Understanding the physiological changes from microgravity

Primary authors: Eric Greenwald, Kristan Armstrong, Patrick Leon, Kevin Reinberger

Motivation

     Ever since man has entered space,  there has been an interest in microgravity and its effect on human physiological growth and function. Studies show that while away from earth's gravitational field, there are defects in cartiliage growth due to the reduced stress on bones. Evidence suggest that while spending an extended period of time in space (~6 months), there is a decrease in cartilage mass that affects the health of astronauts upon the return to earth's gravity [5].  The detriment to physical health due to prolonged exposure to microgravity has put a limit on the duration and allowable travel distance on space missions. A fundamental understanding of the differences in growth and function of cartilage in space will widen the frontiers available for exploration and maintain the health and well being of astronauts.

     In recent years, there has been a renewed interest in space travel that will facilitate funding for new missions and expansion of research projects.  Currently, NASA plans to return to the moon by 2020 in part of what has been named the Constellation Program. The plan is to have a range of new space crafts to replace the function of the existing Space Shuttle and extend the lift capacity and range of space missions.  This will be accomplished by implementing two new boosters Ares I and Ares V, the Orion crew compartment, and the Altair lunar lander. [2] The Bush administration even announced plans to develop a lunar base to use as a way to launch further space missions without the extreme energy requirements to escape Earth's atmosphere. However it has a questionable future due to planned government cutbacks as a result of the economic crisis of 2008. One of the most ambitious plans for space exploration was announced by NASA, which is to send a manned crew to the red planet, Mars, by 2037 [3]. A mission to Mars requires traversing a unprecedented distance, and plans are already being developed that minimize exposure to microgravity.  Exposure will be limited to six months by sending a high-speed return vehicle ahead of the crew.  To stay on the planet for two years, the crew can create methane fuel and water through the Sabatier reaction combined with the reverse water gas shift reaction, while creating oxygen through electrolysis. [4]

      When under the effects of microgravity for extended periods of time it has been noticed that humans and other animals experience “cardiovascular deconditioning, muscle atrophy, immune dysfunction, and bone loss” [5].  Bone is constantly being decomposed and reformed, but when in microgravity, regulatory signal levels are altered and more bone is resorbed than formed [5]. This can be linked to the environment that weight bearing bones, e.g. the tibia, are much more affected than non-weight bearing bones, e.g. radius [5]. The body compensates for the lack stress to achieve homeostasis for the current environment, and makes changes in many human tissues.

     One relation to human physiology growth in space is found in intrauterine development, which occurs in a bouyant environment that resembles microgravity. [1]   For approximately the first 22 weeks of gestation the embryo/fetus grows in a suspended state, allowing protection from forces like movement that would otherwise put it in danger.  Deviation from the resemblance occurs with the gradualally increasing exposure to pseudo-gravitational forces through the last trimester of pregnancy which is extremely important to structural growth. This exposes the fetus to uterine pressure and mechanical resistance to movement, contributing to the development of body tissues and bones.   Infants that remain in the bouyant environment through the third trimester display a similar lack of conditioning as seen in those exposed to microgravity, and are more likely to experience sudden infant death syndrome. [6]  In addition, experiments on embryonic rats developing in space have shown that exposure to microgravity changes tissues structure, and even delays development of some terminal branches and synapses as well as creating behavioral changes [7]. This and other deviations from normal development found in mammalian embryos exposed to microgravity casts questions on the possibility for humans to colonize environments that have less gravity than Earth.  

Previous work 

     Past studies has shown that to maintain a normal ambulatory body weight, typical men require 2,800-3,100 kcal/day.  This suggests that this energy requirement will be the same in a microgravity environment.  It would seem necessary for astronauts in space to maintain an adequate level physical fitness to perform everyday tasks efficiently.  This includes body speed, power, strength, endurance, and cognitive performance.   However, the nature of exercise (type, frequency, duration, and intensity) that should be applied by astronauts before, during, and after flight has not been determined. [8]   

     Among the first physiological responses of weightlessness is fluids shifts that occur within the body.  Fluid shifts occur when the hydrostatic pressure gradient from the head to foot is lost; body fluids redistribute from the legs and lower body to the thorax and head until reaching an equilibrium state.  This is most likely determined by a balance between vascular and tissue pressures.  Long term effects of this fluid shift could result in edema, cardiovascular deconditioning, muscle atrophy, and hyperglycemia. [12]   

     Effects of space travel on human physiology has shown to have correlations to effects shown from bed rest and akinesia. [8]  When restricted to horizontal and head-down-tilt bed rest for more than 24 hours, similar fluid shifts have been observed to those imposed during weightlessness.  Research done at the dawn of the space era involved fluid immersion and complete bed rest, which were researched in short and long time periods. [12]

Preventative Measures

     There are suggestions to have regimented work out programs for astronauts in space to help improve mental and physical ability. Studies have shown that an individual in space loses approximately 170 mg of calcium per day causing major impacts on the ability of the body to maintain cartilage. [8] Daily exercise is imperative for astronauts to maintain healthy bone and muscular structure here on Earth, in addition, to in space. It has been shown that daily exercise helps increase not only the physical ability of the individual, but also increases healthy psychological ability. For these reasons sustaining a rigorous exercise program is essential. This program will be designed around performing isokinetic and isotonic exercises for approximately 40 minutes per day. The isokinetic part of the program helps to maintain leg and arm fitness in case of an emergency situation such as an extra-vehicular activity (EVA). Pilots may need to perform the isokinetic leg exercises for only 20 minutes a day depending on the in-flight situation. [8] Isotonic exercises are focused on endurance training to maintain healthy aerobic performance.  Through implementing a regimented exercise program for astronauts it will reduce the effects of microgravity on human cartilage, in addition to mental and physical health.

