium iodide labelled PRs and RGCs were counted in the retina. Rod density was quantified by counting anti-rod opsin stained outer segments. The fovea and the red field contained significantly higher cone and RGC densities compared with the yellow field. Rods were missing from the fovea, but not in the red field, which suggests that a rod circuitry may be present in this area. The ratio of cones to RGCs was lower in both the fovea and red field, which is consistent with the higher visual acuities that have been reported in these regions. The second study classified the types of DiO-Iabelled bipolar cells in the fovea, central red and yellow fields. Eight bipolar cell types were classified in the retina using a modification of Mariani's (1987) classification scheme. Eight BC types (B1 - B8) had similar dendritic morphology as the ones described by Mariani. Two bipolar cell types, B7 and B8, had comparatively smaller dendritic fields than the other types. It was estimated to receive input from possibly one photoreceptor in the fovea and the central red field. Based on the small dendritic field size, B7 and B8 may be good candidates for being the midget-like BCs in the pigeon retina. The third study classified the RGC groups in the pigeon retina. Classification of RGCs labelled with DiI/DiO in the pigeon retina was based on the dendritic stratification pattern in the inner plexiform layer (IPL). Five morphological RGC groups were identified, the unstratified, monostratified, bistratified, tristratified and tetrastratified. The unstratified group was characterised by vertically oriented dendrites occupying a thick portion of the IPL, whereas the other groups had horizontally oriented dendrites stratifying at narrow portion of the IPL. The unstratified RGC had the narrowest dendritic field (diameter >- 18.5 urn). Based on the unstratified RGC's dendritic field size, it is a good candidate for a 'midget-like' RGC in the pigeon retina. However, it has a different morphology from the primate midget RGC. Further work is required to determine the physiology and differential distribution of the different types of bipolar and ganglion cells in the pigeon retina.
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