Reanalysis of double EphA3 knockin maps in mouse suggests that stochasticity in topographic map formation acts within the retinal cell population rather than between competing mechanisms at the colliculus

Abstract

Double visual maps in the EphA3 knockin mouse have been used extensively to investigate the mechanisms for the formation of ordered nerve connections. Here, EphA3+ and EphA3- retinal ganglion cells, forming a mosaic, each make a separate map on the superior colliculus. We constructed ordered 2D maps from EphA3 knockin Fourier-based intrinsic imaging data. Over both EphA3+/- and EphA3+/+ genotypes, the extent and location in the visual field of each projection of a double map vary considerably. Aided by computer modelling results, we suggest that all EphA3 knockin maps are examples of a single type, with the amount of EphA3 in EphA3+ retinal cells varying between animals. Previous analysis of the same dataset suggested that EphA3+/- maps represent an unstable transition between single (wild type) and double (EphA3+/+) maps, reflecting a balance between molecular and electrical signalling. In contrast, our analysis highlights the importance of molecular-based cues in establishing the retinotopic map.