DEVELOPMENTAL RELATIONS BETWEEN 6TH NERVE MOTOR-NEURONS AND THEIR TARGETS IN THE CHICK-EMBRYO

The developmental relations between abducens (VI) nerves and their tar gets, the lateral rectus, quadratus, and pyramidalis muscles, have bee n examined in the chick embryo from early neural tube stages through 1 0 days of incubation. Sites of myoblast origins were determined by mic roinjection of replication-incompetent retroviruses containing the Lac Z reporter into paraxial mesoderm corresponding to somitomeres 3-5. Mo tor neurons and axons were identified by Bodian staining, immunocytoch emistry, and application of DiI and DiO to dissected peripheral nerves , Anlage of the dorsal oblique originate in somitomere 3, close to the ventrolateral margin of the mid-to-caudal mesencephalon. Precursors o f the lateral rectus arise deep within somitomere 4, beside the future metencephalon (rhombomere ''A''). Quadratus and pyramidalis precursor s are located between and partially segregated from these other two an lage. VIth nerve axons exit rhombomeres 5 and 6 via multiple median ro ots, fasciculate, and by stage 17 have elongated rostrally beneath the hindbrain. Immediately caudal to a mesenchymal pre-muscle condensatio n located deep to rhombomere 2, the VIth nerve separates into two bran ches. One branch enters the rostral portion of the condensation, from which quadratus and pyramidalis muscles will segregate. This branch pr ojects exclusively from rhombomere 5 and is the accessory abducens ner ve. The other branch enters the caudal, presumptive lateral rectus, re gion of the condensation. This is the abducens nerve, and it projects from cell located in both rhombomeres 5 and 6. These findings indicate that specific matching of motor nerves with their presumptive targets begins prior to the differentiation and segregation of myogenic popul ations, and that spatial organization of developing eye muscles is ini tiated well before they interact with connective tissue precursors der ived from the neural crest. (C) 1994 Wiley-Liss, Inc.