Viebahn C, Mayer B, Hrabé de Angelis M
Institute of Anatomy, University of Bonn, Germany.
Anat Embryol (Berl). 1995 Aug;192(2):159-69. doi: 10.1007/BF00186004.
An early common element during anterior-posterior axis formation amongst amniotes is the primitive streak, running longitudinally in the two-layered embryonic disc. In mammals the primordium of this transient structure is the first definite morphological sign of the anterior-posterior axis, while in avian embryos the axis is visible and apparently defined earlier. Here we scrutinize suggestions that in mammals also there are earlier signs of axis formation by using correlative low- and high-resolution light microscopy on tissues from rabbit embryos at 6.3 and 6.5 days post-conception, i.e. immediately before and after primitive streak formation. A series of semithin sections were cut from resin-embedded embryonic discs that had been photographed previously at low power. In embryos at 6.5-days post-conception the primitive streak is as long as up to half the diameter of the embryonic disc, extending anteriorly from a thickening, here called the posterior node, at the posterior margin, which contains the first mesoderm cells ingressing from the epiblast. On both sides of the primitive streak there is a triangular area that appears light in surface views of fixed embryos and correlates with stretches of low-columnar simple epithelium in an otherwise high-columnar pseudostratified epiblast. Within the anterior margin, which has a sharper contour than the rest of the circumference of the embryonic disc, there is a narrow, crescent-shaped dark zone caused by increased cellular height and number in both epiblast and hypoblast. These characteristics of the anterior margin are also found at 6.3 days post-conception, at which stage there is no sign of a primitive streak or a posterior node. The posterior margin, in contrast, is ill-defined in these earlier embryos, or there is a light crescent within the posterior margin, which has the same histological characteristics as the bilateral posterior triangular areas of primitive streak stages. Because the anterior differentiation occurs prior to primitive streak formation and is a sign of both the anterior-posterior and the transverse axes of the embryonic disc, and because some of its histological characteristics are found in primate and human embryos, we propose to name this structure the 'anterior marginal crescent' and to add it to the list of transient structures that gradually establish the principal body axes in mammals. The anterior manifestation of body axes in mammals is thus essentially different from axis development in the avian embryo, where differentiation of these axes is first manifest at the posterior margin.
羊膜动物前后轴形成过程中的一个早期共同要素是原条，它在两层胚胎盘内纵向延伸。在哺乳动物中，这个短暂结构的原基是前后轴的第一个明确形态学标志，而在鸟类胚胎中，轴在更早的时候就可见且明显确定。在这里，我们通过对受孕后6.3天和6.5天的兔胚胎组织进行相关的低分辨率和高分辨率光学显微镜检查，仔细研究了关于在哺乳动物中也存在轴形成早期迹象的观点，即恰好在原条形成之前和之后。从先前已在低倍镜下拍摄过的树脂包埋胚胎盘中切取一系列半薄切片。在受孕后6.5天的胚胎中，原条长达胚胎盘直径的一半，从后缘的一个增厚处向前延伸，此处称为后节点，其中包含从外胚层侵入的第一批中胚层细胞。在原条两侧有一个三角形区域，在固定胚胎的表面观中显得较亮，并且与在其他地方为高柱状假复层外胚层中的低柱状单层上皮伸展区域相关。在胚胎盘圆周其余部分轮廓更清晰的前缘内，有一个狭窄的新月形暗区，这是由外胚层和下胚层中细胞高度和数量增加所致。在受孕后6.3天也能发现前缘的这些特征，此时没有原条或后节点的迹象。相比之下，在这些更早的胚胎中后缘界限不明确，或者在后缘内有一个亮新月形，其组织学特征与原条阶段的双侧后三角区域相同。由于前部分化发生在原条形成之前，并且是胚胎盘前后轴和横轴的标志，还由于其一些组织学特征在灵长类和人类胚胎中也能发现，我们提议将这个结构命名为“前缘新月”，并将其列入在哺乳动物中逐渐确立主要身体轴的短暂结构列表中。因此，哺乳动物身体轴的前部表现与鸟类胚胎中的轴发育本质上不同，在鸟类胚胎中这些轴的分化首先表现在后缘。
