Cell Growth in Microgravity
305
response, cells not only survive, but also
thrive to give a new perspective to biologi-
cal systems.
2
Unique Cell Culture Conditions Offered by
Microgravity
As life evolved on Earth, a multiplicity
of physical and chemical factors invoked
adaptations and participated in the compli-
cated selection process. For many factors,
there are clear examples of the role of phys-
ical forces in determining the pathways in
evolution. A notable exception is gravity.
The force of gravity has been constant
for the 4.8 billion years of life evolutionary
processes on Earth. Therefore, there is lit-
tle or no anticipated genetic memory for
terrestrial life to respond to gravitational
force changes. As we transition terrestrial
life to low gravity environments and study
the adaptive processes in the cell, we will
increase our understanding of gravity’s
role in shaping life on Earth.
The species on Earth evolved to perform
various functions and processes in a grav-
itational force ±eld of 9.8 m s
2
.S
om
e
gravity-dependent processes, such as bal-
ance and the ability of higher organisms
to proceed along a surface, are obvious.
Others, such as the directional growth of
plants (gravitropism), are less obvious but
are demonstrated through experimenta-
tion. For most species (microbial, plant
or animal), only a small proportion of the
functions influenced by gravity are known.
During the past 40 years, the ability to
achieve Earth orbit and experience long-
term microgravity has created a need to
understand the role of gravity in human,
animal, and plant systems in preparation
for exploration class missions into the
solar system.
2.1
The Effects of Microgravity on Cellular
Organelles and Cellular Function
Many biological stresses are induced by
spaceflight and are visible in many areas of
human physiology. The most dramatic re-
sponse to spaceflight is the signi±cant loss
of bone and muscle mass and function,
most likely caused by skeletal unloading
and reduced activity. In addition, cepha-
lad fluid shifting, neurovestibular distur-
bances, and a general malaise termed
space
sickness
are experienced by astronauts soon
after arrival in microgravity. In many in-
stances, the basis of the negative influence
of microgravity on human systems may
be investigated using cell-based systems.
This communication will offer a selected
review of the past work at the cellular level
and invoke models for testing in forth-
coming opportunities on the International
Space Station.
A major challenge in studying the adap-
tation of terrestrial life to space conditions
resides in convincingly delineating the
responses induced by microgravity from
the radiation and stress experienced in
spaceflight. Various
in vitro
parameters of
immune function are suppressed in hu-
mans during and after returning from
space missions. Exposure to cosmic radia-
tion, along with physical and psychological
stresses occurs concurrently with the mi-
crogravity of spaceflight. Understanding
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the physiological changes observed during
spaceflight is a formidable challenge for
space research programs (e.g. for immune
function, which is affected independently
by all three factors: microgravity, stress,
and radiation).
It is unlikely that single cells can ‘‘sense’’
gravity through biophysical changes within
the
cell.
Three
theoretical
approaches
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