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Transient retinofugal pathways in the developing chick
Insights
Early chick embryo development reveals a transient ipsilateral retinal projection to visual nuclei. This temporary pathway disappears by day 15, coinciding with retinal ganglion cell death.
Area of Science:
- Neuroscience
- Developmental Biology
- Ophthalmology
Background:
- The central retinal projections guide visual information processing.
- Understanding early visual pathway development is crucial for identifying developmental abnormalities.
Purpose of the Study:
- To investigate the presence and characteristics of a transient ipsilateral retinal projection in normal chick embryos.
- To determine if this projection is a normal developmental feature or an anomaly.
Main Methods:
- Utilized anterograde transport of horseradish peroxidase (HRP) with tetramethyl benzidine (TMB) chromogen.
- Employed degeneration techniques to trace retinal axon distribution.
- Examined chick embryos at successive developmental stages (6-16 days of incubation).
Main Results:
- A transient ipsilateral retinal projection to primary visual nuclei was observed in embryos between 6 and 12 days of incubation.
- A projection into the contralateral optic nerve was also detected during this period.
- These anomalous projections disappeared by embryonic day 15, coinciding with significant retinal ganglion cell apoptosis.
Conclusions:
- Chick embryos normally exhibit a transient ipsilateral retinofugal projection early in development.
- The observed ipsilateral projection may contribute to the apparent ipsilateral retinofugal projection seen after embryonic enucleation in chicks, similar to findings in rats.
Abstract:
The central retinal projections have been examined in normal chick embryos early in development to determine if a transient ipsilateral projection is present. Anterograde transport of HRP identified using tetramethyl benzidine as a chromogen and degeneration techniques were used on embryos of successive ages between 6 and 16 days of incubation to show the central distribution of the retinal axons from one eye. Besides the anticipated contralateral projection, a small projection was identified to primary visual nuclei on the side of the brain ipsilateral to the injected or lesioned eye in embryos between days 6 and 12 of incubation. A projection from the injected eye into the contralateral optic nerve was also identified in a number of embryos. By embryonic day 15 the retinal projection to the ipsilateral side of the brain and into the contralateral optic nerve had disappeared. This loss of the anomalous projections coincides with a period of substantial cell death in the retinal ganglion cell layer. It appears, therefore, that in the chick, like the rat, the ipsilateral retinofugal projection resulting from an embryonic enucleation may in part be due to retention of a normal ipsilateral projection.