During development, a single cell is transformed into a highly complex organism through progressive cell division, specification and rearrangement. An important prerequisite for the emergence of patterns within the developing organism is to establish asymmetries at various scales, ranging from individual cells to the entire embryo, eventually giving rise to the different body structures. This becomes especially apparent during gastrulation, when the earliest major lineage restriction events lead to the formation of the different germ layers. Traditionally, the unfolding of the developmental program from symmetry breaking to germ layer formation has been studied by dissecting the contributions of different signaling pathways and cellular rearrangements in the in vivo context of intact embryos. Recent efforts, using the intrinsic capacity of embryonic stem cells to self-assemble and generate embryo-like structures de novo, have opened new avenues for understanding the many ways by which an embryo can be built and the influence of extrinsic factors therein. Here, we discuss and compare divergent and conserved strategies leading to germ layer formation in embryos as compared to in vitro systems, their upstream molecular cascades and the role of extrinsic factors in this process.

在发育过程中，单个细胞通过渐进的细胞分裂、规格化和重排转化为高度复杂的生物体。在发育中的生物体中出现模式的一个重要先决条件是在各个尺度上建立不对称，从单个细胞到整个胚胎，最终产生不同的身体结构。这在原肠胚形成期间变得尤其明显，当最早的主要谱系限制事件导致不同胚层的形成时。传统上，通过剖析体内不同信号通路和细胞重排的贡献，研究了从对称性破坏到胚层形成的发育程序的展开。完整胚胎的背景。最近的努力，利用胚胎干细胞自组装和从头生成胚胎样结构的内在能力，为理解胚胎构建的多种方式以及其中外在因素的影响开辟了新的途径。在这里，我们讨论和比较导致胚胎胚层形成的发散和保守策略与体外系统、它们的上游分子级联以及外在因素在该过程中的作用。