Experimental mouse breeding in a near-zero-gravity space simulation suggests making babies is best left to Earthlings.

According to Japanese biologists, defects in their microgravity embryos suggest that "fertilization can occur normally" in space, but standard Earth gravity may be needed for embryo development.

The experiment, published Tuesday in Public Library of Science ONE, is the latest addition to a surprisingly large body of literature on how the space environment affects the cellular basics of reproduction.

Among the animals that have been bred in space are frogs, salamanders, sea urchins — who didn't do so well — and fish. (Birds and bees are, understandably if unfortunately, not on the list.)

Rather less research, however, has been done on mammalian reproduction in space, and there's reason to think the potential effects of low gravity would be pronounced in mammals, whose embryonic development is more complicated and sensitive than other animals.

To test these effects, the researchers artificially fertilized mouse eggs with sperm that had been stored inside a three-dimensional clinostat, a machine that mimics weightlessness by rotating objects in such a way that the effects of gravity are spread in every direction.

Fertilization took place normally, suggesting that microgravity hadn't harmed the sperm. But as the embryos continued to develop inside the clinostat, many developed problems. Their cells had trouble dividing and maturing.

Some were ultimately implanted in female mice and survived to a healthy birth, but at lower numbers than a regular-gravity control group. Part of the difference could be the result of performing tricky procedures on sensitive cells, but the researchers suspect they also reflect the affect of a low-gravity environment on cellular processes that evolved for Earth-specific physics.

"Sustaining life beyond Earth either on space stations or on other planets will require a clear understanding of how the space environment affects key phases of mammalian reproduction," write the researchers.

Citation: "Detrimental Effects of Microgravity on Mouse Preimplantation Development In Vitro." By Sayaka Wakayama, Yumi Kawahara, Chong Li, Kazuo Yamagata, Louis Yuge, Teruhiko Wakayama. Public Library of Science ONE, August 25, 2009.

Images: PLoS ONE

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