ESCs injected into the 8-cell stage mouse embryo modify pattern of cleavage and cell lineage specification

Exposure of chimaeric embryos to exogenous FGF4 leads to the production of pure ESC-derived mice

Producing chimaeras constitutes the most reliable method of verifying the pluripotency of newly established cells. Moreover, forming chimaeras by injecting genetically modified embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) into the embryo is part of the procedure for generating transgenic mice, which are used for understanding gene function. Conventional methods for generating transgenic mice, including the breeding of chimaeras and tetraploid complementation, are time-consuming and cost-inefficient, with significant limitations that hinder their effectiveness and widespread applications. In the present study, we modified the traditional method of chimaera generation to significantly speed up this process by generating mice exclusively derived from ESCs. This study aimed to assess whether fully ESC-derived mice could be obtained by modulating fibroblast growth factor 4 (FGF4) levels in the culture medium and changing the direction of cell differentiation in the chimaeric embryo. We found that exogenous FGF4 directs all host blastomeres to the primitive endoderm fate, but does not affect the localisation of ESCs in the epiblast of the chimaeric embryos. Consequently, all FGF4-treated chimaeric embryos contained an epiblast composed exclusively of ESCs, and following transfer into recipient mice, these embryos developed into fully ESC-derived newborns. Collectively, this simple approach could accelerate the generation of ESC-derived animals and thus optimise ESC-mediated transgenesis and the verification of cell pluripotency. Compared to traditional methods, it could speed up functional studies by several weeks and significantly reduce costs related to maintaining and breeding chimaeras. Moreover, since the effect of stimulating the FGF signalling pathway is universal across different animal species, our approach can be applied not only to rodents but also to other animals, offering its utility beyond laboratory settings.

Generation of Rabbit Chimeras by Eight-Cell Stage Embryo Injection

Pluripotent stem cell (PSC) injection to the blastocyst stage embryos is a widely used method to evaluate the pluripotency through chimeric contribution. It is routinely used to produce transgenic mice. However, PSC injection to the blastocyst stage embryos in rabbits is challenging. At this stage, the in vivo developed rabbit blastocysts possess a thick mucin layer that is inhibitory for microinjection, whereas in vitro developed rabbit blastocysts that lack such mucin layer often fail to implant after embryo transfer. In this chapter, we describe a detailed protocol of rabbit chimera production through mucin-free eight-cell stage embryo injection procedure.

The role of FGF-4 and FGFR-2 on preimplantation embryo development in experimental maternal diabetes

OBJECTIVE

Diabetes mellitus can cause spontaneous abortion, neonatal diseases, congenital malformations, and death. There are many studies related to the damage of in vitro hyperglycemia on embryogenesis in literature, but not enough studies on in vivo hyperglycemia effects on embryogenesis. Fibroblast growth factor (FGF) molecules play an essential role in pre-implantation embryo development and diabetes pathogenesis. In our study, we researched whether FGF-4 and FGFR-2 were playing a role in maternal diabetes' effects on embryo development.

MATERIAL AND METHODS

Thirty adult virgin female BALB/c mice were randomly divided into two groups: control and diabetic. The experimental diabetes model was generated by streptozotocin (55 mg/kg, once, intraperitoneally). The control and the diabetic group were mated. Embryos were collected at the morula and blastocyte stages corresponding to the third and fourth days of pregnancy. Embryo's FGF-4 and FGFR-2 molecules were evaluated by their immunofluorescence staining and immunoreactivity score.

RESULT

The results clearly showed that the FGF-4 and FGFR-2 immunofluorescence reactivity was higher in the diabetes group.

CONCLUSION

We concluded that FGF-4 and FGFR-2 overexpression might impair mouse pre-implantation embryo development in maternal diabetes and suggest investigating whether they have crucial effects on human embryo development and infertility in maternal diabetes.

Developmental capacity is unevenly distributed among single blastomeres of 2-cell and 4-cell stage mouse embryos

During preimplantation development, mammalian embryo cells (blastomeres) cleave, gradually losing their potencies and differentiating into three primary cell lineages: epiblast (EPI), trophectoderm (TE), and primitive endoderm (PE). The exact moment at which cells begin to vary in their potency for multilineage differentiation still remains unknown. We sought to answer the question of whether single cells isolated from 2- and 4-cell embryos differ in their ability to generate the progenitors and cells of blastocyst lineages. We revealed that twins were often able to develop into blastocysts containing inner cell masses (ICMs) with PE and EPI cells. Despite their capacity to create a blastocyst, the twins differed in their ability to produce EPI, PE, and TE cell lineages. In contrast, quadruplets rarely formed normal blastocysts, but instead developed into blastocysts with ICMs composed of only one cell lineage or completely devoid of an ICM altogether. We also showed that quadruplets have unequal capacities to differentiate into TE, PE, and EPI lineages. These findings could explain the difficulty of creating monozygotic twins and quadruplets from 2- and 4-cell stage mouse embryos.

Attempts to obtain fully xenogeneic fetuses in rat ↔ mouse model†,‡

The full-term development of the xenogeneic embryo in the uterus of the mother of different species is very restricted and can occur only in certain groups of closely related mammals. In the case of mouse ↔ rat chimeras, the interspecific uterine barrier is less hostile to interspecific chimeric fetuses. In current work, we tested the development of mouse and rat fetuses in uteri of females of the opposite species. We created chimeric mouse ↔ rat blastocysts by injection of mouse embryonic stem cells (ESCs) into eight-cell rat embryos and rat ESCs into eight-cell mouse embryos. Chimeras were transferred to the foster mothers of the opposite species. Despite a huge number of transferred embryos (>1000 in total for both variants), only one live fetus derived solely from the mouse ESCs was isolated at E13.5 from the rat uterus. All other fetuses and newborns were chimeric or were built only from the cells of the recipient embryo. We examined the possible reason for such an outcome and found that the xenogeneic fetuses are eliminated at the perigastrulation stage of development. Thus, we conclude that in the rat ↔ mouse combination even when extraembryonic tissues of the chimeric embryo are composed solely of the cells of the same species as the female to which embryos are transferred, the full-term development of the pure xenogeneic fetus is very unlikely.

