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 quest ion whether in chick this polarity is determined prior to, or at the o nset of retinal neurogenesis. We addressed the issue by constructing c ompound optic vesicles at Hamburger-Hamilton stages 10 to 11, before t he first ganglion cell progenitors differentiate. The retinae developi ng from such vesicles were examined to find out whether transplanted t issue retained its positional information or whether it was respecifie d in the host eye anlage. Positional information in embryonic day 6 (E 6) retinae was assayed with the stripe assay (Waiter et al., Developme nt 101: 685-696, 1987). This functional in vitro test allows us to dis tinguish temporal from nasal axons. Populations of ganglion cell axons growing out of retinae from operated embryos behaved as predicted fro m the position of their progenitors in the donor optic vesicle. Moreov er, inversion of the anteroposterior axis of optic vesicles led to the development of retinae with an inverted nasotemporal pattern. In orde r to investigate the retinotectal projection in vivo, a retrograde lab eling 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, howe ver, were labeled on both the nasal and temporal side. Together, these in vitro and in vivo results demonstrate that nasotemporal specificit y is determined in the optic vesicle at or prior to HH10-11.