Early Determination of Nasal-Temporal Retinotopic Specificity in the Eye Anlage of the Chick Embryo

Abstract

The retinotectal projection of the chick is established between Embryonic Days 3 and 13 (E3 to E13). Fate mappings of the eye anlage by local injections of the fluorescent dyes DiI and DiA revealed that the anteroposterior axis of the optic vesicle corresponds to the nasotemporal axis of the retina. To investigate possible alterations in retinotopic specificity after ablating parts of the early eye anlage, we resected either most of the presumptive temporal or a large part of the presumptive nasal half of the eye anlage around stage 11 of the Hamburger-Hamilton scale (40-45 hr). After such treatment, the axes are restored in the healed optic vesicle. In the healing process the wound is closed by cells moving in from surrounding areas. After early posterior (i.e., temporal) ablation, the projection from the restored temporal half-retina onto the optic tectum was examined in embryos (E13 to E17) and juvenile chicken (P16) by retrograde and anterograde labeling of ganglion cells and their axons with DiI and DiASP. Normally, only a small fraction of ganglion cells from the temporal retina (between 6.4% on E13 and 0.08% on P16) projects onto the caudal part of the tectum. In experimental embryos and juvenile chicken this fraction is significantly increased (up to 80%). Retrograde double-labeling from the rostral and the caudal tectum reveals that temporal cells project onto either the rostral or the caudal tectum, but not via collaterals upon both areas. The ganglion cells with "displaced nasal" identity within the temporal retina that were backlabeled from the caudal tectum were to a large extent segregated into distinct clusters, indicating their derivation from few or possibly even single progenitor cells. Likewise, ablation of the anterior half of the optic vesicle led to clusters of rostrally projecting cells of "displaced temporal" identity within the restored nasal retina. In these experiments the dorsal-ventral retinotectal relationship remained intact. The simplest, though not exclusive interpretation of the findings is that positional information is imposed as a temporal-to-nasal step function already on the multipotent progenitors of the ganglion cells in the eye anlage by stage 11 or earlier. The positional values remain stable when during the healing of the optic vesicle cells of one specification become surrounded by cells of another specification.