FIBER PATHWAYS AND POSITIONAL CHANGES IN EFFERENT PERIKARYA OF 2.5-DAY TO 7-DAY CHICK-EMBRYOS AS REVEALED WITH DII AND DEXTRAN AMINES

The differentiation of facial motoneurons and inner ear (octaval) effe rents was examined in chicken embryos by applying Dil or dextran amine s to the cut VII/VIII nerve (peripheral label) or to the basal/floor p late of rhombomeres 4/5 (central label). Central labeling found axons of these efferent neurons to leave the brain as early as 2.5 days of i ncubation. Peripheral labeling identified cell bodies ipsilaterally in rhombomeres 4 and 5 at 2.5 days. Central labeling at 3.5 days showed these fibers to have fully segregated into separate pathways to the fa cial nerve and the inner ear and that the octaval efferent axons had r eached the otocyst wall. By 3.5 days many peripherally labeled octaval efferent somata were found in the floor plate and by 5 days they were found bilaterally. At 6 days, selective peripheral labeling of either the VIIth or VIIIth nerve showed that the contralateral population co nsisted of octaval efferents and central label applied to the floor pl ate of rhombomeres 4/5 identified fibers that entered the octaval nerv e via the facial root and entered the vestibular sensory epithelia. To gether these data suggest an initial mingling of two different motoneu ron populations (facial and octaval) in rhombomeres 4/5 and a subseque nt segregation by differential migration. Our data also find a much ea rlier arrival of octaval efferent axons at the otic vesicle than previ ously described and suggest a contralateral migration of many octaval efferents beginning shortly after their axons reach the facial nerve r oot. (C) 1993 John Wiley & Sons, Inc.