Mendonsa E S, Riley B B
Biology Department, Texas A & M University, College Station, Texas, 77843-3258, USA.
Dev Biol. 1999 Feb 1;206(1):100-12. doi: 10.1006/dbio.1998.9134.
Development of the vertebrate inner ear begins during gastrulation with induction of the otic placode. Several embryonic tissues, including cephalic mesendoderm, notochord, and hindbrain, have been implicated as potential sources of otic-inducing signals. However, the relative contributions of these tissues have not been determined, nor have any genes affecting placode induction been identified. To address these issues, we analyzed otic placode induction in zebrafish mutants that are deficient in prospective otic-inducing tissues. Otic development was monitored by examining mutant embryos for morphological changes and, in some cases, by visualizing expression patterns of dlx-3 or pax-2.1 in preotic cells several hours before otic placode formation. In cyclops (cyc-) mutants, which develop with a partial deficiency of prechordal mesendoderm, otic induction is delayed by up to 1 h. In one-eyed pinhead (oep-) mutants, which are more completely deficient in prechordal mesendoderm, otic induction is delayed by 1.5 h, and morphology of the otic vesicles is abnormal. Expression of marker genes in other regions of the neural plate is normal, suggesting that ablation of prechordal mesendoderm selectively inhibits otic induction. In contrast, the timing and morphology of otic development is not affected by mutations in no tail (ntl) or floating head (flh), which prevent notochord differentiation. Similarly, a mutation in valentino (val), which blocks early differentiation of rhombomeres 5 and 6 in the hindbrain, does not delay otic induction, although subsequent patterning of the otic vesicle is impaired. To test whether inductive signals from one tissue can compensate for loss of another, we generated double or triple mutants with various combinations of the above mutations. In none of the multiple mutants do the flh or val mutations exacerbate delays in placode induction, although val does contribute additively to defects in subsequent patterning of the otic vesicle. In contrast, mutants homozygous for both oep and ntl, which interact synergistically to disrupt differentiation of cephalic and axial mesendoderm, show a delay in otic development of about 3 h. These data suggest that cephalic mesendoderm, including prechordal mesendoderm and anterior paraxial mesendoderm, provides the first otic-inducing signals during gastrulation, whereas chordamesoderm plays no discernible role in this process. Because val- mutants are deficient for only a portion of the hindbrain, we cannot rule out a role for that tissue in otic placode induction. However, if the hindbrain does provide otic-inducing signals, they apparently differ quantitatively or qualitatively from the signals required for vesicle patterning, as val disrupts only the latter.
脊椎动物内耳的发育始于原肠胚形成期耳基板的诱导。包括头部中胚层、脊索和后脑在内的几种胚胎组织被认为是耳诱导信号的潜在来源。然而，这些组织的相对贡献尚未确定，也未鉴定出任何影响基板诱导的基因。为了解决这些问题，我们分析了斑马鱼突变体中耳基板的诱导情况，这些突变体缺乏预期的耳诱导组织。通过检查突变胚胎的形态变化来监测耳发育，在某些情况下，通过观察耳基板形成前数小时前耳前细胞中dlx - 3或pax - 2.1的表达模式来监测。在独眼（cyc -）突变体中，其前索中胚层部分缺失，耳诱导延迟长达1小时。在无眼针头（oep -）突变体中，前索中胚层更完全缺失，耳诱导延迟1.5小时，耳泡形态异常。神经板其他区域标记基因的表达正常，这表明前索中胚层的缺失选择性地抑制了耳诱导。相比之下，耳发育的时间和形态不受无尾（ntl）或浮头（flh）突变的影响，这些突变会阻止脊索分化。同样，瓦伦蒂诺（val）突变体中，后脑的菱脑节5和6的早期分化受阻，尽管耳泡的后续模式形成受损，但耳诱导并未延迟。为了测试来自一个组织的诱导信号是否可以补偿另一个组织的缺失，我们构建了上述突变的各种组合的双突变体或三突变体。在所有多重突变体中，flh或val突变均未加剧基板诱导的延迟，尽管val确实会累加地导致耳泡后续模式形成的缺陷。相比之下，oep和ntl均为纯合子的突变体，它们协同作用破坏头部和轴向中胚层的分化，耳发育延迟约3小时。这些数据表明，包括前索中胚层和前轴旁中胚层在内的头部中胚层在原肠胚形成期提供了最初的耳诱导信号，而脊索中胚层在这个过程中没有明显作用。由于val -突变体仅后脑的一部分缺失，我们不能排除该组织在耳基板诱导中的作用。然而，如果后脑确实提供耳诱导信号，它们显然在数量或质量上与耳泡模式形成所需的信号不同，因为val仅破坏后者。
