March 2, 2017

expert reaction to paper on creating an artificial mouse ‘embryo’ from stem cells

Researchers publishing in Science have created a structure resembling a mouse embryo in culture, using two types of stem cells and a 3d scaffold on which they can grow.

Dr Dusko Ilic, Reader in Stem Cell Science, King’s College London, said:

“Another masterpiece of recreating in vitro the earliest steps of embryo development coming from the Zernicka-Goetz lab. A beautifully conceived and executed study demonstrating interplay of different cells in different cellular compartments within the first days of mouse development”

Prof. Robin Lovell-Badge, Group Leader, The Francis Crick Institute, said:

“This is a very interesting paper on a number of counts, showing that it is possible to mimic several aspects of early mouse postimplantation embryo development beginning with just two stem cells lines in vitro. This is a complementary approach to the same group’s previous work on culturing intact embryos through similar stages, permitting study of events that normally take place within the uterus and are therefore difficult to observe, but in this case with an essentially unlimited supply of starting material.

“If has been known for a long time that when mouse embryonic stem (ES) cells are allowed to differentiate in vitro they give many cell types, but in a disorganised fashion, but when they are introduced into normal early preimplantation embryos, usually at blastocyst stages, they can give rise to chimeras where the animals are composed of cells derived from both the host embryo and the ES cells. This shows that the ES cells are able to go through normal embryogenesis in a properly organised way and give rise to all cell types in the resulting a live born animal, at least when they are supported, and presumably patterned by the cells of the host embryo. Furthermore, this patterning must be initiated by the extraembryonic cell types of the early embryo (the outer layer of trophectoderm and an inner layer of extraembryonic endoderm, fated to give the placenta and yolk sac, respectively) because it is possible to bias the contribution of the ES cells such that they completely replace the equivalent pluripotent cells of the host embryo (the epiblast).

“More recently, several groups have used what is known about some of the signals known to pattern the epiblast and to promote gastrulation in attempts to obtain organised development of isolated groups of ES cells in vitro. They have had some success as measured by the expression of relevant genes and the formation of structures referred to as “gastruloids”. However, they do not really look like early postimplantation embryos.

“In the present study the authors have combined a small number of ES cells with another stem cell type corresponding to trophectoderm progenitors (TS cells) within a matrix that supports development of 3D structures. A significant proportion of these “ETS embryos” would appear to develop in a way that is remarkably similar to normal gastrulation, particularly in the way that mesodermal cells form and in the specification of primordial germ cell-like cells. This suggests that it is the combination of the two cell types that is important, and not a history of earlier development of the trophectoderm and epiblast in a precise order and arrangement. The data also suggest that the extraembryonic endoderm is not essential, which is a surprise. The authors propose that its main role may be to produce matrix material, which they had provided artificially. The authors go on to test several signalling pathways for their role in early patterning events, finding that these are similar in the ETS embryos and normal embryos.

“However, the paper is just as interesting for the many unanswered questions. Why did not all the initial ES plus TS structures develop and what happened to those that failed? In normal embryos the epiblast cells undergo very rapid proliferation during gastrulation, but we are not given any information about this. While the authors show mesoderm development, there is no data on the other two germ layers, ectoderm and endoderm: are these present and patterned appropriately?  Why do the primordial germ cell-like cells come to surround the embryonic portion of the ETS embryos, which never happens in a normal embryo?  Is this because of the absence of extraembryonic endoderm which might serve to constrain them to a specific location? How far can the ETS embryos develop? This is unlikely to be very far given the usual constraints about providing adequate supply of nutrients and oxygen in a 3D structure, which is solved in vivo through the development of the placenta. And finally could ETS embryos be developed using human ES cells (or induced pluripotent stem cells)? The simple answer at the moment is probably no, because we do not have the equivalent of TS cells from human embryos. Scientists like to have unanswered questions – they keep us busy!”

Epiblast: The pluripotent cells of the late blastocyst and the embryo just after implantation.

Gastrulation: The process that leads to the formation of the three primary germ layers, ectoderm (skin and central nervous system), mesoderm (muscle, bone, etc), and endoderm (gut, liver, etc) from the single layer of epiblast cells.

Prof. James Adjaye, Chair of Stem Cell Research and Regenerative Medicine, Heinrich Heine University, said:

“Pluripotent stem cells (embryonic stem cells and induced pluripotent stem cells, ESCs and iPSCs) of both human and mouse can—under the correct experimental conditions—be encouraged to transform into a 3D ball or aggregate of cells representing potentially all cell types present in our bodies. With this knowledge, several laboratories have shown that mini brains or so called 3D brain organoids can be generated from pluripotent stem cells. A further development pioneered by a Japanese laboratory [http://www.nature.com/nature/journal/v499/n7459/abs/nature12271.html] showed that 3D liver organoids can be generated by first differentiating the pluripotent stem cells into hepatocytes and them mixing them with the other cell types (mesenchymal stem cells and endothelial cells). These 3D brain and liver organoids are much closer in terms of architecture, physiology and function to the real organs. To date this 3D concept has been used to generate other organoids in the dish: lung, intestine and kidney.”

“In the current study, Prof. Magdalena Zernicka-Goetz’s laboratory has extended and applied the 3D concept shown possible for other organs to generate mouse embryos which are similar to natural embryos in terms of gene expression and architecture. To achieve this, they mixed mouse embryonic stem cells (ESCs) with trophoblast stem cells (TSCs) which are normally generated from trophoblast cells which form the outer layer of a blastocyst, and provides nutrients to the embryo and develop into a large part of the placenta.”

“The ETS embryos were then stimulated with the right chemicals and supportive environment to mimic embryogenesis. The method is highly reproducible since 92.68% of the ETS embryos had the correct architecture seen in native embryos at this early stage of development. “

“The current mouse ETS embryos and the previous publication in 2016 of the same research group [2] clearly paves the way for deriving human ETS embryos. Human pluripotent stem cells (ESCs) and trophoblast stem cells (TSCs) are already established, the signals for inducing embryogenesis as described in this new paper is to a large extent conserved in mouse and man.”

“As always, these types of experiments using human stem cells is regulated but there is no ‘universal regulatory body’. Each country has its own regulatory body which will ultimately decide on whether human ETS embryos can be generated and for how long they can be left in the petri dish to develop further. Of course, there should be an international dialogue on the regulation of such experiments.”

“The ETS embryos were able generate Primordial Germ Cells as one would expect during embryogenesis.  Germ cells are found only in the gonads, i.e. ovaries in females and testes in males. In these organs, females make eggs, and males make sperm.”

“In mice, for example, one week after fertilization, about 50 cells in tissue lying outside the embryo proper are induced by their neighbours to become primordial germ cells. In humans, this occurs after approximately three weeks after fertilization.”

“As an extrapolation, the ETS embryo model is ideal for studying the formation of testes, ovaries and embryogenesis. Furthermore, in combination with genome editing tools it will now be possible to better study and understand why certain gene mutations affect normal embryo development.”

* ‘Assembly of embryonic and extra-embryonic stem cells to mimic embryogenesis in vitro’ by Harrison, SE et al. published in Science on Thursday 2^nd March.

All our previous output on this subject can be seen at this weblink: http://www.sciencemediacentre.org/tag/stem-cells/

Declared interests

Dr Dusko Ilic: I have co-authored another recently published study with Magdalena Zernicka-Goetz and we have an ongoing collaboration.

Prof. Robin Lovell-Badge: Robin Lovell-Badge is employed by the Francis Crick Institute where he works on some aspects of early mouse development and makes use of ES cells, but he has no conflicts of interest with respect to the study reported here.

None others received