decline in strength and the increase in bone resorption found in untreated mice (Bateman
2004).
Mechanical testing data were complimented by serum, messenger ribonucleic acid (mRNA),
and histological analyses that indicated a decline in bone formation and an increase in bone
resorption in addition to an inhibition of mineralization. OPG mitigated the decline in
mechanical strength by preventing increase in resorption
and maintaining mineralization. In addition to this detailed
analysis of skeletal properties, a secondary analysis of calf
muscles from placebo-treated specimens was performed to
collect baseline data to validate space-flown mice as an
appropriate model for sarcopenia (age-related muscle loss).
Spaceflight caused a 15%-30% decline in muscle fiber
diameter size compared to appropriate ground controls
(Harrison 2003).
Data obtained from the mice following return to Earth
indicated some alternations in immune functions. Analysis of
the spleenocytes (immune cells produced by the spleen)
indicated an increase in B-cell (white blood cell that matures
in the bone marrow and, when stimulated by an antigen,
differentiates into plasma cells) production compared to T-
cells (white blood cells that complete maturation in the
thymus and have various roles in the immune system). A slightly lower white blood-cell count in
the flight animals compared to the controls was not statistically significant. The spleen mass
was 18%-28% lower in flight mice compared to controls. Results also indicated that flight mice
weighed 10%-12% less than ground controls (Pecaut 2003).
The ability to survive a major physical trauma in microgravity may be compromised due to an
altered immune system. Platelets (constituent of blood that promotes clotting at the site of
injury) are the primary cells involved in the wound healing process. The animals studied had
significantly higher platelet levels but low volume compared to the controls. This indicates that
the lack of platelets in the wound-healing process is not a problem but that platelets formed in
microgravity have a decreased functionality in the wound healing process. Data indicated that
a short stay in microgravity can induce significant changes in immune defense mechanisms,
hematopoiesis (blood cell formation), and other aspects of health (Gridley 2003).
Analysis of microarray data revealed that 272 mRNAs were significantly altered by spaceflight,
the majority of which displayed similar responses to hindlimb suspension, while reloading
tended to counteract these responses. Several mRNAs altered by spaceflight were associated
with muscle growth, including the PI3 kinase regulatory subunit p85 alpha, insulin response
substrate-1, the forkhead box O1 transcription factor, and MAFbx/atrogin1. Moreover,
myostatin mRNA expression tended to increase while mRNA levels of the myostatin inhibitor
FSTL3 tended to decrease in response to spaceflight. In addition, mRNA levels of the slow-
Fluorescent image of femur
diaphysis from ground control
placebo treated mouse, indicating
greatly decreased bone formation
(calcein label indicates where bone
was forming at the time of launch,
allowing quantification of bone
formation rates during flight. NASA’s
Marshall Space Center image.