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Absence of topography in precociously innervated tecta
C B Chien1, E M Cornel, C E Holt
1Department of Biology, University of California, San Diego, La Jolla 92093-0366 USA.
Development (Cambridge, England)
|August 1, 1995
Summary
Early Xenopus retinal axon development shows topographic mapping cues in the tectum are absent before stage 39. Premature innervation prevents normal ordering, suggesting distinct molecules guide pathfinding versus topographic mapping.
Area of Science:
- Neuroscience
- Developmental Biology
- Axon Guidance
Background:
- The retinotectal map in Xenopus exhibits topographic ordering along the dorsoventral axis by stage 39.
- Retinal axons arrive at the tectum already topographically organized.
Purpose of the Study:
- To investigate the presence and timing of topographic cues in the developing Xenopus retinotectal system.
- To determine if tectal topographic cues are present before retinal axons normally arrive.
Main Methods:
- Heterochronic transplantation of stage 28 eye primordia into stage 20 hosts to induce premature innervation.
- Focal lipophilic dye injections into dorsal and ventral retina of donor embryos (stages 39-40) to assay topography.
- Analysis of axon ordering in the optic tract and tectum in control and experimental groups.
Main Results:
- Control projections showed normal dorsoventral ordering in the optic tract and tectum.
- Prematurely innervating retinal axons were ordered in the tract but failed to show significant dorsoventral ordering in the tectum.
- Axons entering the tectum precociously made incorrect turns, indicating a lack of topographic guidance.
Conclusions:
- Tectal topographic cues are absent a few hours before normal retinal axon arrival (stage 39).
- This absence is temporary, as later-stage transplants showed normal topography.
- Molecules for pathfinding in the diencephalon and target recognition differ from those providing tectal topographic signals.