Ministry of Education Key Laboratory of Marine Genetics and Breeding, College of Marine Life Sciences, Ocean University of China, Qingdao 266003, China.
Ministry of Education Key Laboratory of Marine Genetics and Breeding, College of Marine Life Sciences, Ocean University of China, Qingdao 266003, China; Laboratory for Marine Biology and Biotechnology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China; Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China.
Dev Biol. 2019 Apr 15;448(2):147-153. doi: 10.1016/j.ydbio.2018.11.009. Epub 2018 Nov 17.
The elongation of embryo and tissue is a key morphogenetic event in embryogenesis and organogenesis. Notochord, a typical chordate organ, undergoes elongation to perform its regulatory roles and to form the structural support in the embryo. Notochord elongation is morphologically similar across all chordates, but ascidian has evolved distinct molecular and cellular processes. Here, we summarize the current understanding of ascidian notochord elongation. We divide the process into three phases and discuss the underlying molecular mechanisms in each phase. In the first phase, the notochord converges and extends through invagination and mediolateral intercalation, and partially elongates to form a single diameter cell column along the anterior-posterior axis. In the second phase, a cytokinesis-like actomyosin ring is constructed at the equator of each cell and drives notochord to elongate approximately two-fold. The molecular composition and architecture of the ascidian notochord contractile ring are similar to that of the cytokinetic ring. However, the notochord contractile ring does not impose cell division but only drives cell elongation followed by disassembly. We discuss the self-organizing property of the circumferential actomyosin ring, and why it disassembles when certain notochord length is achieved. The similar ring structures are also present in the elongation process of other organs in evolutionarily divergent animals such as Drosophila and C. elegans. We hereby propose that actomyosin ring-based circumferential contraction is a common mechanism adopted in diverse systems to drive embryo and tissue elongation. In the third phase, the notochord experiences tubulogenesis and the endothelial-like cells crawl bi-directionally on the notochord sheath to further lengthen the notochord. In this review, we also discuss extracellular matrix proteins, notochord sheath, and surrounding tissues that may contribute to notochord integrity and morphogenesis.
胚胎和组织的伸长是胚胎发生和器官发生中的一个关键形态发生事件。脊索是一种典型的脊索动物器官，通过伸长来发挥其调节作用，并在胚胎中形成结构支撑。脊索的伸长在所有脊索动物中形态上相似，但尾索动物已经进化出了不同的分子和细胞过程。在这里，我们总结了尾索动物脊索伸长的当前理解。我们将这个过程分为三个阶段，并讨论每个阶段的潜在分子机制。在第一阶段，脊索通过内陷和中侧插入汇聚并延伸，并部分伸长，沿前后轴形成单个直径的细胞柱。在第二阶段，在每个细胞的赤道处构建一个类似于有丝分裂的肌动球蛋白环，并驱动脊索大约两倍伸长。尾索动物脊索收缩环的分子组成和结构与有丝分裂环相似。然而，脊索收缩环不会引起细胞分裂，而只会驱动细胞伸长，然后解体。我们讨论了周向肌动球蛋白环的自组织性质，以及为什么当达到一定的脊索长度时它会解体。在进化上不同的动物如果蝇和秀丽隐杆线虫等其他器官的伸长过程中也存在类似的环结构。因此，我们提出基于肌动球蛋白环的周向收缩是驱动胚胎和组织伸长的一种普遍机制，存在于不同的系统中。在第三阶段，脊索经历管形成，内皮样细胞在脊索鞘上双向爬行，进一步延长脊索。在这篇综述中，我们还讨论了可能有助于脊索完整性和形态发生的细胞外基质蛋白、脊索鞘和周围组织。
