do they differ, what is the method of choice, what is the best suitable facility for my
scientific object and question?
The idea to alter the influence of gravity and study the impact on basic biological
mechanisms is quite old and started experimentally at the end of the eighteenth
century with plants due to their easy observable gravitropic responses. By putting
them horizontally the restoration of their original growth direction by gravitropic
responses becomes obvious—roots growing downwards and shoots growing
upwards. If such kind of arrangement is equipped with a motor and the plant is
rotated around an axis perpendicular to the gravity vector, the unidirectional influence of gravity is turned into an omnilateral stimulation which in many cases
abolishes the gravitropic response. Under optimal conditions the gravitropic stimulus is neutralized, like in real microgravity, a situation called simulated weightlessness (microgravity). In both cases a plant will no longer show gravitropism, but what
has happened with respect to the underlying gravisensory mechanism—is it permanently stimulated or does it receive no further input? We will come back to this point
later. Such kind of experimental arrangement is called a clinostat.
A 2D clinostat has only a single rotation axis. Wolfgang Briegleb used the 2D
clinostat principle for studying the effect of weightlessness on small plants and
animals and cells. He postulated that speeding up and thus transforming a slow
rotating clinostat, normally rotated with 1–2 rpm (revolutions per minute), into a fastrotating one (in the range of 60–90 rpm) will optimize the simulation of weightlessness conditions (Briegleb 1988; Klaus et al. 1998). Furthermore, not only speed but
also the effective radius (diameter) has to be considered. Under optimal conditions,
the diameter of the sample containers is kept small (in the range of a few mm) and the
objects are placed in the center of rotation in order to keep residual accelerations as
minimal as possible. The latter concerns thresholds for gravity stimulus perception
of the respective organism, which are in most cases not known. A 2D clinostat
constantly runs in one direction inducing a static change of the gravity vector in
relation to the sample. Sedimentation is thereby prevented and small bodies
(e.g. single cells or statoliths within cells) describe floating circles in the media
comparable to the floating conditions in real microgravity. The speed of rotation
determines the circles’ diameter; the faster the rotation, the smaller the circles; too
fast rotation, however, results in radial accelerations. Let us transfer this idea to
statoliths in roots or rhizoids and imagine their movements depending on the speed
of rotations. Having done so, Hensel and Sievers (1980) demonstrated by morphological studies of slowly clinorotated roots (1–2 rpm) strong damages of the
statocytes on the ultrastructural level, e.g. revealed by a considerable increase of
the lytic compartment. They related these changes to the continuously changing
direction of the gravity vector, which is different to the situation in real microgravity.
2D clinostats have been adapted to several experimental demands (for review
see Brungs et al. 2016): clinostats for suspended or adherent organisms and
cell cultures, for aquatic systems and in combination with online analyses using
photomultipliers or microscopy (Fig. 2.1).
Assuming that two rotation axes provide more complex ways to average the
influence of the gravity vector and simulate weightlessness more perfectly, 3D
2.2 Microgravity Simulators—Efforts to Mimic the Effects of Weightlessness
15
scientific object and question?
The idea to alter the influence of gravity and study the impact on basic biological
mechanisms is quite old and started experimentally at the end of the eighteenth
century with plants due to their easy observable gravitropic responses. By putting
them horizontally the restoration of their original growth direction by gravitropic
responses becomes obvious—roots growing downwards and shoots growing
upwards. If such kind of arrangement is equipped with a motor and the plant is
rotated around an axis perpendicular to the gravity vector, the unidirectional influence of gravity is turned into an omnilateral stimulation which in many cases
abolishes the gravitropic response. Under optimal conditions the gravitropic stimulus is neutralized, like in real microgravity, a situation called simulated weightlessness (microgravity). In both cases a plant will no longer show gravitropism, but what
has happened with respect to the underlying gravisensory mechanism—is it permanently stimulated or does it receive no further input? We will come back to this point
later. Such kind of experimental arrangement is called a clinostat.
A 2D clinostat has only a single rotation axis. Wolfgang Briegleb used the 2D
clinostat principle for studying the effect of weightlessness on small plants and
animals and cells. He postulated that speeding up and thus transforming a slow
rotating clinostat, normally rotated with 1–2 rpm (revolutions per minute), into a fastrotating one (in the range of 60–90 rpm) will optimize the simulation of weightlessness conditions (Briegleb 1988; Klaus et al. 1998). Furthermore, not only speed but
also the effective radius (diameter) has to be considered. Under optimal conditions,
the diameter of the sample containers is kept small (in the range of a few mm) and the
objects are placed in the center of rotation in order to keep residual accelerations as
minimal as possible. The latter concerns thresholds for gravity stimulus perception
of the respective organism, which are in most cases not known. A 2D clinostat
constantly runs in one direction inducing a static change of the gravity vector in
relation to the sample. Sedimentation is thereby prevented and small bodies
(e.g. single cells or statoliths within cells) describe floating circles in the media
comparable to the floating conditions in real microgravity. The speed of rotation
determines the circles’ diameter; the faster the rotation, the smaller the circles; too
fast rotation, however, results in radial accelerations. Let us transfer this idea to
statoliths in roots or rhizoids and imagine their movements depending on the speed
of rotations. Having done so, Hensel and Sievers (1980) demonstrated by morphological studies of slowly clinorotated roots (1–2 rpm) strong damages of the
statocytes on the ultrastructural level, e.g. revealed by a considerable increase of
the lytic compartment. They related these changes to the continuously changing
direction of the gravity vector, which is different to the situation in real microgravity.
2D clinostats have been adapted to several experimental demands (for review
see Brungs et al. 2016): clinostats for suspended or adherent organisms and
cell cultures, for aquatic systems and in combination with online analyses using
photomultipliers or microscopy (Fig. 2.1).
Assuming that two rotation axes provide more complex ways to average the
influence of the gravity vector and simulate weightlessness more perfectly, 3D
2.2 Microgravity Simulators—Efforts to Mimic the Effects of Weightlessness
15