AXONAL TARGET CHOICE AND DENDRITIC DEVELOPMENT OF FERRET BETA-RETINALGANGLION-CELLS

We have investigated the relationship between axon targeting and dendr itic morphology in beta retinal ganglion cells in the postnatal ferret . Axonal projections were assessed by making separate injections of di fferent fluorescent retrograde tracers into either the superior collic ulus or lateral geniculate nucleus in vivo. The dendritic morphology o f retrogradely labelled cells was revealed by the in vitro intracellul ar injection of lucifer yellow in fixed retina. In this way, 405 retin al ganglion cells were triple- or double-labelled and characterized by their dendritic branching styles. Both the distinct dendritic morphol ogy of beta cells and the characteristic restriction of their adult ax onal terminals to the lateral geniculate nucleus emerge postnatally. B eta cell dendritic morphology is established between postnatal days 5 and 9. As in the cat (Ramoa et al., 1989), beta cells extend and then retract a projection to the superior colliculus as part of their norma l development. Transient beta axonal collaterals to the superior colli culus persist beyond the period of cell death, but nearly all are with drawn by postnatal day 15. No dendritically distinct beta cell project s to the superior colliculus alone, at any age. Heterochronic injectio ns of different colours of retrograde tracer into the superior collicu lus were used to study changes in the complement of the retinocollicul ar projection over time. A significant proportion of cells (58%) label led at postnatal day 0 from the superior colliculus, which subsequentl y survived the period of cell death, were found to be beta cells that could no longer be demonstrated to have a retinocollicular axon. Neona tal decortication, which reduced the volume of the adult lateral genic ulate nucleus by 36-86%, resulted in the limited stabilization of this normally transient beta cell projection to the superior colliculus. T he fundamental dendritic branching style of beta ganglion cells is unc hanged in decorticate ferrets, suggesting that it develops independent ly of their ultimate axonal target choice.