Adaptation of the macular vestibuloocular reflex to altered gravitational conditions in a fish (Oreochromis mossambicus).

Young fish (Oreochromis mossambicus) were exposed to microgravity (micro g) for 9 to 10 days, or to hypergravity (hg) for 9 days. For several weeks after termination of micro g and hg, the roll-induced static vestibuloocular reflex (rVOR) was recorded. In stage 11/12-fish, the rVOR amplitude (angle between the maximal up and down movement of an eye during a complete 360 degree lateral roll) of micro g-animals increased significantly by 25% compared to 1 g-controls during the first post-flight week but decreased to the control level during the second post-flight week. Microgravity had no effect in stage 14/16 fish on the rVOR amplitude. After 3 g-exposure, the rVOR amplitude was significantly reduced for both groups compared to their 1 g-controls. Readaptation to 1 g-condition was completed during the second post-3 g week. We postulate a critical period during which the development of the macular vestibuloocular reflex depends on gravitational input, and which is limited by the first appearance of the rVOR. At this period of early development, exposure to microgravity sensitizes the vestibular system while hypergravity desensitizes it.     

Exposure to altered gravity affects all stages of endochondral cartilage differentiation.

Chondrogenesis has a number of well-defined steps: (1) condensation, which involves cell aggregation, adhesion and communication; (2) activation of cartilage genes, which is accompanied by rounding up of the cells and intracellular differentiation; and (3) production and secretion of cartilage specific matrix molecules. Our studies show that each of these steps is affected by exposure to gravitational changes. Clinorotation and centrifugation affected initial aggregation and condensation. In the CELLS experiment, where cells were exposed to microgravity after some condensation occurred preflight, intracellular differentiation and matrix production were delayed relative to controls. Once cartilage has developed, in rats, further differentiation (hypertrophy, matrix production) was also affected by spaceflight and hind limb suspension. For the process of chondrogenesis to proceed as we know it, loading and other factors present at 1g are required at each step of the process. This requirement means that not only will skeletal development and bone healing, processes involving chondrogenesis, be altered by long term exposure to microgravity, but that continuous intervention will be necessary to correct any defects produced by altered gravity environments.     

Light conditions affect the roll-induced vestibuloocular reflex in Xenopus laevis tadpoles

In Xenopus laevis tadpoles, effects of asymmetrical light conditions on the roll-induced vestibuloocular reflex (rVOR) were tested for the developmental period between stage 47 and 49. For comparison, the rVOR was tested in dim- and high-symmetrical light environments. Test parameters were the rVOR gain and rVOR amplitude. Under all light conditions, the rVOR increased from tadpole stage 47 to 49. For all stages, the asymmetrical light field induced the strongest response, the dim light field the weakest one. The response for the left and right eye was identical, even if the tadpoles were tested under asymmetrical light conditions. The experiments can be considered as hints (1) for an age-dependent light sensitivity of vestibular neurons, and (2) for the existence of control systems for coordinated eye movements that has its origin in the proprioceptors of the extraocular eye muscles.     

Crickets in space: morphological, physiological and behavioral alterations induced by space flight and hypergravity.

"Crickets in Space" was a Neurolab experiment by which the balance between genetic programs and the gravitational environment for the development of a gravity sensitive neuronal system was studied. The model character of crickets was justified by their external gravity receptors, identified position-sensitive interneurons (PSI) and gravity-related compensatory head response, and by the specific relation of this behavior to neuronal arousal systems activated by locomotion. These advantages allowed to study the impact of modified gravity on cellular processes in a complex organism. Eggs, 1st, 4th and 6th stage larvae of Acheta domesticus were used. Post-flight experiments revealed a low susceptibility of the behavior to micro- and hypergravity while the physiology of the PSI was significantly affected. Immunocytological investigations revealed a stage-dependent sensitivity of thoracic GABAergic motoneurons to 3 g-conditions concerning their soma sizes but not their topographical arrangement. The morphology of neuromuscular junctions was not affected by 3 g-hypergravity. Peptidergic neurons from cerebral sensorimotor centers revealed no significant modifications by microgravity (micro g). The contrary physiological and behavioral results indicate a facilitation of 1 g-readaptation originating from accessory gravity, proprioceptive and visual sense organs. Absence of anatomical modifications point to an effective time window of micro g or 3 g-expo-sure related to the period of neuronal proliferation. The analysis of basic mechanisms of how animals and man adapt to altered gravitational conditions will profit from a continuation of the project "Crickets in Space".     

