Brain Research, 251 (1982) 1-23 1 Elsevier Biomedical Press Research Reports The Retinofugal Pathways in a Primitive Actinopterygian, the Chondrostean A cipenser giildenstiidti. An Experimental Study Using Degeneration, Radioautographic and HRP Methods J. REPI~RANT*, N. P. VESSELKIN, T. V. ERMAKOVA, E. K. RUSTAMOV, J. P. RIO, G. K. PALATNIKOV, J. PEYRICHOUX and R. V. KASIMOV Laboratory of Comparative Physiology of the Central Nervous System, Sechenov Institute of Evolutionary Physiology and Biochemistry Academy of Sciences, 44 Thorez Avenue, Leningrad 194223 (U.S.S.R.) ( N.P.V. and T. V.E.) ; (J.R. and J.P.R.) Laboratoire de Neuromorphologie, INSERM U-106 ttdpital Foch, Suresnes 92.150; (J.R. and J.P.) Laboratoire de Psychophysiologie Sensorielle, Universit~ Paris VI, Paris 75005 (France) ; and (G.M.P., R. V.K. and E.K.R.) Karaev Institute of Physiology, Academy of Sciences, Azerb. SSR, Baku 370100 (U.S.S.R.) (Accepted February 11th, 1982) Key words: retinofugal pathways -- actinopterygian -- horseradish peroxidase -- radioautography Experimental study of the retinofugal pathways in Acipenser giildenstiidti 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 aceessoria). 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. INTRODUCTION Consisting of more than 20,000 living species, the actinopterygii or ray-finned fish constitute the major vertebrate radiation 57. This class is traditionally subdivided into 3 evolutionary levels or grades of organization: chondrostei (or palaeopterygii), holostei and teleostei (or neopterygii)47-49,57, 91. The subclass chondrostei was differentiated by the beginning of the Devonian (palaeonisciformes)5, ~4, 47-49,91,98. During the Paleozoic, the evolution of these fish consisted in certain modifications of the original basic chondrostean condition ~4 and part of the chondrostean lineage evolved to the holostean level in the early MesozoicT,2a,49, 91. The Holostei nearly disappeared at the end of the Mesozoic after having given rise to the teleostei in the upper Jurassic 5,24,49,57. Although the teleostei are nowa- days highly represented, the chondrostei are, in turn, almost nonexistent. In fact, among the 200 genera of known Chondrostei 92, only 6 are still living today: Acipenser, Huso, Scaphirhyncus, Pseudoscaphirhyn- cus (Acipenseridae or sturgeons), Polyodon and Psephurus (Polyodontidae or paddlefish)S,L Both the Acipenseridae and the Polyodontidae (Acipensifor- mes) appeared more recently (upper Cretaceous) and * To whom reprint requests should be addressed at: Laboratoire de Neuromorphologie, INSERM U. 106, 42 rue Desbessayns de Richemont, 92.150 Suresnes, France. 0006-8993/82/~00/$02.75 © 1982 Elsevier Biomedical Press