Carrillo-Baltodano Allan M, Meyer Néva P
Biology Department, Clark University, Worcester, MA 01610, USA.
Biology Department, Clark University, Worcester, MA 01610, USA.
Dev Biol. 2017 Nov 15;431(2):134-144. doi: 10.1016/j.ydbio.2017.09.022. Epub 2017 Sep 21.
In the deuterostomes and ecdysozoans that have been studied (e.g. chordates and insects), neural fate specification relies on signaling from surrounding cells. However, very little is known about mechanisms of neural specification in the third major bilaterian clade, spiralians. Using blastomere isolation in the annelid Capitella teleta, a spiralian, we studied to what extent extrinsic versus intrinsic signals are involved in early neural specification of the brain and ventral nerve cord. For the first time in any bilaterian, we found that brain neural ectoderm is autonomously specified. This occurs in the daughters of first-quartet micromeres, which also generate anterior neural ectoderm in other spiralians. In contrast, isolation of the animal cap, including the 2d micromere, which makes the trunk ectoderm and ventral nerve cord, blocked ventral nerve cord formation. When the animal cap was isolated with the 2D macromere, the resulting partial larvae had a ventral nerve cord. These data suggest that extrinsic signals from second-quartet macromeres or their daughters, which form mesoderm and endoderm, are required for nerve cord specification in C. teleta and that the 2D macromere or its daughters are sufficient to provide the inductive signal. We propose that autonomous specification of anterior neural ectoderm evolved in spiralians in order to enable them to quickly respond to environmental cues encountered by swimming larvae in the water column. In contrast, a variety of signaling pathways could have been co-opted to conditionally specify the nerve cord. This flexibility of nerve cord development may be linked to the large diversity of trunk nervous systems present in Spiralia.
在已研究的后口动物和蜕皮动物（如脊索动物和昆虫）中，神经命运的指定依赖于周围细胞发出的信号。然而，对于第三个主要两侧对称动物分支——螺旋动物门中神经指定的机制却知之甚少。利用环节动物秀丽隐杆线虫（一种螺旋动物）的卵裂球分离技术，我们研究了外在信号与内在信号在脑和腹神经索早期神经指定过程中所涉及的程度。在任何两侧对称动物中，我们首次发现脑的神经外胚层是自主指定的。这发生在第一四分体小卵裂球的子代中，这些子代在其他螺旋动物中也会产生前部神经外胚层。相比之下，分离包括产生躯干外胚层和腹神经索的2d小卵裂球在内的动物帽，会阻止腹神经索的形成。当动物帽与2D大卵裂球一起分离时，产生的部分幼虫有腹神经索。这些数据表明，来自形成中胚层和内胚层的第二四分体大卵裂球或其子代的外在信号是秀丽隐杆线虫神经索指定所必需的，并且2D大卵裂球或其子代足以提供诱导信号。我们提出，前部神经外胚层的自主指定在螺旋动物门中进化，以便它们能够快速响应浮游幼虫在水柱中遇到的环境线索。相比之下，多种信号通路可能已被用于有条件地指定神经索。神经索发育的这种灵活性可能与螺旋动物门中存在的大量多样的躯干神经系统有关。
