Retinal ganglion cells (RGCs) usually increase their dendritic field
area with postnatal retinal growth. The mechanisms that regulate the
postnatal shape of dendritic arbors in the growing retina are not well
understood. Quantitative studies suffer from the difficulty of labeling
specific subpopulations of RGCs selectively including their dendritic
processes. In this study, we labeled displaced retinal ganglion cells
(DGC) that are known to project to the accessory optic system (AOS) in
juvenile and adult chameleons by retrograde transport of dextran
amines. The complete population of DGCs was quantitatively screened for
the effects of postnatal retinal growth on cell morphology, dendritic
field coverage, and dendritic arbor size. The adult eye contained 2000
DGCs/retina. This number was already present at birth. The smaller
size of the hatchling eye (approximately 1/3 of the adult size) led
to higher densities of DGCs. The greatest accumulation of juvenile DGCs
(two-fold higher compared to the adult) was found in the periphery of
the retina where the greatest surface expansion was observed. DGC
dendritic field areas were adjusted proportionally to this expansion in
order to maintain a constant dendritic coverage. The increase of
dendritic fields was mediated by two putative passive mechanisms:
First, an elongation of individual dendrites similar to previous
reports of postnatal RGC development in the retina of goldfish and
chicks. Second, and more prominent, we observed that neighboring
dendrites were pulled apart from each other. This resulted in a looser
spacing of the initially tightly packed dendrites of each dendritic
arbor. This dispersal of dendrites over a larger area was, due to its
passive nature, proportional to the increase of the retinal surface and
preserved a constant dendritic coverage irrespective of the
animal's age and eye size.