Calcium signaling in plant cells in altered gravity.

Changes in the intracellular Ca2+ concentration in altered gravity (microgravity and clinostating) evidence that Ca2+ signaling can play a fundamental role in biological effects of microgravity. Calcium as a second messenger is known to play a crucial role in stimulus-response coupling for many plant cellular signaling pathways. Its messenger functions are realized by transient changes in the cytosolic ion concentration induced by a variety of internal and external stimuli such as light, hormones, temperature, anoxia, salinity, and gravity. Although the first data on the changes in the calcium balance in plant cells under the influence of altered gravity have appeared in 80th, a review highlighting the performed research and the possible significance of such Ca2+ changes in the structural and metabolic rearrangements of plant cells in altered gravity is still lacking. In this paper, an attempt was made to summarize the available experimental results and to consider some hypotheses in this field of research. It is proposed to distinguish between cell gravisensing and cell graviperception; the former is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity (cells not specialized to gravity perception), the latter is related to active use of a gravitational stimulus by cells presumebly specialized to gravity perception for realization of normal space orientation, growth, and vital activity (gravitropism, gravitaxis) in plants. The main experimental data concerning both redistribution of free Ca2+ ions in plant cell organelles and the cell wall, and an increase in the intracellular Ca2+ concentration under the influence of altered gravity are presented. Based on the gravitational decompensation hypothesis, the consequence of events occurring in gravisensing cells not specialized to gravity perception under altered gravity are considered in the following order: changes in the cytoplasmic membrane surface tension --> alterations in the physicochemical properties of the membrane --> changes in membrane permeability, --> ion transport, membrane-bound enzyme activity, etc. --> metabolism rearrangements --> physiological responses. An analysis of data available on biological effects of altered gravity at the cellular level allows one to conclude that microgravity environment appears to affect cytoskeleton, carbohydrate and lipid metabolism, cell wall biogenesis via changes in enzyme activity and protein expression, with involvement of regulatory Ca2+ messenger system. Changes in Ca2+ influx/efflux and possible pathways of Ca2+ signaling in plant cell biochemical regulation in altered gravity are discussed.     

Studies toward birth and early mammalian development in space.

Sustaining life beyond Earth on either space stations or other planets will require a clear understanding of how the space environment affects key phases of mammalian reproduction and development. Pregnancy, parturition (birth) and the early development of offspring are complex processes essential for successful reproduction and the proliferation of mammalian species. While no mammal has yet undergone birth within the space environment, studies spanning the gravity continuum from 0- to 2-g are revealing startling insights into how reproduction and development may proceed under gravitational conditions deviating from those typically experienced on Earth. In this report, I review studies of pregnant Norway rats and their offspring flown in microgravity onboard the NASA Space Shuttle throughout the period corresponding to mid- to late gestation, and analogous studies of pregnant rats exposed to hypergravity (hg) onboard the NASA Ames Research Center 24-ft centrifuge. Studies of postnatal rats flown in space or exposed to centrifugation are reviewed. Although many important questions remain unanswered, the available data suggest that numerous aspects of pregnancy, birth and early mammalian development can proceed under altered gravity conditions.     

Gravitational force regulates elongation growth of Arabidopsis hypocotyls by modifying xyloglucan metabolism.

Growth of dark-grown Arabidopsis hypocotyls was suppressed under hypergravity conditions (300 g), or was stimulated under microgravity conditions in space (Space Shuttle STS-95). The mechanical extensibility of cell walls decreased and increased under hypergravity and microgravity conditions, respectively. The amounts of cell wall polysaccharides (pectin, hemicellulose-I, hemicellulose-II and cellulose) per unit length of hypocotyls increased under hypergravity conditions, and decreased under microgravity conditions. The amount and the molecular mass of xyloglucans also increased under the hypergravity conditions, while those decreased under microgravity conditions. The activity of xyloglucan-degrading enzymes extracted from hypocotyl cell walls decreased and increased under hypergravity and microgravity conditions, respectively. These results indicate that the amount and the molecular mass of xyloglucans are affected by the magnitude of gravity and that such changes are caused by changes in xyloglucan-degrading activity. Modifications of xyloglucan metabolism as well as the thickness of cell walls by gravity stimulus may be the primary event determining the cell wall extensibility, thereby regulating the growth rate of Arabidopsis hypocotyls.     

