308
Cell Growth in Microgravity
a frst in a series oF experiments that
demonstrated shared characteristics be-
tween orbital microgravity and rotational
culture. The MACS provides For a unique
opportunity to investigate the cellular re-
sponsesascandidatesForflightexperiment
opportunities. As with many models in
science, a proportion oF results are shared
with the results From actual conditions in
microgravity.
Hind-limb suspension is a common
ground-based microgravity model used
For experiments with rodents. Antiortho-
static suspension oF rodents provides a 30
head-down tilt, unloading the hind limbs
and weight bearing the Front limbs. This
position provides the cephalad-fluid shiFt
observed in astronauts during spaceflight
and mimics the arm-priority locomotion oF
astronauts. Studies in this model demon-
strated organ-specifc immune changes.
Lymphocytes From lymph nodes and the
peripheral blood oF suspended animals
had a reduced response to phytohemagglu-
tinin (PHA) stimulation, whereas spleen
cells oF suspended animals displayed en-
hanced response. Other suspension ex-
periments, as well as spaceflight stud-
ies, support this organ-specifc response.
In contrast, recent antiorthostatic sus-
pension experiments reported enhanced
responses in animals. Antiorthostatically
suspended mice had enhanced resistance
to primary inFection and an enhanced
protective immunological memory to
Lis-
teria monocytogenes
.Mechan
ismsFo
rth
is
enhancement are not known, but the re-
sults indicate that both suspension and
spaceflight cause complex immune alter-
ations rather than a simple shut down
oF the immune system. Indeed, some
eFFects are related to the altered orienta-
tion to gravity, while others may be the
result oF stress induced by the suspen-
sion model.
The
eFFect
oF
psychoneuroendocrine
changes on immune-response studies is
minimized by perForming experiments on
isolated cell populations in controlled cul-
ture conditions. Ground-based models oF
microgravity enable refning oF hypothe-
ses in advance oF flight. The response oF
cells to microgravity is complex and diFf-
cult to identiFy iF the cell is responding to
the change in gravity or to a culture con-
dition that is created by microgravity. In
the Former resides the posit that cells pos-
sess a ‘‘gravity sensor.’’ No one has as yet
described a gravity sensor in mammalian
cells, despite numerous studies demon-
strating that mammalian cells respond to
changes in gravity. In contrast, cells in the
roots oF plants have geotropic organelles
that identiFy the direction oF the gravity
vector and promote root propagation that
Follows gravity.
Microgravity aFFects fluid systems such
that there are no density-driven phase sep-
arations, no particle sedimentation, and
no gravity-driven convection. Absence oF
these three conditions in cell culture re-
sults in a cell culture environment that is
stressFul or even hostile to many types
oF cells. In microgravity, and without
Facilitation, cell cultures rapidly become
diFFusion-limited, with regard to nutrient
assimilation and waste-product dissipa-
tion. ThereFore, the cellular response in
microgravity may be related to these ad-
verse culture conditions and not directly to
microgravity.
Einstein described gravity in terms oF a
curvature oF space–time in his theory oF
general relativity. In relation to biological
systems, gravity is simply an acceleration
Force (9.8 m s
2
on Earth, G). Astronauts
in orbit typically experience acceleration
Forces oF 10
2
to 10
4
G. The Force oF
gravity at the distance that most orbits take
p
laceinisprac
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ica
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lythesameasi
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