Shih J, Fraser S E
Division of Biology, Beckman Institute, California Institute of Technology, Pasadena, 91125, USA.
Development. 1996 Apr;122(4):1313-22. doi: 10.1242/dev.122.4.1313.
The appearance of the embryonic shield, a slight thickening at the leading edge of the blastoderm during the formation of the germ ring, is one of the first signs of dorsoventral polarity in the zebrafish embryo. It has been proposed that the shield plays a role in fish embryo patterning similar to that attributed to the amphibian dorsal lip. In a recent study, we fate mapped many of the cells in the region of the forming embryonic shield, and found that neural and mesodermal progenitors are intermingled (Shih, J. and Fraser, S.E. (1995) Development 121, 2755-2765), in contrast to the coherent region of mesodermal progenitors found at the amphibian dorsal lip. Here, we examine the fate and the inductive potential of the embryonic shield to determine if the intermingling reflects a different mode of embryonic patterning than that found in amphibians. Using the microsurgical techniques commonly used in amphibian and avian experimental embryology, we either grafted or deleted the region of the embryonic shield. Homotopic grafting experiments confirmed the fates of cells within the embryonic shield region, showing descendants in the hatching gland, head mesoderm, notochord, somitic mesoderm, endoderm and ventral aspect of the neuraxis. Heterotopic grafting experiments demonstrated that the embryonic shield can organize a second embryonic axis; however, contrary to our expectations based on amphibian research, the graft contributes extensively to the ectopic neuraxis. Microsurgical deletion of the embryonic shield region at the onset of germ ring formation has little effect on neural development: embryos with a well-formed and well-patterned neuraxis develop in the complete absence of notochord cells. While these results show that the embryonic shield is sufficient for ectopic axis formation, they also raise questions concerning the necessity of the shield region for neural induction and embryonic patterning after the formation of the germ ring.
胚盾的出现是斑马鱼胚胎背腹极性的最初迹象之一，胚盾是在胚环形成过程中胚盘前缘的轻微增厚。有人提出，胚盾在鱼类胚胎模式形成中所起的作用类似于两栖动物的背唇。在最近的一项研究中，我们对正在形成的胚盾区域的许多细胞进行了命运图谱分析，发现神经和中胚层祖细胞相互混杂（施，J.和弗雷泽，S.E.（1995年）《发育》第121卷，2755 - 2765页），这与在两栖动物背唇处发现的中胚层祖细胞连贯区域形成对比。在这里，我们研究胚盾的命运和诱导潜能，以确定这种细胞混杂是否反映了与两栖动物不同的胚胎模式形成方式。我们使用两栖动物和鸟类实验胚胎学中常用的显微手术技术，要么移植要么切除胚盾区域。同型移植实验证实了胚盾区域内细胞的命运，显示其后代存在于孵化腺、头部中胚层、脊索、体节中胚层、内胚层和神经轴的腹侧。异位移植实验表明胚盾可以构建第二条胚胎轴；然而，与我们基于两栖动物研究的预期相反，移植对异位神经轴有广泛贡献。在胚环形成开始时显微手术切除胚盾区域对神经发育影响很小：在完全没有脊索细胞的情况下，胚胎发育出结构良好且模式正常的神经轴。虽然这些结果表明胚盾足以形成异位轴，但它们也引发了关于胚盾区域在胚环形成后对神经诱导和胚胎模式形成的必要性的问题。