空间飞行与回转器回旋条件下青菜开花与花粉发育的研究

比较研究了SJ-8返回式卫星留轨舱微重力条件与地面三维回转模拟微重力条件下青菜生长与发育情况.研究发现空间微重力条件下青菜开花过程需要大约18 h,明显长于地面对照5 h左右.回转器模拟实验结果表明,改变重力影响了花瓣的伸展与发育及花粉的产量,回转条件下花粉细胞中的微管排列明显不同于静止对照.细胞骨架受到干扰可能是改变重力条件下花粉产量降低的原因之一.本研究首次报道了在空间飞行试验中成功地采用了显微实时图像技术观察植物的开花过程,并获得了从花蕾到开花结束各阶段清晰的图像.      

Leaf senescence under various gravity conditions: relevance to the dynamics of plant hormones.

Effects of simulated microgravity and hypergravity on the senescence of oat leaf segments excised from the primary leaves of 8-d-old green seedlings were studied using a 3-dimensional (D) clinostat as a simulator of weightlessness and a centrifuge, respectively. During the incubation with water under 1-g conditions at 25 degrees C in the dark, the loss of chlorophyll of the segments was found dramatically immediately after leaf excision, and leaf color completely turned to yellow after 3-d to 4-d incubation. In this case kinetin (10 micromolar) was effective in retarding senescence. The application of simulated microgravity conditions on a 3-D clinostat enhanced chlorophyll loss in the presence or absence of kinetin. The loss of chlorophyll was also enhanced by hypergravity conditions (ca. 8 to 16 g), but the effect was smaller than that of simulated microgravity conditions on the clinostat. Jasmonates (JAs) and abscisic acid (ABA) promoted senescence under simulated microgravity conditions on the clinostat as well as under 1-g conditions. After 2-d incubation with water or 5-d incubation with kinetin, the endogenous levels of JAs and ABA of the segments kept under simulated microgravity conditions on the clinostat remained higher than those kept under 1-g conditions. These findings suggest that physiological processes of leaf senescence and the dynamics of endogenous plant hormone levels are substantially affected by gravity.     

Effects of microgravity on c-fos gene expression in osteoblast-like MC3T3-E1 cells.

The paper summarizes the data on proliferation and gravity-related gene expression of osteoblasts that were obtained from an experiment conducted under simulated and real microgravity conditions. Simulated microgravity conditions obtained in a clinostat depress proliferation of both osteoblast-like MC3T3-E1 and HeLa carcinoma cells. This depression of proliferation occurs in a collagen gel culture in which the flow of culture medium by rotation may be reduced. Interestingly, MC3T3-E1 cells which are probably one of target cells to microgravity are more sensitive than the HeLa cells. Simulated microgravity inhibited the epidermal growth factor (EGF)-induced c-fos gene expression in the MC3T3-El cells. To examine in detail the effect of real microgravity on the EGF signal transduction cascade in osteoblasts, MC3T3-E1 cells were cultured in the Cell Culture Experiment Module of the sounding rocket TR-1A6. The EGF-induced c-fos expression in cells was depressed under short-term microgravity conditions in the sounding rocket, while the phosphorylation of mitogen-activated protein kinase (MAPK) was not affected compared with the controls grown on the ground. These results suggest that an action site of microgravity in the signal transduction pathway may be downstream of MAPK.     

Gravisensitivity of cress roots.

The minimum dose (stimulus x time [gs]) eliciting a visible gravitropic response, has been determined using continuous and intermittent stimulation and two different accelerations at 1 g and 0.l g. The minimum dose of 20-30 gs estimated for microgravity roots and of 50-60 gs for roots grown on a 1 g-centrifuge indicated a higher sensitivity of microgravity roots. Applying intermittent stimuli to microgravity-grown roots, gravitropic responses were observed after two stimuli of 13.5 gs separated by a stimulus free interval of 118 s. The curvature of microgravity-grown roots to lateral stimulation by 0.1 g was remarkably smaller than by 1g in spite of the same doses which were applied to the seedlings. Microscopic investigations corresponding to stimulations in the range of the threshold values, demonstrated small displacement (< 2 micrometers) of statoliths in root statocytes. Accepting the statolith theory, one can conclude that stimulus transformation has to occur in the cytoplasm in close vicinity to the statoliths and that this transformation system was affected during seedling cultivation in microgravity.     

