Kil Sung-Hee, Streit Andrea, Brown Stephen T, Agrawal Nidhi, Collazo Andres, Zile Maija H, Groves Andrew K
Gonda Department of Cell and Molecular Biology, House Ear Institute, 2100 West 3rd Street, Los Angeles, CA 90057, USA.
Dev Biol. 2005 Sep 1;285(1):252-71. doi: 10.1016/j.ydbio.2005.05.044.
The hindbrain and cranial paraxial mesoderm have been implicated in the induction and patterning of the inner ear, but the precise role of the two tissues in these processes is still not clear. We have addressed these questions using the vitamin-A-deficient (VAD) quail model, in which VAD embryos lack the posterior half of the hindbrain that normally lies next to the inner ear. Using a battery of molecular markers, we show that the anlagen of the inner ear, the otic placode, is induced in VAD embryos in the absence of the posterior hindbrain. By performing grafting and ablation experiments in chick embryos, we also show that cranial paraxial mesoderm which normally lies beneath the presumptive otic placode is necessary for otic placode induction and that paraxial mesoderm from other locations cannot induce the otic placode. Two members of the fibroblast growth factor family, FGF3 and FGF19, continue to be expressed in this mesodermal population in VAD embryos, and these may be responsible for otic placode induction in the absence of the posterior hindbrain. Although the posterior hindbrain is not required for otic placode induction in VAD embryos, the subsequent patterning of the inner ear is severely disrupted. Several regional markers of the inner ear, such as Pax2, EphA4, SOHo1 and Wnt3a, are incorrectly expressed in VAD otocysts, and the sensory patches and vestibulo-acoustic ganglia are either greatly reduced or absent. Exogenous application of retinoic acid prior to 30 h of development is able rescue the VAD phenotype. By performing such rescue experiments before and after 30 h of development, we show that the inner ear defects of VAD embryos correlate with the absence of the posterior hindbrain. These results show that induction and patterning of the inner ear are governed by separate developmental processes that can be experimentally uncoupled from each other.
后脑和颅旁轴中胚层被认为与内耳的诱导和模式形成有关，但这两种组织在这些过程中的确切作用仍不清楚。我们利用维生素A缺乏（VAD）鹌鹑模型来解决这些问题，在该模型中，VAD胚胎缺乏通常位于内耳旁边的后脑后半部分。通过一系列分子标记，我们发现，在内耳原基耳基板在VAD胚胎中是在没有后脑后半部分的情况下被诱导形成的。通过在鸡胚中进行移植和切除实验，我们还表明，通常位于假定耳基板下方的颅旁轴中胚层对于耳基板的诱导是必需的，而来自其他位置的轴中胚层不能诱导耳基板。成纤维细胞生长因子家族的两个成员FGF3和FGF19在VAD胚胎的这个中胚层群体中持续表达，这可能是在没有后脑后半部分的情况下耳基板诱导的原因。虽然后脑后半部分对于VAD胚胎中耳基板的诱导不是必需的，但内耳随后的模式形成受到严重破坏。内耳的几个区域标记，如Pax2、EphA4、SOHo1和Wnt3a，在VAD耳囊中表达错误，感觉斑和前庭-听觉神经节要么大大减少，要么缺失。在发育30小时之前外源性应用视黄酸能够挽救VAD表型。通过在发育30小时之前和之后进行这样的挽救实验，我们表明VAD胚胎的内耳缺陷与后脑后半部分的缺失相关。这些结果表明，内耳的诱导和模式形成受不同发育过程的控制，这些过程在实验中可以相互分离。
