As far as systems to study embryogenesis go, embryonic stem cell (ESC)-based in vitro culture systems have been a vital tool. But they do not mimic the changes that occur as the embryo implants in the uterus, making it difficult to study this process of dramatic changes in blastocyst architecture in vitro. Now, a report from Magdalena Zernicka-Goetz at the University of Cambridge and colleagues shows that culturing mouse ESCs with extra-embryonic cells leads to formation of embryos that recapitulate this complex process with remarkable veracity (Harrison et al., 2017). The approach is poised to enable unprecedented dissection of the molecular pathways that mediate critical steps in embryogenesis.

An embryo derived from mouse embryonic and trophoblast stem cells, stained for various markers as indicated. Reprinted with permission from Fig. 1b of Harrison et al. (2017).

The beginnings of tissue identity are established during the first days of embryonic development as the epiblast, from which all tissues in the body originate, is defined in parallel to trophectoderm and primitive endoderm, both extra-embryonic tissues that give rise to the placenta and yolk sac. It is the communication among these three tissues that, during implantation, promotes formation of a lumen in the epiblast and another one in the trophectoderm, which eventually unite to form the ‘egg cylinder’. Symmetry then breaks at the tissues’ boundary as development continues.

Studying these critical events in embryogenesis has not been easy. As the embryo implants, engulfment by uterine tissues obscures the changes that occur in the embryo’s architecture, preventing in vivo analysis. In vitro culture of mouse embryos (Bedzhov et al., 2014), previously reported by the Zernicka-Goetz group, has enabled important discoveries by allowing imaging of morphogenetic events but doesn’t enable the degree of molecular manipulation possible with cell-culture-based methods. While culture of ESCs into organoids, embryonic bodies, or patterned colonies has been useful, ESCs alone do not support development of the postimplantation egg cylinder.

To better mimic natural events with cell culture, Zernicka-Goetz and her team mixed two out of the three cell types involved: single ESCs and clusters of trophoblasts, with extracellular matrix in Matrigel providing a 3D scaffold. Reproducibly, 22% of structures contained both cell types, and ∼92% of those had the typical elongated cylindrical structure of a postimplantation embryo. By 96 h post-plating, the cavities of the ESC and trophoblast compartments merged into a single large cavity.

That the stem cells could self organize was no surprise, recalls Zernicka-Goetz, but that they could replicate the embryo’s natural architecture at this developmental stage was. “And they can do this without the third stem cell component for another extra-embryonic tissue that is normally involved in embryo development at that stage,” she says.

Demonstrating the utility and versatility of their approach, the researchers studied molecular and cellular events in embryonic development using reporter ESC lines and synthetic inhibitors. For example, they treated cultured embryos with an inhibitor of the Nodal/Activin pathway and tested embryos from Nodal-deficient ESCs. Seeking insights into the generation of mesoderm and the specification of primordial germ cells, they used reporter ESC lines, which enabled them to demonstrate expression of primordial germ cell markers after 120 h of culture.

The approach does not require exogenous treatment, and “it has a built-in reference point—the boundary between the two cell types,” says Zernicka-Goetz. Whether it would work with human cells is not clear; and if it did, the use of such a system would almost certainly face ethical questions. But the ability to image and manipulate molecular events in mouse embryogenesis in a culture dish provides a tool that will enable important discoveries with implications for both murine and human biology.