Influence of gravity on the circadian timing system.

The circadian timing system (CTS) is responsible for daily temporal coordination of physiological and behavioral functions both internally and with the external environment. Experiments in altered gravitational environments have revealed changes in circadian rhythms of species ranging from fungi to primates. The altered gravitational environments examined included both the microgravity environment of spaceflight and hyperdynamic environments produced by centrifugation. Acute exposure to altered gravitational environments changed homeostatic parameters such as body temperature. These changes were time of day dependent. Exposure to gravitational alterations of relatively short duration produced changes in both the homeostatic level and the amplitude of circadian rhythms. Chronic exposure to a non-earth level of gravity resulted in changes in the period of the expressed rhythms as well as in the phase relationships between the rhythms and between the rhythms and the external environment. In addition, alterations in gravity appeared to act as a time cue for the CTS. Altered gravity also affected the sensitivity of the pacemaker to other aspects of the environment (i.e., light) and to shifts of time cues. Taken together, these studies lead to the conclusion that the CTS is indeed sensitive to gravity and its alterations. This finding has implications for both basic biology and space medicine.     

Effects of spaceflight (STS-87) on tropisms and plastid positioning in protonemata of the moss Ceratodon purpureus.

Apical cells of moss protonemata represent a single-celled system that perceives and reacts to light (positive and negative phototropism) and to gravity (negative gravitropism). Phototropism completely overrides gravitropism when apical cells are laterally irradiated with relatively high red light intensities, but below a defined light intensity threshold gravitropism competes with the phototropic reaction. A 16 day-long exposure to microgravity conditions demonstrated that gravitropism is allowed when protonemata are laterally illuminated with light intensities below 140 nmol m-2s-1. Protonemata that were grown in darkness in microgravity expressed an endogenous tendency to grow in arcs so that the overall culture morphology resembled a clockwise spiral. However this phenomenon only was observed in cultures that had reached a critical age and/or size. Organelle positioning in dark-grown apical cells was significantly altered in microgravity. Gravisensing most likely involves the sedimentation of starch-filled amyloplasts in a well-defined area of the tip cell. Amyloplasts that at 1-g are sedimented were clustered at the apical part of the sedimentation zone in microgravity. Clustering observed in microgravity or during clino-rotation significantly differs from sedimentation-induced plastid aggregations after inversion of tip cells at 1-g.     

Teratogenic effects of gravitational changes.

In investigating the effect of gravitational changes on development, it is instructive to think of altered gravity (delta g) as a teratogen--that is, an environmental factor influencing development. Observed effects on skeletal development include: suppression of morphogenesis in centrifuged mouse limb buds; advanced fusion stages in centrifuged mouse palates; smaller crown rump lengths (CRL) and decreased number of pregnancies in centrifuged rats and mice; altered differentiation of growth plates in young growing rats in space; and decreased length of calcified long bone regions in fetal rats exposed to microgravity in utero. These studies show that delta g is able to alter development in vivo and in vitro and suggest that delta g operates, at least in part, at the cellular level.     

Water movement on the convex surfaces of porous media under microgravity

A plant growth system for crop production under microgravity is part of a life supporting system designed for long-duration space missions. A plant growth in soil in space requires the understanding of water movement in soil void spaces under microgravity. Under 1G-force condition, on earth, water movement in porous media is driven by gradients of matric and gravitational potentials. Under microgravity condition, water movement in porous media is supposed to be driven only by a matric potential gradient, but it is still not well understood. We hypothesized that under microgravity water in void spaces of porous media hardly moved comparing in void spaces without obstacles because the concave surfaces of the porous media hindered water movement. The objective of this study was to investigate water movement on the convex surfaces of porous media under microgravity. We conducted parabolic flight experiments that provided 20–25?s of microgravity at the top of a parabolic flight. We observed water movement in void spaces in soil-like porous media made by glass beads and glass spheres (round-bottomed glass flasks) in the different conditions of water injection under microgravity. Without water injection, water did not move much in neither glass beads nor glass spheres. When water was injected during microgravity, water accumulated in contacts between the particles, and the water made thick fluid films on the particles surface. When the water injection was stopped under microgravity, water was held in the contacts between the particles. This study showed that water did not move upward in the void spaces with or without the water injection. In addition, our results suggested that the difficulty of water movement on the convex (i.e. particle surfaces) might result in slower water move in porous media under microgravity than at 1G-force.     

