Vibration Rather than Microgravity Affects Bone Metabolism in Adult Zebrafish Scale Model

Gravity and mechanical forces cause important alterations in the human skeletal system,
as demonstrated by space flights. Innovative animal models like zebrafish embryos and medaka
have been introduced to study bone response in ground-based microgravity simulators. We used,
for the first time, adult zebrafish in simulated microgravity, with a random positioning machine
(RPM) to study bone remodeling in the scales. To evaluate the effects of microgravity on bone
remodeling in adult bone tissue, we exposed adult zebrafish to microgravity for 14 days using RPM
and we evaluated bone remodeling on explanted scales. Our data highlight bone resorption in
scales in simulated microgravity fish but also in the fish exposed, in normal gravity, to the vibrations
produced by the RPM. The osteoclast activation in both rotating and non-rotating samples suggest that
prolonged vibrations exposure leads to bone resorption in the scales tissue. Stress levels in these fish
were normal, as demonstrated by blood cortisol quantification. In conclusion, vibrational mechanical
stress induced bone resorption in adult fish scales. Moreover, adult fish as an animal model for
microgravity studies remains controversial since fish usually live in weightless conditions because of
the buoyant force from water and do not constantly need to support their bodies against gravity.