Temporal and spatial regulation of α6 integrin expression during the development of the cochlear-vestibular ganglion

The neurons of the cochlear-vestibular ganglion (CVG) that innervate the sensory hair cells of the inner ear are derived from the otic epithelium early in development. Neuroblasts detach from neighboring cells, migrate into the mesenchyme where they coalesce to form the ganglion complex, then send processes back into the epithelium. Cell migration and neuronal process formation involve changes in cellular interactions with other cells and proteins in the extracellular matrix that are orchestrated by cell surface-expressed adhesion molecules, including the integrins. I studied the expression pattern of the alpha 6 integrin subunit during the early development of the CVG using immunohistochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR) in murine tissue sections, otocyst, and ganglion explants. At embryonic day (E)10.5 alpha 6 integrin was expressed in the otic epithelium but not in migrating neuroblasts. Importantly, the loss of alpha 6 was associated with exit from the epithelium, not neuronal determination, revealing differentiation cues acutely associated with the cellular environment. Markers of glial and neuronal phenotype showed that alpha 6-expressing cells present in the CVG at this stage were glia of neural crest origin. By E12.5 alpha 6 expression in the ganglion increased alongside the elaboration of neuronal processes. Immunohistochemistry applied to otocyst cultures in the absence of glia revealed that neuronal processes remained alpha 6-negative at this developmental stage and confirmed that alpha 6 was expressed by closely apposed glia. The spatiotemporal modulation of alpha 6 expression suggests changing roles for this integrin during the early development of inner ear innervation.