The team studied two types of mice, one group with the normal
OPN gene and another group lacking this gene. The mice
experienced three days of hindlimb unloading, a widely used
technique to simulate the physiologic changes that astronauts
experience during spaceflight. With this technique, body fluids
shift similarly to how they do in microgravity (toward the head
instead of toward the extremities) and immune system changes
occur. Mice of both types made up the control groups, which did
not undergo unloading. After three days, the researchers
compared the mice with normal OPN and the OPN-lacking mice. The
normal OPN mice experienced weight loss, spleen and thymus
atrophy, and a reduced number of white blood cells. In addition,
increased levels of corticosterone, a steroid that contributes
to the death of white blood cells, were found only in the normal
OPN mice studied. By contrast, the mice lacking the OPN gene
showed statistically insignificant changes in weight and the
levels of corticosterone, and were mo