Meier S, Jacobson A G
J Exp Zool. 1982 Feb 1;219(2):217-32. doi: 10.1002/jez.1402190211.
On either side of Hensen's node of the fully extended primitive streak of the chick embryo (stage 4) the mesoderm is already organized into circular domains called somitomeres. As Hensen's node regresses, paraxial somitomeres are added in tandem and are early morphological representatives of metameric pattern in the mesoderm. These organized circular domains of mesenchyme cells are best visualized with stereo pair scanning electron microscopy. Experiments suggested that a prepattern of segmentation exists in and around the fully extended primitive streak. Streaks divested of Hensen's node can generate some paraxial somites, but only if surgically split down the midline. We assessed metameric pattern formation in nodeless streaks, both severed and unsevered down the midline. Operated blastoderms were cultured 15 hours, fixed, dissected, processed for scanning electron microscopy, and photographed in stereo. Split nodeless streaks produced a cranial to caudal sequence of somitomeric development. This sequence is similar to the sequential maturational events seen in the segmental plate of older embryos. The least mature somitomeres, toward the posterior end of the severed edge, appear as circular domains of radially oriented cells, looking much like the first somitomeres to emerge near Hensen's node of the stage 4 streak. More cranially along the severed edge, somitomeres are morphologically more mature, being more condensed, with cells oriented about a central myocoele. At the most cranial end of the severed piece, somitomeres are the most mature, having contracted about their centers to create intersomitomeric gaps that permit their identification with light microscopy as individual "somites." Embryos from which the node was removed, but the streak left intact, generated only the most primitive somitomeric pattern repetitively along either side of the primitive groove. We conclude that regression of Hensen's node provides for the timely initiation of morphogenesis of somitomeres from a prepattern of segmentation that already exists.
在鸡胚（第4阶段）完全伸展的原条的亨氏结两侧，中胚层已组织成称为体节球的圆形区域。随着亨氏结后退，成对的体节球依次添加，是中胚层节段模式的早期形态学代表。这些有组织的间充质细胞圆形区域通过立体对扫描电子显微镜能得到最佳观察。实验表明，在完全伸展的原条及其周围存在一种预分割模式。去除亨氏结的原条可以产生一些近轴体节，但前提是要沿中线手术切开。我们评估了无结原条的节段模式形成，包括沿中线切断和未切断的情况。对操作过的胚盘进行15小时培养，固定、解剖、处理用于扫描电子显微镜观察，并进行立体拍照。切断的无结原条产生了从头至尾的体节球发育序列。这个序列类似于在较老胚胎的体节板中看到的连续成熟事件。最不成熟的体节球，朝向切断边缘的后端，表现为径向排列细胞的圆形区域，看起来很像在第4阶段原条的亨氏结附近出现的第一批体节球。沿着切断边缘更靠前的位置，体节球在形态上更成熟，更致密，细胞围绕中央肌腔排列。在切断片段的最前端，体节球最成熟，围绕其中心收缩以形成体节间间隙，通过光学显微镜可将其识别为单个“体节”。去除节点但原条保持完整的胚胎，仅在原沟两侧重复产生最原始的体节球模式。我们得出结论，亨氏结的后退使得从已经存在的预分割模式中及时启动体节球的形态发生。
