Formation of the vertebrate postcranial body axis follows two sequential but distinct phases. The first phase generates pre-sacral structures (the so-called primary body) through the activity of the primitive streak on axial progenitors within the epiblast. The embryo then switches to generate the secondary body (post-sacral structures), which depends on axial progenitors in the tail bud. Here we show that the mammalian tail bud is generated through an independent functional developmental module, concurrent but functionally different from that generating the primary body. This module is triggered by convergent Tgfbr1 and Snai1 activities that promote an incomplete epithelial to mesenchymal transition on a subset of epiblast axial progenitors. This EMT is functionally different from that coordinated by the primitive streak, as it does not lead to mesodermal differentiation but brings axial progenitors into a transitory state, keeping their progenitor activity to drive further axial body extension.

中文翻译：

---

脊椎动物轴向伸长核心的 Tgfbr1/Snai1 依赖性发育模块

脊椎动物颅后体轴的形成遵循两个连续但不同的阶段。第一阶段通过原始条纹对外胚层内轴向祖细胞的活动产生前骶骨结构（所谓的初级体）。然后胚胎转换为产生次级体（后骶结构），这取决于尾芽中的轴向祖细胞。在这里，我们展示了哺乳动物尾芽是通过一个独立的功能发育模块生成的，该模块与生成主要身体的模块同时发生但在功能上有所不同。该模块由会聚的 Tgfbr1 和 Snai1 活动触发，这些活动促进了一部分外胚层轴向祖细胞的不完全上皮到间充质转化。这种 EMT 在功能上与原始条纹协调的不同，