Alternative mammalian strategies leading towards gastrulation: losing polar trophoblast (Rauber's layer) or gaining an epiblast cavity

The guinea pig serves as an alternative model to study human preimplantation development

Preimplantation development is an important window of human embryogenesis. However, ethical constraints and the limitations involved in studying human embryos often necessitate the use of alternative model systems. Here we identify the guinea pig as a promising small animal model to study human preimplantation development. Using single-cell RNA-sequencing, we generated an atlas of guinea pig preimplantation development, revealing its close resemblance to early human embryogenesis in terms of the timing of compaction, early-, mid- and late-blastocyst formation, and implantation, and the spatio-temporal expression of key lineage markers. We also show conserved roles of Hippo, MEK-ERK and JAK-STAT signalling. Furthermore, multi-species analysis highlights the spatio-temporal expression of conserved and divergent genes during preimplantation development and pluripotency. The guinea pig serves as a valuable animal model for advancing preimplantation development and stem cell research, and can be leveraged to better understand the longer-term impact of early exposures on offspring outcomes.

Single-cell analysis of preimplantation embryonic development in guinea pigs

BACKGROUND

Guinea pigs exhibit numerous physiological similarities to humans, yet the details of their preimplantation embryonic development remain largely unexplored.

RESULTS

To address this, we conducted single-cell sequencing on the transcriptomes of cells isolated from the zygote stage through preimplantation stages in guinea pigs. This study identified seven distinct cell types within guinea pig preimplantation embryos and pinpointed the timing of zygotic gene activation (ZGA). Trajectory analysis revealed a bifurcation into two lineage-specific branches, accompanied by alterations in specific pathways, including oxidative phosphorylation and vascular endothelial growth factor (VEGF). Additionally, co-expressed gene network analysis highlighted the most enriched functional modules for the epiblast (EPI), primitive endoderm (PrE), and inner cell mass (ICM). Finally, we compared the similarities and differences between human and guinea pig epiblasts (EPIs).

CONCLUSION

This study systematically constructs a cell atlas of guinea pig preimplantation embryonic development, offering fresh insights into mammalian embryonic development and providing alternative experimental models for studying human embryonic development.

A single-cell atlas of pig gastrulation as a resource for comparative embryology

Cell-fate decisions during mammalian gastrulation are poorly understood outside of rodent embryos. The embryonic disc of pig embryos mirrors humans, making them a useful proxy for studying gastrulation. Here we present a single-cell transcriptomic atlas of pig gastrulation, revealing cell-fate emergence dynamics, as well as conserved and divergent gene programs governing early porcine, primate, and murine development. We highlight heterochronicity in extraembryonic cell-types, despite the broad conservation of cell-type-specific transcriptional programs. We apply these findings in combination with functional investigations, to outline conserved spatial, molecular, and temporal events during definitive endoderm specification. We find early FOXA2 + /TBXT- embryonic disc cells directly form definitive endoderm, contrasting later-emerging FOXA2/TBXT+ node/notochord progenitors. Unlike mesoderm, none of these progenitors undergo epithelial-to-mesenchymal transition. Endoderm/Node fate hinges on balanced WNT and hypoblast-derived NODAL, which is extinguished upon endodermal differentiation. These findings emphasise the interplay between temporal and topological signalling in fate determination during gastrulation.

Development of node architecture and emergence of molecular organizer characteristics in the pig embryo

BACKGROUND

The avian node is the equivalent of the amphibian Spemann's organizer, as indicated by its ability to induce a secondary axis, cellular contribution, and gene expression, whereas the node of the mouse, which displays limited inductive capacities, was suggested to be a part of spatially distributed signaling. Furthermore, the structural identity of the mouse node is subject of controversy, while little is known about equivalent structures in other mammals.

RESULTS

We analyzed the node and emerging organizer in the pig using morphology and the expression of selected organizer genes prior to and during gastrulation. The node was defined according to the "four-quarter model" based on comparative consideration. The node of the pig displays a multilayered, dense structure that includes columnar epithelium, bottle-like cells in the dorsal part, and mesenchymal cells ventrally. Expression of goosecoid (gsc), chordin, and brachyury, together with morphology, reveal the consecutive emergence of three distinct domains: the gastrulation precursor domain, the presumptive node, and the mature node. Additionally, gsc displays a ventral expression domain prior to epiblast epithelialization.

CONCLUSION

Our study defines the morphological and molecular context of the emerging organizer equivalent in the pig and suggests a sequential development of its function.

Lumen expansion is initially driven by apical actin polymerization followed by osmotic pressure in a human epiblast model

Post-implantation, the pluripotent epiblast in a human embryo forms a central lumen, paving the way for gastrulation. Osmotic pressure gradients are considered the drivers of lumen expansion across development, but their role in human epiblasts is unknown. Here, we study lumenogenesis in a pluripotent-stem-cell-based epiblast model using engineered hydrogels. We find that leaky junctions prevent osmotic pressure gradients in early epiblasts and, instead, forces from apical actin polymerization drive lumen expansion. Once the lumen reaches a radius of ∼12 μm, tight junctions mature, and osmotic pressure gradients develop to drive further growth. Computational modeling indicates that apical actin polymerization into a stiff network mediates initial lumen expansion and predicts a transition to pressure-driven growth in larger epiblasts to avoid buckling. Human epiblasts show transcriptional signatures consistent with these mechanisms. Thus, actin polymerization drives lumen expansion in the human epiblast and may serve as a general mechanism of early lumenogenesis.

