Role of the hindbrain in patterning the otic vesicle: A study of the zebrafish vhnf1 mutant

Abstract

The vertebrate inner ear develops from an ectodermal placode adjacent to rhombomeres 4 to 6 of the segmented hindbrain. The placode then transforms into a vesicle and becomes regionalised along its anteroposterior, dorsoventral and mediolateral axes. To investigate the role of hindbrain signals in instructing otic vesicle regionalisation, we analysed ear development in zebrafish mutants for vhnf1, a gene expressed in the caudal hindbrain during otic induction and regionalisation. We show that, in vhnf1 homozygous embryos, the patterning of the otic vesicle is affected along both the anteroposterior and dorsoventral axes. First, anterior gene expression domains are either expanded along the whole anteroposterior axis of the vesicle or duplicated in the posterior region. Second, the dorsal domain is severely reduced, and cell groups normally located ventrally are shifted dorsally, sometimes forming a single dorsal patch along the whole AP extent of the otic vesicle. Third, and probably as a consequence, the size and organization of the sensory and neurogenic epithelia are disturbed. These results demonstrate that, in zebrafish, signals from the hindbrain control the patterning of the otic vesicle, not only along the anteroposterior axis, but also, as in amniotes, along the dorsoventral axis. They suggest that, despite the evolution of inner ear structure and function, some of the mechanisms underlying the regionalisation of the otic vesicle in fish and amniotes have been conserved.

Hedgehog Signaling Governs the Development of Otic Sensory Epithelium and Its Associated Innervation in Zebrafish

“…The decrease of cell proliferation could account for the smaller size of the otic vesicle after CyA treatment. The possibility that the lack of space within the otic vesicle of treated embryos could impair the development of the PM appeared however unlikely, as other mutants with small otic vesicles, such as vhnf1 hi2169 , had laterally displaced but otherwise well developed PM (Lecaudey et al, 2007;supplemental Fig. 2, available at www.…”

Axial patterning in the developing vertebrate inner ear

“…A similar anteriorisation of the ear is seen in homozygous tcf2 (vhnf1) zebrafish mutants, which also have disrupted rhombomere patterning and lack expression of val/mafb in the hindbrain (Hernandez et al, 2004;Lecaudey et al, 2007). In tcf2 mutant embryos, expression domains of anterior otic markers (fgf8, pax5, hmx3 ) extend posteriorly or are duplicated at the posterior of the otic vesicle, while expression of fst (follistatin ), a posterior otic marker, is often absent (Lecaudey et al, 2007). Note that the tcf2 and val/mafb mutant phenotypes, although similar, are not identical: duplications (rather than ex- Both mutants show a similar wide spectrum of defects; examples of relatively strong phenotypes are shown for each.…”

“…The proneural gene atoh1b is initially expressed in a single domain (the sensory equivalence group) across the entire AP axis of the otic placode, but this is later segregated into two domains, one at the anterior and one at the posterior of the placode, which prefigure the appearance of hair cell foci (Millimaki et al, 2007). Refinement of the expression domain is dependent on Notch signalling (Millimaki et al, 2007) and may also require an inhibitory signal from rhombomere 5 (r5), preventing hair cell differentiation in the middle of the placode (Lecaudey et al, 2007). The two foci of hair cells that arise at the A and P poles of the ear (the precursors of the utricular and saccular maculae, respectively) initially appear relatively symmetric, and they comprise the same differentiated cell types (hair cells and supporting cells), but the shapes, positions, sizes and polarity patterns of the two maculae will be very different in the mature ear as they develop according to their A or P position.…”

Patterning and cell fate in ear development

“…B A zebrafish, defects have been found primarily along the AP axis, recent data have shown that this difference may be only apparent, since a closer examination of the mutants has allowed the detection of DV patterning defects in both vhnf1 and val zebrafish mutants (Fig. 4B, Lecaudey et al, 2007; Schneider-Maunoury and Pujades, unpublished results), and AP defects in the otic vesicle of mice kreisler mutants (Vazquez-Echeverria et al, 2008). It has to be kept in mind that the process of patterning is also concomitant to growth and large morphogenetic movements all of which will surely require of more elaborated models to be understood.…”

“…As expected, AP patterning phenotypes are observed in the inner ear of vhnf1 mutants, which display an expansion or a duplication of the expression of anterior otic genes such as hmx3, fgf8 and pax5. However, vhnf1 mutants also show DV patterning defects, and a dorsal shift of intermediate markers such as atoh1a, which marks the future maculae (Lecaudey et al, 2007). val and vhnf1 mutants display hair cells at ectopic positions all along the AP axis of the otic vesicle, suggesting that either an intact r5 identity or r5-signals are essential to restrict early hair cell specification to the otic region lateral to r4 and r6.…”