Poznanski A, Keller R
Molecular and Cell Biology, University of California, Berkeley 94720, USA.
Dev Biol. 1997 Apr 15;184(2):351-66. doi: 10.1006/dbio.1996.8500.
In this paper we examine the contributions of planar and vertical signaling to the patterning of gene expression in neural development and we examine the routes of this neural induction. We have examined how the expression of Xenopus homeobox gene, Hoxb-1, is regulated by instruction from the mesoderm and/or endoderm and ask whether this instruction is by the vertical or planar routes. We investigated normal expression patterns of Hoxb-1 during early Xenopus development and Hoxb-1 expression in sandwich explants of the dorsal marginal zone, which putatively allow only planar signals to pass from the mesodermal and endodermal tissue (Spemann's organizer) to the prospective neural tissue. In the latter case we found significant variability of expression. Observations during dissections suggested that variable degrees of invasion of the mesodermal-endodermal tissue at the leading edge of the mesodermal mantle might be the cause of this variability. Alternatively, differing lengths of time that the prospective neural region spends in planar contact with tissues of the lateral or ventral regions of the embryo could also contribute to this variability. Analysis of staged Keller sandwich explants, "skewered" sandwiches, in which the degree of contact with underlying, involuted mesoderm-endodermal tissues was marked, and "over-the-pole" and "giant" sandwich explants, in which the degree of planar contact with lateral or ventral tissues was normalized, suggests that both planar and vertical signals are involved in induction and patterning of Hoxb-1 expression. The shift in Hoxb-1 expression from a broad, diffuse pattern to a local, focused pattern, characteristic of the ultimate expression pattern in vivo, does not reflect variable degrees of contact with ventral or lateral tissues, but rather reflects early vertical contact with underlying mesodermal-endodermal tissues. We observed such contact at early gastrula stages (stages 10 to 10+), stages commonly assumed not to have the potential for vertical signaling. As the bottle cells first begin to form, at stage 10-, a massive rotation of the lower involuting marginal zone occurs around an internal lip ("levre interne," Nieuwkoop and Florschutz, 1950). This rotation initiates the formation of the Cleft of Brachet from the floor of the blastocoele and brings the prospective mesoderm and endoderm at the leading edge of the marginal zone into vertical apposition with the prospective neural region quite early in gastrulation. The consequence and importance of recognizing these early internal rearrangements are that it pushes backward the time at which potential vertical inductive interactions between mesoderm and neurectoderm can occur. This means that a purely planar inductive situation can cease to exist as early as the inception of bottle cell formation and that neural patterning through vertical induction starts at the very beginning of gastrulation.
在本文中，我们研究了平面信号和垂直信号在神经发育过程中基因表达模式形成中的作用，并探讨了这种神经诱导的途径。我们研究了非洲爪蟾同源框基因Hoxb-1的表达如何受到中胚层和/或内胚层指令的调控，并询问这种指令是通过垂直途径还是平面途径传递的。我们研究了Hoxb-1在非洲爪蟾早期发育过程中的正常表达模式，以及其在背侧边缘区三明治外植体中的表达情况，该外植体被认为仅允许平面信号从中胚层和内胚层组织（斯佩曼组织者）传递到预期的神经组织。在后一种情况下，我们发现表达存在显著差异。解剖过程中的观察表明，中胚层套膜前缘的中胚层-内胚层组织不同程度的侵入可能是这种差异的原因。或者，预期神经区域与胚胎侧部或腹侧区域组织进行平面接触的时间长短不同也可能导致这种差异。对分期的凯勒三明治外植体、“串烤”三明治（其中与下方内卷的中胚层-内胚层组织的接触程度有标记）以及“极上”和“巨型”三明治外植体（其中与侧部或腹侧组织的平面接触程度已标准化）的分析表明，平面信号和垂直信号都参与了Hoxb-1表达的诱导和模式形成。Hoxb-1表达从广泛、弥散的模式转变为局部、集中的模式，这是体内最终表达模式的特征，它并不反映与腹侧或侧部组织接触程度的差异，而是反映了与下方中胚层-内胚层组织的早期垂直接触。我们在原肠胚早期阶段（10至10+期）观察到了这种接触，通常认为这些阶段不具备垂直信号传递的潜力。在10-期，当瓶状细胞刚开始形成时，下方内卷的边缘区围绕一个内唇（“内唇”，Nieuwkoop和Florschutz，1950）发生大规模旋转。这种旋转引发了从囊胚腔底部形成布拉凯特裂，并使边缘区前缘的预期中胚层和内胚层在原肠胚形成早期就与预期神经区域垂直并置。认识到这些早期内部重排的后果和重要性在于，它将中胚层和神经外胚层之间潜在的垂直诱导相互作用可能发生的时间向后推移。这意味着早在瓶状细胞形成开始时，纯粹的平面诱导情况就可能不再存在，并且通过垂直诱导的神经模式形成在原肠胚形成一开始就开始了。
