Location of retinal ganglion cells contributing to the early imprecision in the retinotopic order of the developing projection to the superior colliculus of the wallaby (Macropus eugenii)

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Abstract
The position of ganglion cells contributing to the early imprecision in retinotopic order in the developing retinocollicular projection in the wallaby (Macropus eugenii) has been determined.Deposits of horseradish peroxidase conjugated to wheat germ agglutinin (WGA‐HRP) were made in the caudal pole of the superior colliculus (SC) at ages ranging from 22 days after birth, when sparse retinal axons have only just reached the caudal pole of the SC and are yet to cover its surface completely, to 96 days when the retinotopy of ganglion cell terminals in the SC is precise (Marotte, '90). From 30 days onwards, the deposit of WGA‐HRP resulted in a dense patch of retrogradely labelled retinal ganglion cells that could be seen to be appropriately positioned in nasal retina. However, at all ages prior to 92 days, there were inappropriately positioned labelled cells between the densely labelled patch and the central retina and both dorsal and ventral to the patch. They were not found in far distant regions of retina and composed a relatively small proportion of labelled cells. They reached a peak at 45 days, had decreased to low levels by 63 days, were rare by 81 days, and by 92 days were absent. This latter age fits with the time when retinotopy was judged to be precise in a previous study (Marotte, '90).Inappropriately projecting cells never originate from the entire retina but only from regions adjacent to the appropriate region. Thus, during development there are no gross projection errors. Initially, ganglion cell axons are distributed on the colliculus in a coarse retinotopy. Refinement of the projection then follows, revealed by this technique as a loss of inappropriately projecting ganglion cells. This is complete by 92 days well before eye opening at around 140 days.