306
Cell Growth in Microgravity
address this problem and are reviewed
by Cogoli and Cogoli–Greuter: (1) a direct
effect: the direct interaction of gravity with
one or more cellular organelles of a density
different from that of the cytoplasm gener-
ates a pressure on neighboring structures
(e.g. the cytoskeleton) and consequently a
signal that is transduced into a biologi-
cal event; (2) a nonequilibrium thermody-
namic effect: the interaction of gravity with
a few organelles is not sufFcient to trigger
one event, but a series of small changes
is ampliFed to generate an important ef-
fect; and (3) an indirect effect: alterations
in gravity induce changes in the microen-
vironment of the cell and the cell responds
to the new environmental conditions.
Studies by Pollard suggest that in cells
smaller than 10
µ
mindiameter,Brownian
motion caused by impacts from molecules
in the culture fluid are sufFcient to coun-
teract gravity-induced sedimentation. How
much sedimentation can occur in cells
larger than 10
µ
m is not known. ±urther-
more, this approach assumes that cells
act like a bag of water containing solutes
and undissolved particles (starch gran-
ules, oil vacuoles, and organelles). This
theory does not agree with the obser-
vations that the aqueous phase of the
cell is heterogenous, containing solution
properties, colloidal regions, suspended
and attached organelles, and contractile
cytoskeletal structures. Studies by Nace
suggest that organelles of the cell may
impart signiFcant torque on the cytoskele-
ton using the force of gravity. It is not
known what effect the microgravity-related
loss of this torque has on cellular physi-
ology. Nevertheless, the possibility of a
direct gravity sensor within mammalian
cells exists although it is not known as yet.
The
likelihood that
gravity,
a small
force, would have an effect on cell func-
tion is questionable. The nonequilibrium
thermodynamic effect, or the bifurcation
theory, provides the reasoning to allow
such small changes to exert a signiFcant
effect on biochemical processes of the cell.
This theory, outlined by Mesland, argues
that the biochemical processes of a cell are
described in terms of nonlinear, nonequi-
librium thermodynamics with chaotic out-
comes. Therefore, the direction in which
these reactions proceed is extremely sen-
sitive to environmental conditions. This
sensitivity may allow the seemingly small
effect of gravity changes to have dramatic
effects on cellular processes.
Microgravity can possibly alter the bio-
physical microenvironment around cells,
and in turn, affect the shape, metabolism,
and function of the cell. ±or example,
the lack of sedimentation and thermal
convection in microgravity leaves only
simple diffusion and possibly surface ten-
sion–driven phenomena to move waste
products away from the cell and bring
nutrients toward the cell. Depending on
the metabolic rate of the cell, diffusion
and surface tension may not be sufFcient
to fulFll the metabolic requirements of the
cell. These and other microgravity-induced
alterations in the physical and chemical
microenvironment alter cellular function.
3
Modeling Microgravity
Modeling microgravity for investigation of
cell biological phenomena is a challenge
that the space agency and scientists ad-
dress in many familiar venues: parabolic
flight, suborbital rocketry, drop towers,
and neutral buoyancy. The former three
strategies provide only short duration ana-
log conditions, while the latter permits
extended observations. A novel model
emerged from early attempts to keep
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