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Otic vesicle, or auditory vesicle, consists of two sac-like invaginations formed during embryonic development. It is part of the neural ectoderm, which will develop into the membranous labyrinth of the inner ear. The membranous labyrinth of the inner ear is a continuous epithelium, giving rise to the vestibular and auditory components of the inner ear. During the earlier stages of embryogenesis, the otic placode invaginates to produce the otic cup. Subsequently, the otic cup closes off, creating the otic vesicle. Once formed, the otic vesicle will reside next to the neural tube medially, and on the lateral side will be somitomeric mesoderm. Neural crest cells will migrate rostral and caudal to the placode.

The sequence in formation of the otic vesicle is relatively conserved across vertebrates. The patterning during morphogenesis into the distinctive inner ear structures is determined by homeobox transcription factors, Pax2, Dlx5 and Dlx6, with the former specifying for ventral otic vesicle derived auditory structures and the latter two specifying for dorsal vestibular structures.

Gene Signaling

The Fgf, Bmp, Wnt and Pax genes are likely to be involved in otic induction. Ffg and BMP signals help control patterning in the early otic vesicle. Fgf3 and Fgf10 were suggested to play a role in otic induction in mice, as were Msx genes suggested to play a role in otic vesicle formation in chicks. Pax8 is expressed during the entirety of otic vesicle formation. Other genes found in the otic vesicle across species that may play a role in patterning include Hmx, Fox, Dlx, and Gbx genes. Signaling during inner ear development involves typical developmental signaling cascades involving genes such as the Tgfbeta super family, fibroblast growth factor family (Fgf), Sonic hedgehog (Shh), forkhead genes (Fox), a family of winged-helix transcription factors and T-box genes (Tbx). Notable signaling cascades such as the Wnt and Notch signaling cascades are also involved in inner ear development following formation of the otic vesicle.. Signaling cues not only extend from the inner ear, but also the hindbrain, neural tube and notochord, where sources for Shh, Wnt and Gbx2 can be found.

Parcellation, divisions and fates

The otic vesicle is uniquely derived from the neural ectoderm. The early otic vesicle is characterized as having broad competence and can be subdivided into sensory, non-sensory, and neurogenic components. Sensory epithelial cells and neurons are derived from the proneurosensory domain. This domain can be further sub-categorized into the neurogenic sub-domain and prosensory sub-domain. Prosensory sub-domain eventually gives rise to the support cells and hair cells while the neurogenic sub-domain gives rise to the auditory neuron and vestibular neuron.

Variation across Species

Formation of the otic vesicle has been studied extensively in developmental model organisms including chick, Xenopus, zebrafish, axolotyl, & mouse. The transition from the otic placode to the otic vesicle occurs during the 19th somite stage in Zebrafish, Xenopus, and chick. In chick, invagination of the otic placode occurs passively due to the the movements of the surrounding placode. The otic placode in zebrafish, on the other hand, occurs by cavitation; the ectodermal placode condenses and forms an ovoid ball directly below the embryo surface. Otic vesicle formation occurs later, during the 25-30 somite stage in mice.

References

References:

1 Chatterjee S, Krausl P, Lufkin T: A symphony of inner ear developmental control genes. BMC Genet 2010, 11:68 doi:10.1186/1471-2156-11-68

2 Noramly, S. and Grainger, R. M. (2002), Determination of the embryonic inner ear. J. Neurobiol., 53: 100–128. doi: 10.1002/neu.10131

3 Appler JM, Goodrich LV. Connecting the ear to the brain: molecular mechanisms of auditory circuit assembly. Progress in neurobiology. 2011;93(4):488-508. doi:10.1016/j.pneurobio.2011.01.004.

4 Freyer L, Aggarwal V, Morrow BE. Dual embryonic origin of the mammalian otic vesicle forming the inner ear. Development (Cambridge, England). 2011;138(24):5403-5414. doi:10.1242/dev.069849.

5 Whitfield, T. T., Riley, B. B., Chiang, M.-Y. and Phillips, B. (2002), Development of the zebrafish inner ear. Dev. Dyn., 223: 427–458. doi: 10.1002/dvdy.10073