Coordination between patterning and morphogenesis ensures robustness during mouse development

The mammalian preimplantation embryo is a highly tractable, self-organizing developmental system in which three cell types are consistently specified without the need for maternal factors or external signals. Studies in the mouse over the past decades have greatly improved our understanding of the cues that trigger symmetry breaking in the embryo, the transcription factors that control lineage specification and commitment, and the mechanical forces that drive morphogenesis and inform cell fate decisions. These studies have also uncovered how these multiple inputs are integrated to allocate the right number of cells to each lineage despite inherent biological noise, and as a response to perturbations. In this review, we summarize our current understanding of how these processes are coordinated to ensure a robust and precise developmental outcome during early mouse development. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Growth-factor-mediated coupling between lineage size and cell fate choice underlies robustness of mammalian development

Precise control and maintenance of population size is fundamental for organismal development and homeostasis. The three cell types of the mammalian blastocyst are generated in precise proportions over a short time, suggesting a mechanism to ensure a reproducible outcome. We developed a minimal mathematical model demonstrating growth factor signaling is sufficient to guarantee this robustness and which anticipates an embryo's response to perturbations in lineage composition. Addition of lineage-restricted cells both in vivo and in silico, causes a shift of the fate of progenitors away from the supernumerary cell type, while eliminating cells using laser ablation biases the specification of progenitors toward the targeted cell type. Finally, FGF4 couples fate decisions to lineage composition through changes in local growth factor concentration, providing a basis for the regulative abilities of the early mammalian embryo whereby fate decisions are coordinated at the population level to robustly generate tissues in the right proportions.

Production of F0 mice from embryonic stem cells injected eight-cell stage embryos which stored at refrigeration temperature

At refrigeration temperature, mouse embryos can retain their developmental ability for a couple of days. Previous research reports have focused on the effect of cool temperature on the development of 2-cell stage embryos, morulae or blastocysts and determined that the embryo still has the ability to produce offspring after about 48 h storage at refrigeration temperature. Here we examined whether refrigeration temperature affects the development of the eight-cell stage and if the stored eight-cell stage embryo can still be used as a host embryo for ES cell injection. Our results show that eight-cell stage embryos can develop into blastocysts and yield pups after cold storage for 24 and 48 h. After ES cell injection, stored eight-cell stage embryos can support ES cells developing to F0 pups. In summary, cool storage can preserve the developmental ability of eight-cell stage embryos for at least 48 h, allowing transportation of the embryos at refrigeration temperature between different labs and their subsequent use as host embryos for ES cell injection.

Pluripotent stem cells secrete Activin A to improve their epiblast competency after injection into recipient embryos

It is not fully clear why there is a higher contribution of pluripotent stem cells (PSCs) to the chimera produced by injection of PSCs into 4-cell or 8-cell stage embryos compared with blastocyst injection. Here, we show that not only embryonic stem cells (ESCs) but also induced pluripotent stem cells (iPSCs) can generate F0 nearly 100% donor cell-derived mice by 4-cell stage embryo injection, and the approach has a “dose effect”. Through an analysis of the PSC-secreted proteins, Activin A was found to impede epiblast (EPI) lineage development while promoting trophectoderm (TE) differentiation, resulting in replacement of the EPI lineage of host embryos with PSCs. Interestingly, the injection of ESCs into blastocysts cultured with Activin A (cultured from 4-cell stage to early blastocyst at E3.5) could increase the contribution of ESCs to the chimera. The results indicated that PSCs secrete protein Activin A to improve their EPI competency after injection into recipient embryos through influencing the development of mouse early embryos. This result is useful for optimizing the chimera production system and for a deep understanding of PSCs effects on early embryo development.

Mouse blastomeres acquire ability to divide asymmetrically before compaction

The mouse preimplantation embryo generates the precursors of trophectoderm (TE) and inner cell mass (ICM) during the 8- to 16-cell stage transition, when the apico-basal polarized blastomeres undergo divisions that give rise to cells with different fate. Asymmetric segregation of polar domain at 8–16 cell division generate two cell types, polar cells which adopt an outer position and develop in TE and apolar cells which are allocated to inner position as the precursors of ICM. It is still not know when the blastomeres of 8-cell stage start to be determined to undergo asymmetric division. Here, we analyze the frequency of symmetric and asymmetric divisions of blastomeres isolated from 8-cell stage embryo before and after compaction. Using p-Ezrin as the polarity marker we found that size of blastomeres in 2/16 pairs cannot be used as a criterion for distinguishing symmetric and asymmetric divisions. Our results showed that at early 8-cell stage, before any visible signs of cortical polarity, a subset of blastomeres had been already predestined to divide asymmetrically. We also showed that almost all of 8-cell stage blastomeres isolated from compacted embryo divide asymmetrically, whereas in intact embryos, the frequency of asymmetric divisions is significantly lower. Therefore we conclude that in intact embryo the frequency of symmetric and asymmetric division is regulated by cell-cell interactions.