In primates, the amnion emerges through cavitation of the epiblast during implantation, whereas in other species it does so later at gastrulation by the folding of the ectoderm. How the mechanisms of amniogenesis diversified during evolution remains unknown. Unexpectedly, single-cell analysis of primate embryos uncovered two transcriptionally and temporally distinct amniogenesis waves. To study this, we employed the naive-to-primed transition of human pluripotent stem cells (hPSCs) to model peri-implantation epiblast development. Partially primed hPSCs transiently gained the ability to differentiate into cavitating epithelium that transcriptionally and morphologically matched the early amnion, whereas fully primed hPSCs produced cells resembling the late amnion instead, thus recapitulating the two independent differentiation waves. The early wave follows a trophectoderm-like pathway and encompasses cavitation, whereas the late wave resembles an ectoderm-like route during gastrulation. The discovery of two independent waves explains how amniogenesis through cavitation could emerge during evolution via duplication of the pre-existing trophectoderm program.

在灵长类动物中，羊膜在植入过程中通过外胚层的空化出现，而在其他物种中，羊膜在原肠胚形成时通过外胚层折叠而出现。羊膜形成机制在进化过程中如何多样化仍然未知。出乎意料的是，灵长类胚胎的单细胞分析揭示了两个转录和时间上不同的羊膜发生波。为了研究这一点，我们采用人类多能干细胞 (hPSCs) 的幼稚到启动过渡来模拟植入周围外胚层发育。部分引发的 hPSC 瞬时获得了分化为在转录和形态上与早期羊膜相匹配的空洞上皮的能力，而完全引发的 hPSC 则产生了类似于晚期羊膜的细胞，从而概括了两个独立的分化波。早期波遵循类似滋养外胚层的途径并包含空化，而晚期波在原肠胚形成期间类似于外胚层样途径。两个独立波的发现解释了在进化过程中如何通过复制先前存在的滋养外胚层程序而出现通过空化的羊膜发生。