Overlapped and differential expression of cAMP-dependent kinase-inhibitor isoforms during avian organogenesis period

Otic placode cell specification and proliferation are regulated by Notch signaling in avian development

BACKGROUND

The entire inner ear including the cochlear-vestibular ganglion arises from a simple epithelium, the otic placode. Precursors for the placode originate from a pool of progenitors located in ectoderm next to the future hindbrain, the pre-otic field, where they are intermingled with future epibranchial and epidermal cells. While the importance of secreted proteins, such as FGFs and Wnts, in imparting otic identity has been well studied, how precursors for these different fates segregate locally is less well understood.

RESULTS

(1) The Notch ligand Delta1 and the Notch target Hes5-2 are expressed in a part of pre-otic field before otic commitment, indicative of active Notch signaling, and this is confirmed using a Notch reporter. (2) Loss and gain-of-function approaches reveal that Notch signaling regulates both proliferation and specification of pre-otic progenitors.

CONCLUSIONS

Our results identify a novel function of Notch signaling in cell fate determination in the pre-otic field of avian embryos.

Mutual repression between Pax3 and Pax6 is involved in the positioning of ophthalmic trigeminal placode in avian embryo

Cranial sense organs and a subset of cranial sensory neurons are generated from placodes, thickenings of the ectoderm. Pax3 has been known as a marker for ophthalmic trigeminal placode specification, and also an important regulator of trigeminal placode neuron differentiation. In this study, I show that Pax6 is initially expressed in the preplacodal region at the level of ophthalmic trigeminal placode, and that this expression gradually regresses in a medial-to-lateral direction as Pax3 expression expands in the same direction. Misexpression studies revealed that Pax6 represses Pax3 expression indirectly as a transcriptional activator in a cell-autonomous manner. Pax3-misexpression represses Pax6 expression in an indirect fashion, suggesting that unknown factor(s) downstream of Pax3 may repress Pax6 expression, and thereby allow an expansion of Pax3-positive ophthalmic trigeminal placode region. These results indicate that the mutual repression between Pax3 and Pax6 has important roles in the specification and the positioning of the ophthalmic trigeminal placode.