Mayor R, Morgan R, Sargent M G
Laboratory of Developmental Biology, National Institute for Medical Research, Mill Hill, London, UK.
Development. 1995 Mar;121(3):767-77. doi: 10.1242/dev.121.3.767.
The earliest sign of the prospective neural crest of Xenopus is the expression of the ectodermal component of Xsna (the Xenopus homologue of snail) in a low arc on the dorsal aspect of stage 11 embryos, which subsequently assumes the horseshoe shape characteristic of the neural folds as the convergence-extension movements shape the neural plate. A related zinc-finger gene called Slug (Xslu) is expressed specifically in this tissue (i.e. the prospective crest) when the convergence extension movements are completed. Subsequently, Xslu is found in pre- and post-migratory cranial and trunk neural crest and also in lateral plate mesoderm after stage 17. Both Xslu and Xsna are induced by mesoderm from the dorsal or lateral marginal zone but not from the ventral marginal zone. From stage 10.5, explants of the prospective neural crest, which is underlain with tissue, are able to express Xslu. However expression of Xsna is not apparently specified until stage 12 and further contact with the inducer is required to raise the level of expression to that seen later in development. Xslu is specified at a later time. Embryos injected with noggin mRNA at the 1-cell stage or with plasmids driving noggin expression after the start of zygotic transcription express Xslu in a ring surrounding the embryo on the ventroposterior side. We suggest this indicates (a) that noggin interacts with another signal that is present throughout the ventral side of the embryo and (b) that Xslu is unable to express in the neural plate either because of the absence of a co-inducer or by a positive prohibition of expression. The ventral co-inducer, in the presence of overexpressed noggin, seems to generate an anterior/posterior pattern in the ventral part of the embryo comparable to that seen in neural crest of normal embryos. We suggest that the prospective neural crest is induced in normal embryos in the ectoderm that overlies the junction of the domains that express noggin and Xwnt-8. In support of this, we show animal cap explants from blastulae and gastrulae, treated with bFGF and noggin express Xslu but not NCAM although the mesoderm marker Xbra is also expressed. Explants treated with noggin alone express NCAM only. An indication that induction of the neural plate border is regulated independently of the neural plate is obtained from experiments using ultraviolet irradiation in the precleavage period. At certain doses, the cranial crest domains are not separated into lateral masses and there is a reduction in the size of the neural plate.
非洲爪蟾预期神经嵴的最早迹象是Xsna（蜗牛的非洲爪蟾同源物）的外胚层成分在11期胚胎背侧呈低弧形表达，随后随着汇聚延伸运动塑造神经板，其呈现出神经褶特有的马蹄形。一个相关的锌指基因Slug（Xslu）在汇聚延伸运动完成时在该组织（即预期的嵴）中特异性表达。随后，在迁移前和迁移后的颅神经嵴和躯干神经嵴中发现Xslu，在17期后也存在于侧板中胚层。Xslu和Xsna均由背侧或外侧边缘区的中胚层诱导产生，而腹侧边缘区则不能诱导。从10.5期开始，位于组织下方的预期神经嵴外植体能够表达Xslu。然而，Xsna的表达直到12期才明显确定，需要与诱导物进一步接触才能将表达水平提高到发育后期所见的水平。Xslu在较晚时间确定。在单细胞期注射noggin mRNA或在合子转录开始后注射驱动noggin表达的质粒的胚胎，在腹后侧围绕胚胎的环状区域表达Xslu。我们认为这表明（a）noggin与胚胎腹侧普遍存在的另一种信号相互作用，（b）Xslu无法在神经板中表达，要么是因为缺乏共诱导物，要么是由于表达受到正向抑制。在过表达noggin的情况下，腹侧共诱导物似乎在胚胎腹侧产生了一种前后模式，类似于正常胚胎神经嵴中的模式。我们认为，在正常胚胎中，预期神经嵴是在覆盖表达noggin和Xwnt - 8区域交界处的外胚层中诱导产生的。为此，我们展示了来自囊胚和原肠胚的动物帽外植体，用bFGF和noggin处理后表达Xslu但不表达NCAM，尽管也表达中胚层标记物Xbra。仅用noggin处理的外植体仅表达NCAM。使用卵裂前期紫外线照射的实验表明，神经板边界的诱导独立于神经板进行调节。在某些剂量下，颅嵴区域不会分离成外侧团块，神经板的大小会减小。