Longevity in space; experiment on the life span of Paramecium cell clone in space.

Life span is the most interesting and also the most important biologically relevant time to be investigated on the space station. As a model experiment, we proposed an investigation to assess the life span of clone generation of the ciliate Paramecium. In space, clone generation will be artificially started by conjugation or autogamy, and the life span of the cell populations in different gravitational fields (microgravity and onboard 1 x g control) will be precisely assessed in terms of fission age as compared with the clock time. In order to perform the space experiment including long-lasting culture and continuous measurement of cell division, we tested the methods of cell culture and of cell-density measurement, which will be available in closed environments under microgravity. The basic design of experimental hardware and a preliminary result of the cultivation procedure are described.     

Longevity in space; Experiment on the life span of Paramecium cell clone in space

Life span is the most interesting and also the most important biologically relevant time to be investigated on the space station. As a model experiment, we proposed an investigation to assess the life span of clone generation of the ciliate Paramecium. In space, clone generation will be artificially started by conjugation or autogamy, and the life span of the cell populations in different gravitational fields (microgravity and onboard 1 x g control) will be precisely assessed in terms of fission age as compared with the clock time. In order to perform the space experiment including long-lasting culture and continuous measurement of cell division, we tested the methods of cell culture and of cell-density measurement, which will be available in closed environments under microgravity. The basic design of experimental hardware and a preliminary result of the cultivation procedure are described.     

Cytoskeleton and gravity at work in the establishment of dorso-ventral polarity in the egg of Xenopus laevis

The establishment of polarities during early embryogenesis is essential for normal development. Amphibian eggs are appropriate models for studies on embryonic pattern formation. The animal-vegetal axis of the axially symmetrical amphibian egg originates during oogenesis and foreshadows the main body axis of the embryo. The dorso-ventral polarity is epigenetically established before first cleavage. Recent experiments strongly suggest that in the monospermic eggs of the anuran Xenopus laevis both the cytoskeleton and gravity act in the determination of the dorso-ventral polarity. In order to test the role of gravity in this process, eggs will be fertilized under microgravity conditions during the SL-D1 flight in 1985. In a fully automatic experiment container eggs will be kept under well-defined conditions and artificially fertilized as soon as microgravity is reached; eggs and embryos at different stages will then be fixed for later examination. Back on earth the material will be analysed and we will know whether fertilization under microgravity conditions is possible. If so, the relation of the dorso-ventral axis to the former sperm entry point will be determined on the whole embryos; in addition eggs and embryos will be analysed cytologically.     

Auxin polar transport in Arabidopsis under simulated microgravity conditions — Relevance to growth and development

Activity of auxin polar transport in inflorescence axes of Arabidopsis thaliana grown under simulated microgravity conditions was studied in relation to the growth and development. Seeds were germinated and allowed to grow on an agar medium in test tubes on a horizontal clinostat. Horizontal clinostat rotation substantially reduced the growth of inflorescence axes and the productivity of seeds of Arabidopsis thaliana (ecotypes Landsberg erecta and Columbia), although it little affected seed germination, development of rosette leaves and flowering. The activity of auxin polar transport in inflorescence axes decreased when Arabidopsis plants were grown on a horizontal clinostat from germination stage, being ca. 60% of 1 g control. On the other hand, the auxin polar transport in inflorescence axes of Arabidopsis grown in 1 g conditions was not affected when the segments were exposed to various gravistimuli, including 3-dimensional clinorotation, during transport experiments. Pin-formed mutant of Arabidopsis, having a unique structure of the inflorescence axis with no flower and extremely low levels of the activity of auxin polar transport in inflorescence axes and endogenous auxin, did not continue its vegetative growth under clinostat rotation. These facts suggest that the development of the system of auxin polar transport in Arabidopsis is affected by microgravity, resulting in the inhibition of growth and development, especially during reproductive growth.