Breaking embryonic symmetry is an essential prerequisite to shape the initially symmetric embryo into a highly organized body plan that serves as the blueprint of the adult organism. This critical process is driven by morphogen signaling gradients that instruct anteroposterior axis specification. Despite its fundamental importance, what triggers symmetry breaking and how the signaling gradients are established in time and space in the mammalian embryo remain largely unknown. Stem cell-based in vitro models of embryogenesis offer an unprecedented opportunity to quantitatively dissect the multiple physical and molecular processes that shape the mammalian embryo. Here we review biochemical mechanisms governing early mammalian patterning in vivo and highlight recent advances to recreate this in vitro using stem cells. We discuss how the novel insights from these model systems extend previously proposed concepts to illuminate the extent to which embryonic cells have the intrinsic capability to generate specific, reproducible patterns during embryogenesis.

打破胚胎对称性是将最初对称的胚胎塑造成高度组织的身体计划的必要先决条件，该计划充当成年生物的蓝图。这个关键过程是由指示前后轴规范的形态发生素信号梯度驱动的。尽管它具有根本的重要性，但在哺乳动物胚胎中是什么触发了对称性破坏以及信号梯度如何在时间和空间中建立，但在很大程度上仍然未知。基于干细胞的体外胚胎发生模型为定量剖析塑造哺乳动物胚胎的多种物理和分子过程提供了前所未有的机会。在这里，我们回顾了控制体内早期哺乳动物模式的生化机制并强调使用干细胞在体外重现这一点的最新进展。我们讨论了这些模型系统的新见解如何扩展先前提出的概念，以阐明胚胎细胞在胚胎发生过程中具有生成特定、可重复模式的内在能力的程度。