Innervation of the medial rectus muscle in the ratfish, Hydrolagus colliei

Oculomotor organization in elasmobranch fish (sharks, skates, and rays) differs from that in other vertebrates in that the medial rectus muscle is innervated by contralateral rather than ipsilateral motoneurons. Distinguishing whether this innervation pattern is unique to the elasmobranchs, or is the ancestral pattern for cartilaginous fishes, requires examination of a representative of the sister group to the elasmobranchs, the holocephalans (ratfish). In the present study, the innervation pattern of the medial rectus was examined in a ratfish, Hydrolagus colliei, by using biotinylated dextran amines (BDA, 3,000 MW). Labeled cells were revealed in the contralateral oculomotor nucleus. Therefore, an innervation pattern in which the medial rectus muscle is innervated by contralateral motoneurons is the primitive condition for cartilaginous fishes.