Early Development of the Spiral Ganglion

The spiral ganglion develops in parallel with the patterning and morphogenesis of the inner ear (Fig. 2.1). The inner ear arises from the otic placode, an ectodermal thickening that forms adjacent to the 5th and 6th rhombomeres of the hindbrain in vertebrates other than the lamprey (Kuratani et al., 1998). This occurs at the 8–10 somite stage of development, which corresponds to E8.5 in mouse (Mus musculus) (Anniko & Wikstrom, 1984) and stage 10 in chicken (Gallus gallus) (Hemond & Morest, 1991a). In birds and rodents, the placode subsequently invaginates and deepens to become an otic cup (Knowlton, 1967; Marovitz et al., 1977; Anniko & Schacht, 1984). The otic cup then detaches from the ectoderm and seals to form an ovoid otic vesicle that is closely apposed to the hindbrain and surrounded by mesenchyme. Inner ear neurons develop from precursors in the anteroventral quadrant of the otic vesicle, which leave the epithelium and proliferate to form the cochlear-vestibular ganglion (CVG) just rostral to the developing inner ear. At this stage, the cochlear and vestibular ganglia are distinct yet closely associated with each other, as well as with the geniculate ganglion (Fig. 2.1), forming a three-ganglion complex. Morphologically, neuroblasts can be recognized during otic cup stages as otic epithelial cells that lose their columnar morphology and delaminate from the epithelium (Carney & Silver, 1983; Hemond & Morest, 1991b), forming a distinct CVG by the 22–24 somite stage in mice (Wikstrom &