Efficient derivation of embryonic stem cells and primordial germ cell-like cells in cattle

The induction of the germ cell lineage from pluripotent stem cells (in vitro gametogenesis) will help understand the mechanisms underlying germ cell differentiation and provide an alternative source of gametes for reproduction. This technology is especially important for cattle, which are among the most important livestock species for milk and meat production. Here, we developed a new method for robust induction of primordial germ cell-like cells (PGCLCs) from newly established bovine embryonic stem (bES) cells. First, we refined the pluripotent culture conditions for pre-implantation embryos and ES cells. Inhibition of RHO increased the number of epiblast cells in the pre-implantation embryos and dramatically improved the efficiency of ES cell establishment. We then determined suitable culture conditions for PGCLC differentiation using bES cells harboring BLIMP1-tdTomato and TFAP2C-mNeonGreen (BTTN) reporter constructs. After a 24-h culture with bone morphogenetic protein 4 (BMP4), followed by three-dimensional culture with BMP4 and a chemical agonist and WNT signaling chemical antagonist, bES cells became positive for the reporters. A set of primordial germ cells (PGC) marker genes, including PRDM1/BLIMP1, TFAP2C, SOX17, and NANOS3, were expressed in BTTN-positive cells. These bovine PGCLCs (bPGCLCs) were isolated as KIT/CD117-positive and CD44-negative cell populations. We anticipate that this method for the efficient establishment of bES cells and induction of PGCLCs will be useful for stem cell-based reproductive technologies in cattle.

Using extended growth of cattle embryos in culture to gain insights into bovine developmental events on embryonic days 8 to 10

We have previously described an extended embryo culture system, based on uterine composition, growth factors and the cell culture additive B27, for growing cattle embryos in vitro beyond embryonic day 7. Here, extended in vitro embryos are compared to embryos developed in the uterus and are used to establish a developmental staging framework useful for understanding developmental events occurring until Day 10. Immunofluorescence or mRNA expression of the ICM/epiblast markers OCT4, SOX2 and NANOG, hypoblast markers GATA6, SOX17 and GATA4 and trophoblast genes CDX2, GATA3, ASCL2 and IFNT revealed the presence of four stages during this period that can be molecularly distinguished. These are expanded blastocyst, hatched blastocyst, hypoblast layering and early hypoblast migration. Interestingly NANOG and SOX17 show reciprocal expression at the expanded blastocyst stage, well before SOX2 and GATA6 expression refines to a similar so-called "salt and pepper" mutually exclusive expression in the ICM at the hatched blastocyst stage. GATA4 expression is only seen from stages when the hypoblast starts migrating around the blastocyst cavity. Intriguingly, trophoblast still expresses GATA6 and OCT4 in all cells during the expanded blastocyst phase, while SOX2 and SOX17 are seen in only some trophoblast cells. By the hypoblast-epiblast layering stage no trophoblast expression remains except for that of OCT4 protein, which starts waning in trophoblast once the hypoblast begins migrating. Lastly, it is shown that cultured embryos exhibit increased expression of the stress marker TP53 in the epiblast and hypoblast at late stages in comparison to embryos produced in the uterine environment.

Interspecies control of development during mammalian gastrulation

Gastrulation represents a pivotal phase of development and aberrations during this period can have major consequences, from minor anatomical deviations to severe congenital defects. Animal models are used to study gastrulation, however, there is considerable morphological and molecular diversity of gastrula across mammalian species. Here, we provide an overview of the latest research on interspecies developmental control across mammals. This includes single-cell atlases of several mammalian gastrula which have enabled comparisons of the temporal and molecular dynamics of differentiation. These studies highlight conserved cell differentiation regulators and both absolute and relative differences in differentiation dynamics between species. Recent advances in in vitro culture techniques have facilitated the derivation, maintenance and differentiation of cell lines from a range of species and the creation of multi-species models of gastrulation. Gastruloids are three-dimensional aggregates capable of self-organising and recapitulating aspects of gastrulation. Such models enable species comparisons outside the confines of the embryo. We highlight recent in vitro evidence that differentiation processes such as somitogenesis and neuronal maturation scale with known in vivo differences in developmental tempo across species. This scaling is likely due to intrinsic differences in cell biochemistry. We also highlight several studies which provide examples of cell differentiation dynamics being influenced by extrinsic factors, including culture conditions, chimeric co-culture, and xenotransplantation. These collective studies underscore the complexity of gastrulation across species, highlighting the necessity of additional datasets and studies to decipher the intricate balance between intrinsic cellular programs and extrinsic signals in shaping embryogenesis.