Viebahn C, Mayer B, Miething A
Institute of Anatomy, University of Bonn, Germany.
Acta Anat (Basel). 1995;154(2):99-110. doi: 10.1159/000147756.
Mesoderm formation is a hallmark of vertebrate gastrulation and, at the same time, one of the prime examples for epithelio-mesenchymal transformation. Recent advances in experimental embryology and molecular biology have clarified the role of growth factors and genes in this process; however, its microscopic anatomy in higher vertebrates is still far from clear. Therefore, the present study describes the morphology of mesoderm formation in the rabbit embryo, a species which may be representative for both the avian and the mammalian embryo in this respect. Serial semithin sections were correlated with topographical landmarks in surface views of embryonic discs at 6.4, 6.5, and 6.6 days post conceptionem, and selected semithin sections were reembedded for ultrastructural analysis. Mesoderm cells are shown to be generated by ingression of bottle-shaped epiblast cells in the area of the posterior node and the primitive streak. Here, basal endocytotic pits and absence or discontinuity of the basal lamina are taken as suggestive evidence for specific removal of extracellular matrix material. Within the bottle-shaped cells most organelles are concentrated in a narrow apical neck which will subsequently constitute the 'trailing end' of the ingressing mesoderm cells. These features support the assumption that most principles of epithelio-mesenchymal transformation seen during primary mesenchyme formation in the sea urchin also apply to mesoderm formation in vertebrates. However, transient tripartite zonula adherens-type junctions are formed apically between ingressing mesoderm cells and the neighboring epiblast cells. They are interpreted here as being responsible for maintaining supracellular integrity of the embryonic disc during the shedding of mesoderm cells in the amniote embryo.
中胚层形成是脊椎动物原肠胚形成的一个标志，同时也是上皮-间充质转化的主要例子之一。实验胚胎学和分子生物学的最新进展已经阐明了生长因子和基因在这一过程中的作用；然而，高等脊椎动物中胚层形成的微观解剖结构仍远未明确。因此，本研究描述了兔胚胎中胚层形成的形态，在这方面兔胚胎可能代表鸟类和哺乳动物胚胎。对妊娠6.4、6.5和6.6天胚胎盘表面视图中的连续半薄切片与地形标志进行了关联，并对选定的半薄切片进行重新包埋以进行超微结构分析。结果表明，中胚层细胞是由后节点和原条区域中瓶状上胚层细胞的内陷产生的。在这里，基底内吞小窝以及基底膜的缺失或不连续被视为细胞外基质物质被特异性去除的提示性证据。在瓶状细胞内，大多数细胞器集中在一个狭窄的顶端颈部，该颈部随后将构成内陷中胚层细胞的“尾端”。这些特征支持了这样一种假设，即海胆初级间充质形成过程中所见的上皮-间充质转化的大多数原理也适用于脊椎动物的中胚层形成。然而，在侵入的中胚层细胞和相邻的上胚层细胞顶端形成了短暂的三方紧密连接型连接。在这里，它们被解释为在羊膜动物胚胎中胚层细胞脱落期间维持胚胎盘超细胞完整性的原因。
