Shamim, Khondker Manzare; (1993) Frog retinal ganglion cells. Morphological and spatial analyses. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Retinal ganglion cells have previously been classified using morphological, electrophysiological and biochemical criteria that are sometimes correlated. However, emphasis on morphological details has often led to complicated classification schemes, which risk creating artificial cell types rather than discovering natural types. This study addresses the problem by exploiting the population characteristic of an independent, regular spatial distribution (mosaic) as a classification criterion for ganglion cells, alongside individual morphological features. Retrogradely labelled large ganglion cells were studied light- microscopically in five frog species from four genera. These were the aquatic Xenopus laevis, semi-aquatic Rana esculenta and Rana pipiens, terrestrial Bufo marinus and arboreal Hyla (Litoria) moorei. Three large (a) ganglion cell types were identified in each of the first four species. Each formed an independent mosaic and was morphologically distinct. One had a dendritic arborisation in the most scleral sublamina (a) of the inner plexiform layer, the second in both that sublamina and the middle sublamina (b), and the third mostly in the vitreal sublamina (c). Accordingly, a nomenclature was devised in which they are termed aa, aab respectively. In Hyla moorei, a larger and a smaller aab type formed independent mosaics and were termed a1ab and a2ab. The ac type was present (although not labelled consistently enough to permit spatial analysis) but no aa type was found. Members of each mosaic were analysed for within-type and cross-type similarities, and variations in mosaic properties and morphological features. The observations are compared with those of other authors to identify possible cross-species homologies and formulate a broad classification scheme. Possible functional roles and developmental mechanisms for different large ganglion cell types and their mosaics across species are discussed, taking into consideration both potential phylogenetic links and preliminary evidence for dendritic interactions. The importance of further mosaic studies is emphasised.

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