Spaceflight and the fantasy of colonising other planets have fascinated mankind for centuries. With more mixed-sex crews launching into space—Christina Hammock Koch being expected as the first female astronaut to land on the moon in 2024—extra-terrestrial reproduction is becoming an increasingly greater possibility. But the road from conception to birth of healthy, fertile offspring is long and arduous, particularly at microgravity, exposed to extreme doses of radiation and subjected to countless behavioural stressors like impaired sleep, the space diet and social isolation. To find out whether these factors prevent procreation in space the European Space Agency has funded SciSpacE white papers to lay out a systematic roadmap of what must be researched in the next years. One of the first items on this scientific to-do list has recently been ticked off by Sayaka Wakayama and co-workers: proving that early embryonic development can occur in the extreme environment of space.
Studying reproduction in space is a difficult task: test animals cannot be kept on the International Space Station (ISS) for extended periods of time as disinfectant use is limited, and specimens are subjected to both microgravity in space and hypergravity during the launch. To overcome some of these obstacles, the team at the University of Yamanashi, China developed a novel “Embryo Thawing and Culturing Unit”, in which two-cell stage embryos are cryopreserved and launched to the ISS. Here, they are cultured for four days before being returned to Earth for examination.
To overcome some of these obstacles, the team at the University of Yamanashi, China developed a novel “Embryo Thawing and Culturing Unit”, in which two-cell stage embryos are cryopreserved and launched to the ISS.
Before assessing the results of this examination, some background on embryonic development is needed. The two-cell embryo divides rapidly to form a ball of cells known as a blastocyst. Next, the homogenous cell mass differentiates into two cell types: the inner cell mass (ICM) destined to become foetal tissue, and the trophectoderm (TE) essential for implantation and placenta formation. Whether this differentiation can occur in space was a key concern for the team at Yamanashi, but the experiments showed that the relative number of ICM and TE cells was the same in space microgravity as in the two control conditions (1g simulated on the ISS and 1g on the ground). Another essential step in embryonic development is the clustering of ICM cells in a distinct region of the blastocyst. Separation of the ICM cells from one another causes multiple pregnancies associated with high risks to both the mother and offspring. This process was previously suspected to be dependent on gravity. However, although there were slightly more misplaced ICM cells at microgravity, clustering did not seem greatly impaired overall.
Additionally, many previous studies argued that space radiation would prevent embryonic development. Radiation exposure in the first two weeks post-conception is a critical determinant of embryo survival. By staining for a cellular marker indicating radiation-induced DNA damage, Wakayama and colleagues were able to show that within the first four days, space travel does not cause significant DNA damage. Gene expression patterns also did not significantly differ between the embryos cultured in space and those on Earth.
Overall, this study indicates that the early phases of human reproduction are possible in space.
Overall, this study indicates that the early phases of human reproduction are possible in space. This data is further supported by previous studies showing that space does not affect male fertility or the fertilisation process. The Dutch entrepreneur Edelbroek is planning to confirm these findings by launching a novel device into space, in which both fertilisation and early embryonic development occur before the blastocysts are cryogenically frozen and returned to Earth for analysis.
Of course, a human pregnancy usually lasts around 40 weeks, and while the first few days are crucial, there are many more obstacles to be overcome before a healthy foetus is born. A lower absolute number of TE cells suggests that implantation and placental development might prove problematic. Additionally, the organogenesis period, when organs develop eight to fifteen weeks post-conception, is also significantly impacted by radiation, as seen by the congenital malformations of individuals in utero during the Hiroshima and Nagasaki disasters. This was not examined in the discussed study.
Besides foetal implications, maternal health during pregnancy must be considered. While complication rates of pregnancies conceived post space travel seem unchanged, mice sent to space during pregnancy deliver babies with a lower birth weight and higher mortality rate. Uterine muscle atrophy, the wasting of muscle occurring from the lack of gravity, might further complicate birth and a lack of a multi-disciplinary team on board the spacecraft could endanger maternal wellbeing.
Therefore, not only are there physiological barriers to extra-terrestrial procreation, but the ethical aspects must be thoroughly discussed. Simulations of space radiation on Mars suggest that exposure during the critical organogenesis period lowers IQ and brain size, causes mental health issues and raises the risk of cancers.
Not only are there physiological barriers to extra-terrestrial procreation, but the ethical aspects must be thoroughly discussed.
Equally, little is known about the long-term genetic implications of space travel. Mouse experiments indicate that outer space alters DNA structure and gene expression in sperm cells, by changing epigenetic methylation patterns – this feature would likely be carried through future generations. If these are the risks to the offspring, we may begin to question the ethics of attempting reproduction in space. Some may argue that just because we can, does not mean that we should.
Regardless of ethical dilemmas, the study by Sayaka Wakayama and their team has made waves in the field of space science. The development of the new “Embryo Thawing and Culturing Unit”, together with the robust study design, has enabled the answering of pertinent questions in the exploration of normal development of mammalian foetuses away from Earth. Early embryos can develop on the International Space Station without any apparent developmental or genetic defects. Nevertheless, although the possibility of humans colonising outer space now seems a step closer, there is certainly still a long way to go.