Humanity’s ongoing quest for knowledge has revealed unseen realms and unimaginable places, but there are still a few things that we know little about – the inner workings of a black hole, the composition of McDonald’s special sauce, and what goes on in the early post-implantation human embryo.
There are good reasons for that latter knowledge gap, including ethical and legal concerns that limit the supply of assayable material. However, many studies have hinted that human development is unlike that of animal models such as mice or even non-human primates. Therefore, revealing what goes in the early post-implantation embryo, when the cell layers that give rise to every bodily structure form, could lead to improved reproductive technologies and enhanced stem cell cultivation/differentiation techniques.
Insight from a Petri Dish
Two new studies, from Ali H. Brivanlou (Rockefeller University, NY, USA)  and Magdalena Zernicka-Goetz (University of Cambridge, UK) , have now provided much-wanted insight into the human post-implantation period, following the optimization of a mouse in vitro embryo culture technique [3, 4]. This new system allowed human embryo growth for up to 13 days (more on that later!) and demonstrated that human in vitro fertilization (IVF)-derived embryos self-assemble and have the innate ability to begin early post-implantation development.
Human-Specific Embryo Development Revealed
Both studies began by assessing the presence of various cell lineages in the pre-implantation human blastocyst at roughly 6-7 days post-fertilization (d.p.f). At that stage, their embryos consisted of an inner cell mass (ICM) containing epiblast precursors (OCT4HI/NANOG+) and primitive endoderm (PE) precursors (OCT4HI/GATA6HI), and a single population of trophectodermal cells (OCT4LO/GATA6LO/GATA3+/CDX2+).
Subsequently, the epiblast and PE lineages segregated after the embryos attached to the petri dish culture substrate at around d.p.f 7-8, (so becoming post-implantation embryos). This is distinct to that observed in the mouse, where the epiblast and PE segregate before implantation. The embryos then continued to develop to day 13, at which point they had undergone human-specific tissue morphogenesis and architectural reorganization without the need for any maternal input.
Previous studies into these changes studied rhesus monkey embryos using electron microscopy, highlighting the importance of programmed death (apoptosis) of certain cells. However, these new studies suggest that polarization of epiblast cells is the main driving force behind body plan organization, so discounting the role of apoptosis.
Pluripotent Stem Cells – An Alternative Model System
Excitingly, one of the studies  also used human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) in place of human embryos to pick apart the molecular mechanisms behind post-implantation development. The culture of single stem cells in a 3D matrix led to morphological changes and polarization in the dividing cells similar to that observed in human embryos. This strategy also confirmed that apoptosis was not required for embryo development, and demonstrated the potential utility of this model system for the delineation of the molecular mechanisms at play in the human post-implantation embryo.
Revisiting the Two-Week Limit?
One of the most interesting points to come from this study concerns the 14-day limit for in vitro embryo development set by international policy. At this stage, the primitive streak, a band of cells marking an embryo’s head-to-tail axis, begins to form. In these studies, all experiments were “terminated” at day 13 to comply with said policy. However, an associated Nature Comment (Embryology policy: Revisit the 14-day rule) argues that now that we have the technological tools to break this barrier, policy should be revisited to recognize this fact. For more on that, see the recent Updated Guidelines for Stem Cell Research and Clinical Translation released by the ISSCR.
The Petri Dish and Beyond
The clear conclusion is that post-implantation embryo development is different for humans compared to animal models, and now we have the ability to see what these differences are. The authors also hope that this system will allow us to understand placental and embryonic disorders and early pregnancy loss, and also to devise better iPSC-reprogramming, ESC-derivation, and stem cell differentiation strategies that will aid cell replacement therapies.
- Deglincerti A, Croft GF, Pietila LN, et al. Self-organization of the in vitro attached human embryo. Nature 2016;533:251-254.
- Shahbazi MN, Jedrusik A, Vuoristo S, et al. Self-organization of the human embryo in the absence of maternal tissues. Nat Cell Biol 2016;
- Bedzhov I, Leung CY, Bialecka M, et al. In vitro culture of mouse blastocysts beyond the implantation stages. Nat Protoc 2014;9:2732-2739.
- Bedzhov I and Zernicka-Goetz M Self-organizing properties of mouse pluripotent cells initiate morphogenesis upon implantation. Cell 2014;156:1032-1044.