Early development of retinal ganglion cell dendrites in the marsupial Setonix brachyurus, Quokka

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Abstract
The dendritic morphology of retinal ganglion cells was studied in flatmounted retinae of the marsupial Setonix brachyurus, quokka. In the adults, horseradish peroxidase (HRP) was applied to the vitread surface of flattened retinae. Wide‐, large‐, medium‐, and small‐field classes appeared to correspond to gamma, alpha, delta, and beta cells, respectively, in the cat (Boycott and Wässle, J. Physiol.249:397‐419, 1974). To reveal the early stages of dendritic development, HRP was placed on the optic nerve of isolated eye cups from the day of birth to postnatal day (P) 63 when the area centralis is beginning to form (Dunlop and Beazley, Dev. Brain Res.23:81‐90, 1985). Youngest cells lacked dendrites and had an elongate soma in the cytoblastic layer with an endfoot contacting the ventricular surface. Once in the ganglion cell layer, the soma was rounded and dendrites appeared as short, unbranched processes. Most cells were asymmetric or „polarised”︁ with the axon arising from the side nearest the optic disk and dendrites from the opposite side. Polarity was maintained in cells with longer, branched dendrites. A small proportion of cells exhibited a reversed polarity in which the axon arose from the side nearest the retinal edge and dendrites towards the disk. Cells appeared to acquire an approximately symmetric, adult‐like tree by the addition of new primary dendrites between the existing ones and the axon hillock. Wide‐, large‐, medium‐, and small‐field cells were evident from P6, P25, P31, and P40, respectively. Spines were observed on dendrites and axons during development but were rare in the adult. Some den‐dro‐axons were seen at all ages examined. The existence of an initial axodendritic polarity in retinal ganglion cells supports the hypothesis that the axon hillock is the determinant of dendritic geometry (Maffei and Perry, Dev. Brain Res. 41:185‐194, 1988). Polarity may also contribute to the establishment of „radial orientation”︁ in which the long axis of the elliptical dendritic tree of cells outside the area centralis points towards central retina and the weighted centre is displace towards the retinal periphery (Leventhal and Schall, J. Comp. Neurol. 220:465‐475, 1983).