Mutational analysis of tissue-tissue interaction required for otic placode induction in zebrafish

Abstract

Development of the vertebrate inner ear begins with aphics. the induction of the otic placate. Several embryonic tissues have been implicated as potential sources of otic inducing signals, including cephalic mesoderm, notochord, and hindbrain. However, the relative contribution of each tissue has not been determined, nor have any genes controlling placate induction been identified. To address these issues, we analyzed otic placate induction in zebrafish mutants that are deficient in prospective otic inducing tissues. The induction of placates, as monitored by morphological observations, was delayed by 3h in mutants that are deficient in cephalic mesoderm. In contrast, there were no perturbations in the timing of otic development in mutants deficient in notochord hindbrain differentiation. Similarly, induction of the otic placate, as seen by upregulation of dlx-3 and pax-2 in pre-otic cells, was delayed by up to 313 in mutants that are deficient in cephalic mesoderm while the timing of otic development in mutants deficient in notochord or hindbrain differentiation was normal. In all mutants studied, expression of marker genes in other regions of the neural plate was not delayed, indicating that otic induction was specifically delayed in mesoderm deficient mutants. Our data suggest that the cephalic mesoderm provides the first signals required for otic induction during gastrulation. In contrast, the data suggest that the notochord plays is not required for this process. Because the hindbrain mutant studied here is deficient for only a portion of the hindbrain, we cannot rule out a role for that tissue in the regulation of otic placate development. However, the observation that this mutation did not potentiate defects caused by mesoderm deficiency suggests that the hindbrain is not a source of otic inducing signals.