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MicroCT images of the femoral head bone and cartilage in ground control (GC) versus spaceflight (FLT), showing premature secondary ossification in microgravity. Credit Eduardo Almeida, Rukmani Cahill and Elizabeth Blaber,
Human beings evolved on Earth under the 1G pull of gravity. Travelling out into space has profound effects on the body, challenging it in ways that Earth-bound life never does. Microgravity or weightlessness causes muscle atrophy and bone density loss, as the body no longer needs to support its own weight. There is further damage from the prolonged exposure to cosmic radiation which increases the risk of cancer and can damage the nervous system. In addition to this, astronauts experience fluid shifts that lead to vision problems and even cardiovascular changes. Of course this is just the physical aspect but there is a psychological impact too from the isolation and confinement which just adds another level of complexity. Understanding the impacts of space travel is what has driven a team of scientists to try and learn more.
Astronauts exercise for around 2 hours every day on board the ISS (Credit : NASA)
The team led by Rukmani Cahill from the Blue Marble Space Institute of Science have published their findings in the Public Library of Science. They explored wanted to explore if bone loss during spaceflight in Low Earth Orbit is primarily due to the microgravity induced unloading on weight-bearing skeletal sites.
To test this they sent a plucky bunch of mice off to the International Space Station for 37 days as part of the NASA Rodent Research-1 experiment. The team were then able to analyse the bones from the mice on their return to Earth using microcomputed tomography, a high resolution 3D imaging technique very similar to hospital CT scans but on a much finer scale. They were able to study the bone structure and composition and hoped to understand how spaceflight would effect the integrity of their skeleton.
NASA’s Rodent Habitat module with both access doors open (Credit : NASA/Dominic Hart)
Their study showed that there was significant bone loss in the femur of the mice but not in vertebrae. This suggests that Low Earth Orbit radiation or systemic stresses aren't major contributors to bone degradation. Interestingly, the microgravity environment actually seemed to accelerate the transformation of cartilage in the rounded upper end of the thigh bone into bone tissue! This seems to indicate that space conditions may promote premature progression of secondary bone formation during late skeletal maturation stages at 21 weeks.
The research also showed a surprising benefit of the ISS Rodent Habitat design: control mice housed in these special wire-mesh enclosures down on Earth maintained or increased their bone mass, while those in standard laboratory cages showed significant bone deterioration. The team attribute this to the enclosures 3D structure, which encourages more elaborate movement patterns increasing mechanical loading on weight-bearing bones, a natural stimulus for maintaining healthy bone density. The outcome demonstrates how environmental design can surprisingly have a positive impact on bone health even under normal gravity conditions.
Concluding their paper, the study reveals that bone loss in space primarily affects weight-bearing sites in nearly mature female mice, while muscle-activated areas like the spine remain largely unaffected. This confirms mechanical unloading as the main culprit rather than radiation or other factors relating to space travel. The team also concluded that microgravity unexpectedly accelerates bone formation in femoral head growth plates, potentially leading to premature cessation of bone lengthening growth, a previously unknown effect of skeletal unloading in space environments.
Source : 37-Day microgravity exposure in 16-Week female C57BL/6J mice is associated with bone loss specific to weight-bearing skeletal sites
Mark Thompson
Science broadcaster and author. Mark is known for his tireless enthusiasm for making science accessible, through numerous tv, radio, podcast and theatre appearances, and books. He was a part of the aware-nominated BBC Stargazing LIVE TV Show in the UK and his Spectacular Science theatre show has received 5 star reviews across UK theatres. In 2025 he is launching his new pocast Cosmic Commerce and is working on a new book 101 Facts You Didn't Know About Deep Space In 2018, Mark received an Honorary Doctorate from the University of East Anglia.
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