Analysis of EphB receptors and their ligands in the developing retinocollicular system of the wallaby reveals dynamic patterns of expression in the retina.

The expression of EphB1 and B2 receptors and ephrins-B1, -B2 and -B3 in the retina and superior colliculus of the wallaby (Macropus eugenii) was examined during the development of the retinocollicular projection, using reverse transcription-polymerase chain reaction and immunohistochemistry. There was an early transient differential expression of EphB2 that was higher in ventral retina and restricted to the outer neuroblast layer, whereas a high ventral to low dorsal gradient of ephrin-B2 expression occurred there throughout the study period. However, there was no dorsoventral gradient of receptors or ligands in retinal ganglion cells or a mediolateral gradient of ephrins in the colliculus. These findings suggest a limited role for these molecules in topographic mapping across the mediolateral colliculus in the wallaby. Early in retinal development there is a complementary pattern of expression of ephrin-B1 and -B2 in the outer neuroblast layer that overlaps with expression of EphB2. Ganglion and amacrine cells also express EphB2. As development proceeds subpopulations of putative horizontal and bipolar cells, also expressing EphB2, come to reside in the inner nuclear layer and ephrin-B1 is expressed throughout the outer nuclear layer. At the same time cells expressing ephrin-B2, and subpopulations of horizontal and bipolar cells come to reside in the inner nuclear layer and there is a corresponding decrease in ephrin-B2 expression in the outer nuclear layer. This pattern of coexpression of receptors and ligands suggests a role for them in cell migration and maintenance of laminar boundaries.