The building blocks of embryo models: embryonic and extraembryonic stem cells

Abstract

The process of a single-celled zygote developing into a complex multicellular organism is precisely regulated at spatial and temporal levels in vivo. However, understanding the mechanisms underlying development, particularly in humans, has been constrained by technical and ethical limitations associated with studying natural embryos. Harnessing the intrinsic ability of embryonic stem cells (ESCs) to self-organize when induced and assembled, researchers have established several embryo models as alternative approaches to studying early development in vitro. Recent studies have revealed the critical role of extraembryonic cells in early development; and many groups have created more sophisticated and precise ESC-derived embryo models by incorporating extraembryonic stem cell lines, such as trophoblast stem cells (TSCs), extraembryonic mesoderm cells (EXMCs), extraembryonic endoderm cells (XENs, in rodents), and hypoblast stem cells (in primates). Here, we summarize the characteristics of existing mouse and human embryonic and extraembryonic stem cells and review recent advancements in developing mouse and human embryo models.

Fig. 1. A schematic diagram of the relationship between cell states in different culture systems and their correspondence to in vivo development in humans and mice.

The timeline is segmented into three key phases: fertilization, pre-implantation, and post-implantation, indicating specific embryonic days (E) for each species. The upper part of the axis corresponds to mouse development, from fertilization up to E6.5, while the lower part corresponds to human development, from fertilization up to E14. Along the timeline, various stem cell types, primarily ESCs, are listed with their respective culture systems and developmental periods. The periods when embryonic tissues such as TSCs and XENs appear are also correlated with their in vivo counterparts. Different colors signify different stem cell types or conditions. For instance, green shades represent various PSCs, while blue and red shades indicate extraembryonic stem cells such as TSCs and XENs. The gray arrowed area denotes the process from fertilization to gastrulation. Red, pathway inhibition. Green, pathway activation. 2C-LCs 2-cell-like cells, PrESCs primitive endoderm stem cells, FTW-XENs extraembryonic endoderm stem cells cultured in the medium with FGF2, Activin A and CHIR99021, FTW-TSCs trophectoderm stem cells cultured in the medium with FGF2, Activin A and CHIR99021; XENs extraembryonic endoderm stem cells, TSCs trophectoderm stem cells, 2i/LIF ESCs embryonic stem cells cultured in the medium with MEK inhibitor PD0325901, the GSK3 inhibitor CHIR99021, and LIF, RSC rosette-like stem cells, A_loXR-FSCs formative stem cells cultured in the medium with a relatively low concentration of Activin A, the WNT inhibitor XAV939, and the retinoic acid antagonist BMS493, F/A EpiSCs epiblast-derived stem cells cultured in the medium with FGF2 and Activin A, NBFR EpiSCs epiblast-derived stem cells cultured in the N2B27 basal medium containing FGF2 and IWR-1, FTW-ESCs embryonic stem cells cultured in the medium with FGF2, Activin A, and CHIR99021, fPSCs formative state pluripotent stem cells cultured in the medium with FGF2, Activin A, and XAV939, C/X EpiSCs epiblast-derived stem cells cultured in CHIR99021/XAV939 or CHIR99021/IWR-1 condition, 8C-LCs 8-cell-like cells cultured in enhanced medium with higher concentrations of DZNep and TSA, 5i/LA PSCs pluripotent stem cells cultured in five inhibitors medium with LIF, PXGL PSCs: PSCs cultured in the medium with PD0325901, XAV939, Gö6983, and LIF, t2iLGö PSCs PCSs cultured in two inhibitors medium with Gö6983, 4CL PSCs PSCs cultured in the medium with PD0325901, CHIR99021, IWR-1, Activin A, LIF, and two epigenetic factors, A_loXR-FSCs formative stem cells cultured in the medium with a relatively low concentration of Activin A, the WNT inhibitor XAV939, and the retinoic acid antagonist BMS493, FTW-iPSCs induced pluripotent stem cells cultured in the medium with CHIR99021, Activin A, FGF2, 4i ESCs embryonic stem cells cultured in four inhibitors medium with LIF, FGF2, and TGFβ, mTeSR PSCs PSCs cultured in mTeSR medium, NBFR ESCs cultured in the N2B27 basal medium with IWR-1, FGF2.

Fig. 2. A diagram illustrating the correspondence between developmental stages in mice and humans and their alignment with current embryonic model periods.

This detailed diagram illustrates the various stages of early embryonic development in mice and humans, including assembled embryo models and induced embryo models. The timeline at the top marks the embryonic development stages in mice from the zygote to the E14 stage, as well as the corresponding stages in human development. The lower half of the image categorizes two main methods of in vitro emulation. In the “Assembled embryo models” section, different cell types, such as ESCs and TSCs, are combined to simulate the structures and functions of early embryos. In the “Induced embryo models” section, cells differentiate and organize into structures without the guidance of extraembryonic cells. As shown, gastruloid models for both mice and humans self-organize from stem cells and can simulate the gastrulation stage of the embryo. The numbers in the diagram refer to the cited scientific references. TE trophectoderm, PrE primitive endoderm, Epi epiblast, ESC embryonic stem cell, TSC trophoblast stem cell, XEN extraembryonic endoderm, EXMC extraembryonic mesoderm cells, EPSC extended pluripotent stem cell, 2C-LC 2-cell-like cells, EpiSC epiblast stem cell, PSC pluripotent stem cell, PS primitive streak, 8C-LC 8-cell-like cells.