Selective tracing of auditory fibers in the avian embryonic vestibulocochlear nerve

Axonal Cleaved Caspase-3 Regulates Axon Targeting and Morphogenesis in the Developing Auditory Brainstem

Caspase-3 is a cysteine protease that is most commonly associated with cell death. Recent studies have shown additional roles in mediating cell differentiation, cell proliferation and development of cell morphology. We investigated the role of caspase-3 in the development of chick auditory brainstem nuclei during embryogenesis. Immunofluorescence from embryonic days E6–13 revealed that the temporal expression of cleaved caspase-3 follows the ascending anatomical pathway. The expression is first seen in the auditory portion of VIIIth nerve including central axonal regions projecting to nucleus magnocellularis (NM), then later in NM axons projecting to nucleus laminaris (NL), and subsequently in NL dendrites. To examine the function of cleaved caspase-3 in chick auditory brainstem development, we blocked caspase-3 cleavage in developing chick embryos with the caspase-3 inhibitor Z-DEVD-FMK from E6 to E9, then examined NM and NL morphology and NM axonal targeting on E10. NL lamination in treated embryos was disorganized and the neuropil around NL contained a significant number of glial cells normally excluded from this region. Additionally, NM axons projected into inappropriate portions of NL in Z-DEVD-FMK treated embyros. We found that the presence of misrouted axons was associated with more severe NL disorganization. The effects of axonal caspase-3 inhibition on both NL morphogenesis and NM axon targeting suggest that these developmental processes are coordinated, likely through communication between axons and their targets.

[Microanatomy of the cranial segment of the vestibulocochlear nerve. Possible correlations with the symptoms of neurovascular compression syndrome]

The objective of the present study was to elucidate the topographic features of the nerve fibers belonging to the acoustic and vestibular analyzers located in the intracranial cranial segment of human vestibulocochlear nerve (VCN). A total of 16 samples of the intracranial cranial segment of the human vestibulocochlear nerve isolated from the region enclosed between the exit of VCN from the brainstem and its entrance into the internal acoustic meatus were available for the investigation. Prior to fixation of the samples, the VCN segments were marked in correspondence with their intravital anatomical location in the posterior cranial fossa. Cross sections of the PCN segments were stained with hematoxylin and eosin as well as according to the van-Hison method. The cross sections were made either at the exit of the nerve from the brainstem (N1), its entrance into the internal acoustic meatus (N3) or in-between these sites (N2). The morphometric analysis of the sections and the statistical treatment of the data obtained were performed with the use of the Diamfor hardware and software complex («Diamfor», Russia). The digitized images of the PCN sections were prepared using amVizo 103 microvisor (Russia). It was shown that the intracranial segment of the human vestibulocochlear nerve consists of two isolated groups of nerve fibers differing in terms of staining density, size, and the degree of myelinization. The mutual location of the fibers forming the cochlear and vestibular nuclei (CN and VN respectively) varies. Namely, CN near the internal acoustic foramen occupies the antero-posterior position with respect to VN. In the middle part of VCN, CN-forming fibers are located at the anetro-inferoposterior surface of the nerve. The nerve fibers of both CN and VN are similarly arranged near the lateral surface of the brain stem.

Differential roles for EphA and EphB signaling in segregation and patterning of central vestibulocochlear nerve projections

Auditory and vestibular afferents enter the brainstem through the VIIIth cranial nerve and find targets in distinct brain regions. We previously reported that the axon guidance molecules EphA4 and EphB2 have largely complementary expression patterns in the developing avian VIIIth nerve. Here, we tested whether inhibition of Eph signaling alters central targeting of VIIIth nerve axons. We first identified the central compartments through which auditory and vestibular axons travel. We then manipulated Eph-ephrin signaling using pharmacological inhibition of Eph receptors and in ovo electroporation to misexpress EphA4 and EphB2. Anterograde labeling of auditory afferents showed that inhibition of Eph signaling did not misroute axons to non-auditory target regions. Similarly, we did not find vestibular axons within auditory projection regions. However, we found that pharmacologic inhibition of Eph receptors reduced the volume of the vestibular projection compartment. Inhibition of EphB signaling alone did not affect auditory or vestibular central projection volumes, but it significantly increased the area of the auditory sensory epithelium. Misexpression of EphA4 and EphB2 in VIIIth nerve axons resulted in a significant shift of dorsoventral spacing between the axon tracts, suggesting a cell-autonomous role for the partitioning of projection areas along this axis. Cochlear ganglion volumes did not differ among treatment groups, indicating the changes seen were not due to a gain or loss of cochlear ganglion cells. These results suggest that Eph-ephrin signaling does not specify auditory versus vestibular targets but rather contributes to formation of boundaries for patterning of inner ear projections in the hindbrain.