Fiber pathways and positional changes in efferent perikarya of 2.5‐to 7‐day chick embryos as revealed with dil and dextran amiens

Abstract

The differentiation of facial motoneurons and inner ear (octaval) efferents was examined in chicken embryos by applying DiI or dextran amines to the cut VII/VIII nerve (peripheral label) or to the basal/floor plate 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 incubation. 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 facial nerve and the inner ear and that the octaval efferent axons had reached 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 consisted of octaval efferents and central label applied to the floor plate of rhombomeres 4/5 identified fibers that entered the octaval nerve via the facial root and entered the vestibular sensory epithelia. Together these data suggest an initial mingling of two different motoneuron populations (facial and octaval) in rhombomeres 4/5 and a subsequent segregation by differential migration. Our data also find a much earlier arrival of octaval efferent axons at the otic vesicle than previously described and suggest a contralateral migration of many octaval efferents beginning shortly after their axons reach the facial nerve root.

Timing and topography of nucleus magnocellularis innervation by the cochlear ganglion

“…This path is the same as previously described by Boord (1961) for the cochlear and lagenar efferents in caiman and pigeon. Since others have studied the efferent projection in detail (Boord, 1961;Fritzsch et al, 1993;Simon and Lumsden, 1993;Code, 1995;Fritzsch, 1996;Kaiser and Manley, 1996;Manley and Köppl, 1998), we did not examine it except to see if any efferents sent collateral axons to NM or NA. We did not observe any labeled efferents projecting to NM or NA as a result of our injections and conclude that NM and NA do not receive collaterals from any inner ear efferent projections during the time examined in this study.…”

T-Box transcription factor Tbx20 regulates a genetic program for cranial motor neuron cell body migration

“…Facial BM neurons migrate tangentially from r4 into r6 orthogonal to the radial glial fibers, whereas trigeminal BM neurons in r2 migrate dorsolaterally along a non-radial pathway. Vestibuloacoustic cells display a third type of cell body movement, migrating across the midline at r4 to the contralateral side of the hindbrain (Chandrasekhar, 2004;Fritzsch et al, 1993;Simon and Lumsden, 1993). The last step in the migration of these motor neuron subtypes is shared, comprising the radial migration of the cell bodies toward their final settling position near the pial surface of the neural tube.…”

Development of vestibular afferent projections into the hindbrain and their central targets

“…3), suggesting that mammalian auditory nuclei derive from rhombomeres 2 to 6. Unfortunately, no such specific basilar papilla data on initial projections into rhombomeres exist in birds in which only the entire ear projections have been mapped during early development when they can be related to rhombomeres [40].…”