Transplantations of the chick eye anlage reveal an early determination of nasotemporal polarity.

Cells in the optic vesicle must acquire positional values in order to express the molecular cues necessary for the later establishment of a topographically organized retinotectal projection. It is an open question whether in chick this polarity is determined prior to, or at the onset of retinal neurogenesis. We addressed the issue by constructing compound optic vesicles at Hamburger-Hamilton stages 10 to 11, before the first ganglion cell progenitors differentiate. The retinae developing from such vesicles were examined to find out whether transplanted tissue retained its positional information or whether it was respecified in the host eye anlage. Positional information in embryonic day 6 (E6) retinae was assayed with the stripe assay (Walter et al., Development 101: 685-696, 1987). This functional in vitro test allows us to distinguish temporal from nasal axons. Populations of ganglion cell axons growing out of retinae from operated embryos behaved as predicted from the position of their progenitors in the donor optic vesicle. Moreover, inversion of the anteroposterior axis of optic vesicles led to the development of retinae with an inverted nasotemporal pattern. In order to investigate the retinotectal projection in vivo, a retrograde labeling experiment was employed. In control retinae, most ganglion cells labeled from the caudal tectum were confined to the nasal side on E13. Retinae that had developed from double-anterior optic vesicles, however, were labeled on both the nasal and temporal side. Together, these in vitro and in vivo results demonstrate that nasotemporal specificity is determined in the optic vesicle at or prior to HH10-11.