We have investigated the relationship between axon targeting and dendritic morphology in beta retinal ganglion cells in the postnatal ferret. Axonal projections were assessed by making separate injections of different fluorescent retrograde tracers into either the superior colliculus or lateral geniculate nucleus in viva The dendritic morphology of retrogradely labelled cells was revealed by the in vitro intracellular injection of Lucifer yellow in fixed retina. In this way, 405 retinal ganglion cells were triple- or double-labelled and characterized by their dendritic branching styles. Both the distinct dendritic morphology of beta cells and the characteristic restriction of their adult axonal terminals to the lateral geniculate nucleus emerge postnatally. Beta 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 normal development. Transient beta axonal collaterals to the superior colliculus persist beyond the period of cell death, but nearly all are withdrawn by postnatal day 15. No dendritically distinct beta cell projects to the superior colliculus alone, at any age.
Heterochronic injections of different colours of retrograde tracer into the superior colliculus were used to study changes in the complement of the retinocollicular projection over time. A significant proportion of cells (58%) labelled at postnatal day O from the superior colliculus, which subsequently survived the period of cell death, were found to be beta cells that could no longer be demonstrated to have a retinocollicular axon. Neonatal decortication, which reduced the volume of the adult lateral geniculate nucleus by 36–86%, resulted in the limited stabilization of this normally transient beta cell projection to the superior colliculus. The fundamental dendritic branching style of beta ganglion cells is unchanged in decorticate ferrets, suggesting that it develops independently of their ultimate axonal target choice.