In a recent study published within the Cell journal, researchers showed that post-gastrulation synthetic embryos (sEmbryos) may very well be synthesized outside the uterus using mouse naïve embryonic stem cells (ESCs).  

Study: ¬Post-Gastrulation Synthetic Embryos Generated Ex Utero from Mouse Naïve ESCs. Image Credit: Anusorn Nakdee/Shutterstock

Background

Recent studies demonstrated that stem cells cultured in vitro featuring various developmental capacities after being microinjected into mammalian pre-implantation embryos could contribute to extra-embryonic or embryonic tissues.

Nonetheless, it’s technically not possible to return embryos on the post-implantation phase to the host uterus, and suitable platforms for embryogenesis outside the uterus after such embryonic stages haven’t yet been made available, including for natural embryos. Hence, the developmental capability of such entities lacks assessments and optimizations. Besides, it’s unknown if cultured stem cells could independently grow to be fully gastrulating embryo-like entities comprising extra-embryonic and embryonic compartments.

The current study’s authors recently developed dynamic and static culture platforms and growth environments enabling continuous recording of natural mouse embryogenesis. It captured embryogenesis from pre-gastrulation to late organogenesis phases outside the uterus. Moreover, the scientists established the best growth circumstances for post-implantation mouse embryos, often called Ex Utero Culture Media (EUCM). They demonstrated that the mechanisms of organogenesis and gastrulation in a mammalian species may very well be jointly and constantly reenacted satisfactorily in a petri dish.

Concerning the study

In the present study, the scientists adapted their newly developed platform for prolonged ex utero development of natural embryos to create mouse post-gastrulation synthetic whole embryo models, i.e., sEmbryos. The team evaluated whether, when incubated in these simulated ex utero experimental settings, stem cells cultured in vitro may very well be used to recreate the organogenesis and gastrulation processes from the beginning. They termed such assumed advanced post-gastrulation embryo-like models because the Synthetic Whole Embryoids (SWEMs) or sEmbryos.

The scientists investigated whether co-aggregation for about five days of the three naïve ESC lines, iGata4, WT, and iCdx2, after pre-treatment with doxycycline (DOX), can produce egg-cylinder-like aggregates in vitro and if the cells segregated in keeping with their transgenic priming. They checked out how well naïve ESC starting populations could yield egg-cylinder-shaped sEmbryos that would grow to be advanced post-gastrulation phases. These populations were then separated into three fractions depending on a transient pre-treatment before co-aggregation.

The authors analyzed whether sEmbryos replicate the morphological alterations occurring throughout natural embryo growth. They assessed advanced sEmbryos for appropriate spatial and temporal expression of lineage markers. Further, the team explored whether advanced sEmbryos can promote self-organization using embryo-derived TSC lines (eTSC). They aimed to well-characterize the formation of extra-embryonic compartments across post-gastrulation sEmbryos.

Finally, the investigators conducted a single cell ribonucleic acid-sequencing (scRNA-seq) evaluation to confirm the complexity of the mouse sEmbryo.

Results and discussions

The researchers documented that their recently formulated electrically assisted ex utero embryo culture conditions and platform, which enable accurate and continuous capture of mouse natural gastrulation and organogenesis outside the uterus, also supported ex utero self-organization of post-gastrulation sEmbryos produced by stem cells co-aggregation. 

The aggregated naïve pluripotent stem cells cluster during lineage priming to form egg-cylinder-shaped and fully developed embryo-like models. These embryo-like structures achieved gastrulation and continued significantly farther to ascertain neural folds, the brain, gut tube, neural tube, somite, beating heart, and progenitors of other organs.

The sEmbryos depicted within the study developed inside extra-embryonic membranes much like natural embryos without the need for externally induced specific signaling pathways. These results highlight the naïve pluripotent stem cells’ latent capability to self-organize into sophisticated, organized, whole embryo-like structures outside the uterus.

The authors exhibit that the trophoblast-stemmed extra-embryonic compartment across advanced sEmbryos may very well be produced exclusively from mouse naïve pluripotent stem cells and want not be acquired only from TSC lines derived from the natural embryos. They identified that the one method to broaden the experimental platforms accessible to analyze early embryonic growth biology from many species was by creating post-gastrulation mouse sEmbryos and only initiating with naïve ESCs.

Conclusions

The study findings indicate that the presently developed sEmbryos harbored each extra-embryonic and embryonic compartments, starting exclusively from naïve ESCs. For this, non-transduced ESCs were co-aggregated with naïve ESCs transiently expressing Gata4 and Cdx2 to stimulate their priming toward primitive endoderm and trophectoderm lineages, respectively. Comparable to mouse embryos on the E8.5 stage, sEmbryos satisfactorily complete gastrulation, progress through critical growth milestones, and generate organ progenitors inside intricate extra-embryonic compartments.

As well as, the study data demonstrated that SWEMs weren’t much like embryos. Nonetheless, the scientists mentioned that SWEMs may be utilized in the longer term as a priceless model for difficult-to-reach temporal windows in early growth embryology studies for modeling human growth abnormalities.

Overall, the current study illustrated that naïve pluripotent cells have the flexible ability to self-organize, functionally reconstruct, and model the entire mammalian embryo after gastrulation.

Journal reference:

• Tarazi, S., Aguilera-Castrejon, A., Joubran, C., Ghanem, N., Ashouokhi, S., Roncato, F., Wildschutz, E., Haddad, M., Oldak, B., Gomez-Cesar, E., Livnat, N., Viukov, S., Lukshtanov, D., Naveh-Tassa, S., Rose, M., Hanna, S., Raanan, C., Brenner, O., Kedmi, M., Keren-Shaul, H., Lapidot, T., Maza, I., Novershtern, N., Hanna, J.H., (2022). ¬Post-Gastrulation Synthetic Embryos Generated Ex Utero from Mouse Naïve ESCs. Cell. doi: https://doi.org/10.1016/j.cell.2022.07.028 https://www.cell.com/cell/fulltext/S0092-8674(22)00981-3