If the future of humanity lies in the vast expanse of space, could humans actually perpetuate the species beyond Earth?
As a team of Chinese scientists has discovered – the outlook is not very positive.
The researchers used two of China’s Tianzhou cargo spacecraft missions to culture and study the differentiation of human reproductive cells in space.
Early-stage human reproductive cells did not grow or develop nearly as well in space as on Earth, the team from the Chinese Academy of Sciences’ Shanghai Institute of Technical Physics and Beijing’s Tsinghua University found.
In space, the success rate of generating the earliest precursor germ cells falls by around half, while early sperm-producing cells multiply over 25 per cent more slowly, according to their findings published in Science Advances on July 15. The outcome was attributed largely to microgravity and cosmic radiation.
The study marks the world’s first successful differentiation of human embryonic stem cells into germ cells in a space environment, according to the paper.
The South China Morning Post has contacted the authors for comment.
Co-corresponding author Zhang Tao, a professor at the Shanghai Institute of Technical Physics, has been working on experimental instruments for space science since 1998.
“As long-duration missions increase the prospect of pregnancy in space, prenatal germ cells emerge as a critical vulnerability in reproductive risk assessment,” Zhang wrote in the paper.
During the Cold War space race, the Soviet Union carried out animal reproduction experiments in space. Besides China, scientists from the United States and Japan have in recent years also conducted germ cell experiments in space – testing the activity of mouse and human sperm – aboard the International Space Station.
For their experiment, Zhang and the team developed an automated cell culture bioreactor with integrated real-time imaging capabilities, designed for use in space.
This equipment was first put to practical use during the Tianzhou-1 mission launched in April 2017.
Once in orbit, the device completed a 30-day stem cell culture and induction experiment.
The in vivo imaging device confirmed that the induced differentiated germ cells survived and maintained germ cell characteristics.
However, it could not recover complete samples, making it impossible to conduct more complex experiments and quantitative analyses.
Subsequently, the space cell culture system was upgraded. By the time the Tianzhou-6 mission to supply China’s Tiangong space station was launched in May 2023, all of the hardware had been optimised.
The advanced culture system could simultaneously induce three types of germ cells, perform real-time fluorescence imaging, and send data to Earth – generating comprehensive, matched “multi-omic” or multiple biological data sets for both the space flight and ground control groups.
During the Tianzhou-6 mission, astronauts installed a complete set of automated cell culture equipment in the space station’s Wentian experiment module.
The fully automated differentiation was initiated via a preset pump-control programme, with time-lapse imaging captured every 24 hours for morphological analysis.
In June 2023, before the return of the Shenzhou-15 crewed mission, the samples were transferred to a minus 80 degrees Celsius (minus 112 Fahrenheit) storage module to bring them back to Earth.
When researchers compared their experiment to a matching set-up on Earth, they found that the success rate of growing these special stem cells – human-induced pluripotent stem cell-derived primordial germ cells (hiPGCs) – halved. The proportion of hiPGC-positive cells also dropped, to just 2 to 6 per cent, compared with 6 to 15 per cent for the ground group.
Both indicators were significantly lower than the control group, confirming that the space environment inhibits the specialisation process of primordial germ cells – that is, being in outer space makes it much harder for primordial cells to turn into eggs or sperm.
In the experiment capturing the proliferation capacity of human-induced spermatogonial stem cells in space, the total cell count of the space-exposed group was reduced by as much as 26 per cent compared with the ground control set.
The findings indicate that the space environment causes lineage-specific damage to germ cell development.
However, the findings were not entirely negative. The activity and overall count of human-induced ovarian follicles did not show a notable decline, suggesting that space radiation has a limited short-term impact on germ cells.
The Tianzhou-6 specimens were subjected to cell culture experiments on Tiangong for up to 15 days, with the help of the Shenzhou-15 crew. During that time, the cumulative radiation dose recorded by an in-cabin dosimeter was within the typical low-dose range for low Earth orbit.
Post-return analysis showed that while genomic damage in germ cells increased with longer exposure, a 15-day window did not produce large-scale genomic damage.
A separate Chinese research team has sent artificial embryos to Tiangong to also study the possibility of human reproduction in zero gravity, according to a report by state broadcaster CCTV in May.
Zhang wrote: “Future studies in extended on-orbit systems, human organoids and adult-relevant models will be needed to determine the broader significance of these responses for reproductive biology.” -- SOUTH CHINA MORNING POST
