Dendritic remodelling of retinal ganglion cells during development of the rat

Investigation of the morphology of ganglion cells in the cat retina has shown that a remarkable reduction in the number of dendritic spines and branches occurs during development of the alpha and beta cell classes. To learn whether dendritic remodelling represents a generalized mechanism of mammalian retinal ganglion cell development, we have examined the morphology of ganglion cells in the retina of the developing rat. The present study has concentrated on type II cells, which retain a great number of dendritic spines and branches in the adult and comprise a large proportion of the population of rat retinal ganglion cells. To reveal fine dendritic and axonal processes, Lucifer yellow was injected intracellularly in living retinae maintained in vitro. Size and complexity of the dendritic trees were found to increase rapidly during an initial stage of development lasting from late fetal life until approximately postnatal day 12 (P12). Dendrites and axons of immature ganglion cells expressed several transient morphological features comprising an excessive number of dendritic branches and spine-like processes, and short, delicate axonal sidebranches. The following developmental stage was characterized by a remarkable decrease in the morphological complexity of retinal ganglion cells and a slowed growth of their dendritic fields. The number of dendritic branches and spines of types I and II retinal ganglion cells declined after P12 to reach a mature level by the end of the first postnatal month. Thus, even cells that retain a highly complex dendritic tree into the adult state undergo extensive remodelling. These results suggest that regressive modifications at the level of the dendritic field constitute a generalized mechanism of maturation in mammalian retinal ganglion cells.