Shook David R, Majer Christina, Keller Ray
Department of Biology, University of Virginia, Gilmer Hall, Charlottesville, VA 22903, USA.
Dev Biol. 2004 Jun 1;270(1):163-85. doi: 10.1016/j.ydbio.2004.02.021.
The mesoderm, comprising the tissues that come to lie entirely in the deep layer, originates in both the superficial epithelial and the deep mesenchymal layers of the early amphibian embryo. Here, we characterize the mechanisms by which the superficial component of the presumptive mesoderm ingresses into the underlying deep mesenchymal layer in Xenopus tropicalis and extend our previous findings for Xenopus laevis. Fate mapping the superficial epithelium of pregastrula stage embryos demonstrates ingression of surface cells into both paraxial and axial mesoderm (including hypochord), in similar patterns and amounts in both species. Superficial presumptive notochord lies medially, flanked by presumptive hypochord and both overlie the deep region of the presumptive notochord. These tissues are flanked laterally by superficial presumptive somitic mesoderm, the anterior tip of which also appears to overlay the presumptive deep notochord. Time-lapse recordings show that presumptive somitic and notochordal cells move out of the roof of the gastrocoel and into the deep region during neurulation, whereas hypochordal cells ingress after neurulation. Scanning electron microscopy at the stage and position where ingression occurs suggests that superficial presumptive somitic cells in X. laevis ingress into the deep region as bottle cells whereas those in X. tropicalis ingress by "relamination" (e.g., [Dev. Biol. 174 (1996) 92]). In both species, the superficially derived presumptive somitic cells come to lie in the medial region of the presumptive somites during neurulation. By the early tailbud stages, these cells lie at the horizontal myoseptum of the somites. The morphogenic pathway of these cells strongly resembles that of the primary slow muscle pioneer cells of the zebrafish. We present a revised fate map of Xenopus, and we discuss the conservation of superficial mesoderm within amphibians and across the chordates and its implications for the role of this tissue in patterning the mesoderm.
中胚层由完全位于深层的组织构成，起源于早期两栖动物胚胎的表层上皮层和深层间充质层。在此，我们描述了热带爪蟾中预定中胚层的表层成分侵入其下方深层间充质层的机制，并扩展了我们之前对非洲爪蟾的研究结果。对原肠胚前期胚胎的表层上皮进行命运图谱分析表明，表面细胞以相似的模式和数量侵入到轴旁中胚层和轴中胚层（包括脊索）。预定脊索的表层位于内侧，两侧是预定脊索，两者都覆盖在预定脊索的深层区域之上。这些组织的外侧是预定体节中胚层的表层，其前端似乎也覆盖在预定的深层脊索之上。延时记录显示，在神经胚形成过程中，预定体节和脊索细胞从原肠腔顶部移出并进入深层区域，而脊索细胞在神经胚形成后侵入。在侵入发生的阶段和位置进行扫描电子显微镜观察表明，非洲爪蟾的预定体节表层细胞以瓶状细胞的形式侵入深层区域，而热带爪蟾的细胞则通过“重新分层”侵入（例如，[《发育生物学》174 (1996) 92]）。在这两个物种中，源自表层的预定体节细胞在神经胚形成过程中位于预定体节的内侧区域。到早期尾芽阶段，这些细胞位于体节的水平肌隔处。这些细胞的形态发生途径与斑马鱼的初级慢肌先驱细胞的途径非常相似。我们展示了一个修订后的爪蟾命运图谱，并讨论了两栖动物以及整个脊索动物中表层中胚层的保守性及其对该组织在中胚层模式形成中作用的影响。
