From time immemorial, three things have been needed for a new life to enter this world: sperm, an egg and a womb to carry the fertilized egg until birth.
Breakthrough research carried out at the Weizmann Institute of Science in Rehovot has shown that it’s possible to create life without this primal threesome. Scientists there have created a mouse embryo without an egg, sperm or a womb. According to their findings, reported last week in the journal Cell, a synthetic "embryoid" created solely from the stem cells of an adult mouse survived for eight and a half days.
The team was led by Jacob (Yaqub) Hanna of the Department of Molecular Genetics of the institute, located in Rehovot. In earlier research, Prof. Hanna showed that a natural mouse embryo created by insemination of an egg by sperm could grow in an artificial womb that he developed in his lab. That study still left a big question unanswered: Is it possible to create a synthetic embryo from stem cells cultured in a petri dish?
“To answer this question, we had to first of all develop the conditions for growing a natural embryo in the gastrulation stage [when differentiation of the embryo begins],” Hanna says. “Then we examined what happens if you put embryonic stem cells that were grown in a petri dish in these conditions to see if they would develop into embryos. And the answer is yes.”
Another innovative aspect of his study has to do with the three types of stem cells from which all mammalian embryos are composed: the cells of the embryo itself, the cells from which the yolk sac develops and the cells from which the placenta develops. Up to now, scientists believed that in order to create a synthetic embryo, the three types of stem cells would have to be developed separately.
The present study shows that embryonic stem cells in their incipient (i.e., naïve) state are sufficient – that the yolk sac and placenta develop from them independently. “That was another major surprise,” the professor notes. “The earliest stem cells can also manufacture the surrounding tissues, and later all of the organs.”
“The early steam cells are called ‘naïve stem cells.’ They are virgin stem cells, tabula rasa,” Hanna explains. “These cells can develop into any type of cell. In mice, these cells develop in the first three days after fertilization, and in humans in the first six days, to the 32-cell embryo stage.”
Another problem cracked by the researchers was how to maintain these naïve stem cells in their primal condition. “Blood and skin cells, for example, are something we have all the time,” Hanna explains. “But embryonic stem cells are something that only exist for a few days in our development, and then disappear.”
The scientists thus had to develop a medium in which the naïve state of the stem cell would be perpetuated, by blocking its paths of development. “Normally, the naivete disappears within 12 hours,” he says. “But we take the cell and trap it using inhibitor molecules and hormones.”
Hanna’s lab at the Weizmann Institute lab was also the first in the world to have developed naïve growth conditions for human stem cells – a process that took eight years – and one of the first two labs in the world to do so with mouse stem cells.
Stem cell scientist Prof. Alexander Meissner, director of the Max Planck Institute in Berlin, says that the Weizmann study “is an incredible step forward and shows the amazing potential of pluripotent cells. There is still a natural limit of how far one can take these embryos ex utero, but I am confident that these boundaries will be continuously pushed back. The Hanna study is a powerful example of that.”
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Two months ago, Prof. Hanna and a partner founded a company called Renewal Bio, which will develop possible applications of the stem cell technology, primarily for treating infertility problems and genetic diseases. “There are patents on all of our discoveries,” he says. “This technology shows that if you place naïve stem cells in the right culture, they have the capability to self-organize. You don’t need to do anything.”
One possible application can be growth of tissues for transplantation in humans, Hanna says: “If a person needs a transplant of blood cells, or egg cells, you’ll be able to take skin cells from them, use them to grow a short-lived model synthetic embryoid, and take from it the blood cells or very early gonad cells and eventually cause an egg to develop from them.”
'3D biological printer'
At present, scientists cannot easily differentiate stem cells into mature cell types, so that they can develop into a specific type of cell needed for transplant. Hanna says that this has only been accomplished with a few types of cells: “We haven’t been able to recreate the differentiation within the embryo, because there are a lot of very specific stages in the timing. Here we’re saying that you don’t need to do any of that, because if you put the naïve stem cells in a supportive environment they will self-organize and start to produce all the organs and tissues.
"The embryo’s development process is essentially a sophisticated mechanism for manufacturing organs – the best 3D biological printer there is,” he adds.
The synthetic embryos that were created in the lab were defective when compared to naturally developed embryos. Out of 10,000 starting cellular aggregates, only about 50 ultimately self-organized into embryos after eight days. The synthetic ones also only survived eight and a half days. (The full gestational period is 20 days, among mice.)
“Even after eight and a half days, all of the organs’ earliest cells are there,” Hanna emphasizes. “There are brain stem cells, early heart cells, blood stem cells, the digestive system began to develop, there were even progenitor cells of the eyes. We definitely want to reach more advanced stages, but even if we don’t succeed, it is known that if you implant early embryonic blood cells from these stages back into adult mice, they can implant and replace the blood cells in the transplanted host.”
The professor stresses that his lab team does not have the capability of producing adult mice, and certainly not human beings, nor does it aspire to such goals.
“This is very, very far from us," he says. "But even if we reach the point of organized tissue resembling a synthetic human embryo that is 40 or 50 days old, all of the stem cells of the organs will already exist in it – the stem cells of the eggs, of the heart, and so on, and this could be a source of tissues for the creation of cells that could be used in transplants. In mice we’ve shown that it is possible, and now we need to see it in humans.”
Development of artificial models of embryos could also reduce the use of animals in scientific experiments, Hanna notes. “For instance, experiments with monkeys are quite problematic. If there are early synthetic embryos, they will be very helpful for research, for example, as a platform for testing drugs,” he says.
Prof. Alfonso Martinez Arias of Cambridge University, a world expert in the field of embryonic modeling, says that the Weizmann research constitutes “an important landmark in our understanding of how embryos build themselves.”
He adds: “The last few years have seen the emergence of technologies based on embryonic stem cells that allow the differentiation of all cell types of an organism in the lab. This raised the possibility that, perhaps, it might be possible to reconstruct the embryo from these cells and a small number of laboratories have been trying to do this with interesting results but never obtaining a complete specimen. The group of Jacob Hanna has now achieved this. It is the first time that a complete embryo is built from its component parts in vitro.”
He notes, however, that “it is important to understand that this is a proof of concept experiment, and that much work remains to be done.”
Martinez Arias praised the detail with which the Israeli scientists described every aspect of their research, adding that “this has been missing in previous attempts, which may have hampered the reproducibility of the system.”
As to whether this technology would make it possible to create synthetic human embryoids, Martinez Arias says that he expects that it will happen, but not anytime soon: “It is important to notice that these synthetic embryos do not progress very far and have defects and the same should be expected from any human counterpart. Nevertheless, an important issue raised by the potential of this study is the need to think about the ethical issues that it raises in the human experimental context.”
The study involving the synthetic mice embryo was co-led at Weizmann’s Molecular Genetics Department by Shadi Tarazi, Alejandro Aguilera-Castrejon and Carine Joubran in collaboration with Dr. Nadir Ghanem, Dr. Itay Maza of the Rambam Health Care Campus, in Haifa.