Arendt D, Nübler-Jung K
Institut für Biologie I (Zoologie), Hauptstrasse 1, 79104, Freiburg, Germany.
Mech Dev. 1999 Mar;81(1-2):3-22. doi: 10.1016/s0925-4773(98)00226-3.
Gastrulating birds and mammals form a primitive streak in lieu of a circular blastopore, and a conspicuous underlying tissue layer, the hypoblast. In an attempt to understand the evolution of these amniote characteristics, pregastrula and gastrulation stages in selected amniotes are compared with the more ancestral situation in amphibians. At blastula/blastoderm stages, the overall fate maps and the arrangement of tissues around the organizer are rather similar, as is exemplified by a comparison of gene expression and fate maps in the frog and chick. Compared with amphibians, however, the eggs of reptiles, birds and monotreme mammals have a disproportionately large yolk that alters gastrulation morphology. During amphibian gastrulation, the organizer moves from anterior to posterior, to lay down the dorsal axis around the vegetal hemisphere (Arendt, D., Nübler-Jung, K., 1997. Dorsal or ventral: similarities in fate maps and gastrulation patterns in annelids, arthropods and chordates. Mech. Dev. 61, 1-15). In contrast, in amniote eggs, the large yolk impedes the organizer from moving around the entire vegetal hemisphere so that axis formation begins and ends at the same side of the egg. This has apparently provoked an evolutionary transformation of an amphibian-like blastopore, first into the 'blastoporal canal' of reptiles, and then into the birds' and mammals' primitive streak. The blastopore divides into two functionally divergent parts, one as the site of mesoderm internalization ('intraembryonic blastopore') and the other as the site of ectodermal epiboly ('extraembryonic blastopore'). The hypoblast is proposed to derive from the 'endodermal wedge' that is seen already in the amphibian gastrula. Hypoblast formation would then represent a special kind of gastrulation movement that also exists in the amphibians, and for which the term 'hypoboly' is introduced.
处于原肠胚形成阶段的鸟类和哺乳动物会形成一条原条，以替代圆形的胚孔，以及一个明显的下方组织层，即下胚层。为了理解这些羊膜动物特征的进化过程，研究人员将选定羊膜动物的原肠胚前期和原肠胚形成阶段与两栖动物中更为原始的情况进行了比较。在囊胚/胚盘阶段，整体的命运图谱以及组织围绕组织者的排列方式颇为相似，青蛙和小鸡的基因表达与命运图谱的比较就例证了这一点。然而，与两栖动物相比，爬行动物、鸟类和单孔目哺乳动物的卵含有不成比例的大量卵黄，这改变了原肠胚形成的形态。在两栖动物原肠胚形成过程中，组织者从前向后移动，在植物半球周围奠定背轴（阿伦特，D.，纽布勒 - 荣格，K.，1997年。背侧还是腹侧：环节动物、节肢动物和脊索动物命运图谱和原肠胚形成模式的相似性。《发育机制》61卷，第1 - 15页）。相比之下，在羊膜动物的卵中，大量的卵黄阻碍了组织者围绕整个植物半球移动，使得轴的形成在卵的同一侧开始和结束。这显然引发了类似两栖动物胚孔的进化转变，首先转变为爬行动物的“胚孔管”，然后转变为鸟类和哺乳动物的原条。胚孔分为两个功能不同的部分，一个是中胚层内陷的部位（“胚胎内胚孔”），另一个是外胚层外包的部位（“胚胎外胚孔”）。下胚层被认为源自两栖动物原肠胚中已经可见的“内胚层楔”。下胚层的形成将代表一种特殊的原肠胚形成运动，这种运动在两栖动物中也存在，为此引入了“下包”这一术语。
