Netrin is a remarkably conserved midline landmark, serving as a chemotactic factor that organizes the bilateral neural architecture in the post-gastrula bilaterian embryos. Netrin signal also guides cell migration in many other neural and non-neural organogenesis events in later developmental stages but has never been found to participate in gastrulation - the earliest cell migration in metazoan embryogenesis. Here, we found that the netrin signaling molecules and their receptors are expressed during gastrulation of the leech Helobdella. Intriguingly, Hau-netrin-1 was expressed in the N lineage, which gives rise in part to the ventral midline of ectoderm, at the onset of gastrulation. We demonstrated that the N lineage is required for the entrance of mesoderm into the germinal band and that misexpression of Hau-netrin-1 in early gastrulation prevented mesoderm from entering the germinal band. Together, these results suggested that Hau-netrin-1 secreted by the N lineage guides mesoderm migration during germinal band assembly. Furthermore, ectopic expression of Hau-netrin-1 after the completion of germinal band assembly disrupted the epibolic migration of the germinal bands in a later stage of gastrulation. Thus, Hau-netrin-1 is likely involved in two distinct events in sequential stages of leech gastrulation: the assembly of germinal bands in early gastrulation and their epibolic migration in mid-gastrulation. Given that the leech netrin is expressed in the precursor cells of the ventral midline during gastrulation, we propose that a heterochronic change from the midline netrin expression had taken place in the evolution of a novel mode of gastrulation in the directly developing leech embryos.

中文翻译：

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由腹外胚层谱系表达的Netrin指导水ech的表皮胃肠道中胚层迁移。

Netrin是一个非常保守的中线地标，是一个趋化因子，可以组织胃后双侧胚胎中的双侧神经结构。Netrin信号还可在后期发育阶段指导许多其他神经和非神经器官发生事件中的细胞迁移，但从未发现其参与胃祖细胞形成-子生后胚胎发生中最早的细胞迁移。在这里，我们发现netrin信号分子及其受体在水ech Helobdella的胃形成过程中表达。有趣的是，Hau-netrin-1在N谱系中表达，在胚芽开始时，其部分出现在外胚层的腹中线。我们证明了中胚层进入生发带需要N谱系，并且在早期胃化过程中Hau-netrin-1的错误表达阻止了中胚层进入生发带。总之，这些结果表明，由N谱系分泌的Hau-netrin-1在生发带组装过程中指导中胚层的迁移。此外，在发芽带组装完成后，Hau-netrin-1的异位表达破坏了发芽带的表皮迁移。因此，Hau-netrin-1可能参与了水gas胃化的连续阶段中的两个不同事件：早期胃化过程中的生发带的组装以及中胃化过程中它们的表皮迁移。假设在胃造瘘过程中水netnetrin在腹中线的前体细胞中表达，