Morphological diversity of displaced retinal ganglion cells in the rat: A lucifer yellow study

Abstract

Displaced retinal ganglion cells (DRGCs) were retrogradely labelled by injections of the fluorescent dye Fast Blue into the superior colliculi of pigmented rats. Following fixation these cells were intracellularly injected with Lucifer Yellow to determine their dendritic morphology and distribution. Graphic reconstruction of Lucifer Yellow‐filled prelabelled neurones revealed a heterogeneous population of DRGCs. Their stratification within the inner plexiform layer was diverse and cells were classified according to their dendritic morphology. The present sample consists largely of unistratifying neurones, the dendrites of which arborized within a narrow sublamina of the inner plexiform layer. They were characterized by a centrally located soma and densely branched dendritic network with little overlap within the branching pattern. In contrast, bistratifying DRGCs possessed a loose and sparsely branched dendritic structure, while diffusely stratifying neurones contained a high degree of dendritic crossing, culminating in a complex network, in which the soma position was biased toward the periphery. One type of DRGC bore a striking resemblance to type 1 neurones (Perry, 1979; Proc. R. Soc. Lond. [Biol.] 204:363–375) in the ganglion cell layer. They were characterized by a large soma (15.8 μm ± 2.2 μm s.d.) and a dendritic field diameter averaging 288 μm (s.d. ± 62 μm) and were on average larger than the rest of the displaced population but smaller than type 1 cells in the ganglion cell layer. Since the stratification patterns of the displaced and nondisplaced type 1 neurones were indistinguishable, it is reasonable to assume that the Lucifer Yellow‐filled cells in the present study represent the displaced counterpart of regular type 1 ganglion cells. Therefore, it appears that despite the aberrant location of the soma within the inner nuclear layer, the dendritic morphology of the cell develops unimpeded into a normal stratification pattern, although both soma and dendritic field size are reduced. If displacement is a developmental aberration, this size decrease may arise from overcrowding within the inner nuclear layer where the soma becomes lodged erroneously during its migration to the ganglion cell layer. This would also account for the heterogeneity of displaced cells found in the present study, since the randomness of a developmental error would ensure that a variety of ganglion cell types was included in the displaced population.