DEVELOPMENT AND ORGANIZATION OF THE OCULAR MOTOR NUCLEI IN THE LARVALSEA LAMPREY, PETROMYZON-MARINUS L - AN HRP STUDY

Retrograde transport of horseradish peroxidase (HRP) after its applica tion into the orbit was used to investigate the development of the dif ferent ocular motor nuclei in larvae of the sea lamprey (Petromyzon ma rinus) and to identify their regions of origin. In the smallest larvae studied (10-19 mm in length), the oculomotor and abducens neurons wer e ipsilateral to the site of HRP application, whilst trochlear neurons were contralateral. These motoneurons did not have dendritic processe s. In larvae more than 19 mm in length, both ipsilateral and contralat eral components were found in the oculomotor and trochlear nuclei; den drites were present, and their length and branching increased with lar val age. An adult-like pattern of topographic organization and dendrit ic arborization was reached in larvae of about 45-60 mm in length. In oculomotor neurons, medial dendrites appear first, then dorsolateral d endrites, and finally ventral dendrites. Similarly, in trochlear neuro ns ventral and ventrolateral dendrites develop first, followed by dors al dendrites that course either to the caudal optic tectum or to the t erminal fields of the octaval and lateral line nerves in the cerebella r plate. Dorsal and ventral dendrites of the abducens neurons arise at the same time, but dorsal dendrites attain an adult-like morphology e arlier. A few motoneurons showed ventricular attachments in larvae lon ger than 40 mm. The significance of these processes and their possible usefulness as a marker for the regions of origin of the ocular motor nuclei are discussed. Finally, the results presented here indicate tha t differentiation of the ocular motor nuclei in larval lampreys preced es and is independent of the maturation of the eye at transformation. (C) 1994 Wiley-Liss, Inc.