The diencephalon of the Pacific herring, Clupea harengus: retinofugal projections to the diencephalon and optic tectum.

Abstract

The pattern of retinofugal projections to nuclei in the diencephalon and to the optic tectum was analyzed with horseradish peroxidase and autoradiographic methods in Clupea harengus, a clupeomorph teleost, for comparison with osteoglossomorph, elopomorph, and euteleost teleosts and with non-teleost actinopterygians. Most retinal fibers decussate in the optic chiasm and project to nuclei in the preoptic area, ventral and dorsal thalamus, posterior tuberculum, synencephalon, and pretectum, as well as to the accessory optic nuclei and optic tectum. Some ipsilateral projections do not decussate in the optic chiasm, while others decussate and recross via the supraoptic (minor) and posterior commissures. The pattern of projections is similar to that seen in other actinopterygian fishes with several exceptions. The terminal field usually present lateral to nucleus anterior in the dorsal thalamus is extremely reduced despite the relatively large size of the nucleus. A dense terminal field lies within the cell plate of nucleus corticalis in the pretectum rather than dorsal to it. The tectal hemisphere is composed of two distinct lobules, and the dorsal optic tract projects to the more rostromedial lobule while the ventral optic tract projects to the more caudolateral lobule. The lack of a significant projection to nucleus anterior and the lobular morphology of the optic tectum appear to be apomorphic for Clupea. Other features of the pattern of retinal projections are also analyzed in actinopterygian fishes including Clupea, and several hypotheses are advanced as to which traits are plesiomorphic for actinopterygians and/or for teleosts.