The retinofugal pathways in a primitive actinopterygian, the chondrostean Acipenser güldenstädti. An experimental study using degeneration, radioautographic and HRP methods.

ABSTRACT

Experimental study of the retinofugal pathways in Acipenser güldenstädti was carried out on 109 specimens using 3 experimental tracing methods: Fink-Heimer (after retinal ablation), radioautography (after intraocular injection of tritiated markers) and HRP (after intraocular injection of HRP or iontophoretic deposit of HRP on the optic nerve). The optic fiber was found to partially decussate at the chiasm and to project to 5 contralateral regions (1) hypothalamus (area optica hypothalami); (2) thalamus (area optica dorsalis thalami, area optica mediale thalami, nucleus thalamicus tractus optici marginalis, nucleus laminaris ventralis); (3) pretectum (nucleus pretectalis ventralis, nucleus commissurae posterioris, nucleus intercalaris lateralis); (4) optic tectum (superficial layers); and (5) mesencephalic tegmentum (area optica accessoria). The ipsilateral component was well developed and innervated the same regions mentioned above. A few contralateral optic fibers crossed again in the posterior commissure and terminated within ipsilateral visual pretectal structures. Although the architecture of the visual centers was less elaborated in this paleopterygian than in neopterygians (holosteans, teleosteans), we observed that the general organization of the retinal contralateral projections in this fish was comparable to that of more advanced actinopterygians. Our results indicate that this pattern was probably set at a very early date, perhaps as soon as the emergence of the chondrostean grade at the beginning of the Devonian.