Differential retinal growth appears to be the primary factor producing the ganglion cell density gradient in the rat.

We have studied the distribution of retinal ganglion cells (RGCs) that had been retrogradely labelled from bilateral injections of horseradish peroxidase into the retino-recipient nuclei of albino rats aged from the 22nd postconceptional day (22PCD-day of birth) to adulthood. During the period in which most (85%) of the naturally occurring RGC loss takes place (22-26 PCD) the distribution of RGCs remains almost uniform. Between the 26 and 32PCD (11th postnatal day), the peak RGC density decreases by only 20% while the RGC density at the superior retinal periphery decreases by 80%. In the same period a centro-peripheral RGC density difference of 4:1 becomes apparent. We have interpreted these changes to be due to a phase of rapid differential retinal growth (with more growth occurring at the retinal periphery). Thereafter the reduction in RGC density (and presumably retinal growth) is more uniform resulting by adulthood in a centro-peripheral RGC density ratio of 5:1.