Research Strategies

     Cartilage is comprised of a matrix of collagen, glycosaminoglycan and sparsely populated with chondrocites.[11] The collagen provides the tensile strength of cartilage[11]. Aggrecan is a proteoglycan that is made up of two glycosaminoglyan (GAG) chains (chondroitin sulfate and keratan sulfate) and a protein core. The GAGs are carbohydrate chains that have many negative charges on them and provide the compressive strength that cartilage needs by way of  repulsive negative ion-ion forces that occur when the many aggrecan molecules are compressed together.[11] The collagen and the GAGs in cartilage are the two most important components to study because they are have the largest role in cartilage’s structural components.

Tissue engineering of cartilage in space - The Mir experiment [1]

     An experiment was done using  Mir space station where articular cartilage is grown on earth and in space  and compared the GAG and collagen deposition as well as mechanical properties such as aggregate modulus and dynamic stiffness [1]. First, chondrocytes  were extracted from bovine calves and encapsulated in PGA[1]. These were then cultured for 3 months and then some of the samples were sent up to the Mir space station and the rest were left on earth. These samples were cultured for 7 months and then compared to each other and to natural cartilage [1]. It was found that the Mir constructs were smaller, more spherical and mechanically inferior. 

GAG deposition

     The constructs from both the Mir and Earth groups maintained the structure throughout the cross section that would suggest having the properties of being firm and durable with flexible structure.  However, the overall GAG weight percent of the Mir construct is less than half of the Earth construct, and the Mir group showed GAG deposition in the outer layer of the cartilage sample.  The deposition can be seen in figure C below, where the GAG is stained red with safranin-O.  The loss of total weight percent of GAG could mean that the cartilage is not able to withstand as much stress before breakage.  The GAG in the outside layer marks the presence of fibrous capsules that were unique to the Mir group could affect its tensile strength, making the cartilage less pliable and reducing its elasticity.  

Collagen II

     Constructs grown in normal gravity have more organization in their extracellular matrix (Figures E and F).  As shown in the figure, the collagen orientation is more uniform for the Earth construct, as the cartilage grown on the Mir space station show random spacial orientation due to disruption of fibrillogenesis, the creation of fine fibrils from tropocollagen.  Despite the inhibition of fibril formation, the average collagen fiber diameter was found to be similar.  The effect of reduced organization of collagen II on tensile strength is that the cartilage grown in microgravity would not be able to resist breaking over as large of a stress range as natural cartilage.

Conclusions and Recomendations

• Tissue Growth
  • Currently not a necessity to grow tissue in space but with upcoming space colonies, and growth of tissue engineering, technique may need to be developed to provide functional tissue.
  • If it were found to be advantageous to grow cartilage in microgravity conditions, then microgravity reactors could produce cartilage for medical use.
  • Mostly useful as experiments to understand the that lead to the physiological changes seen in humans exposed to microgravity

     

Figure G.  A simulated microgravity bioreactor used at the University of Amsterdam.

     While it is important to be able to understand the entirety of variables that affect cartilage and tissue growth while in space, tissue growth can be examined on earth using a microgravity reactor. Nadia Rucci et al. simulated microgravity using a reactor developed by NASA called a Rotating Wall Vessel Bioreactor where microgravity is simulated by “randomizing the gravitational vector in response to the rotation of the culture vessel”. [10] Their experiment examined the effect of microgravity on bone growth. Freed and Vunjak-Novakovic also used a microgravity bioreactor to study the growth of cartilage while on earth [9].  These studies are important because they limit the variables to microgravity and exclude many of the other factors, as well as being much more cost effective. 

     One limitation of these studies is that they only analyze the effect of microgravity on cartilage growth in PEG. Now, another area that could be investigated is the rate of growth and effectiveness of engineered cartilage in vivo. Not only would the cartilage have issues with growth in culture but there are probably more issues with a lack of stress applied to the cartilage in growth. This also brings up another issue with long term stays in space that may be achieved by living at a space station on the moon. If children are to be raised in space they might have decreased musculoskeletal strength and would most likely not be able to go back to Earth.

Citations

[1] Freed, Lisa E., Robert Langer, Ivan Martin, Neal R. Pellis, and Gordana Vanjak-Novakovic. "Tissue Engineering of Cartilage in Space". The Proceedings of the National Academy of Sciences. Vol. 94. pp 13558-13590, December 1997.

[2] Witt, Howard. "NASA prepares for return to moon". Chigago Tribune. Oct. 8, 2007.

[3] AFP. "NASA aims to put man on Mars by 2037". http://afp.google.com/article/ALeqM5jkmdP908t7rFtnuI4rNSCpCl3TTQ . Sept. 24 2007.

[4] Canton, Gilbert.  "Sizing of a Combined Sabatier Reaction and Water Electrolysis Plant for Use in In-Situ Resource Utilization on Mars". University of Florida Journal of Undergraduate Research. Vol. 3 Issue 1, September 2007.

[5] Zayzafoon, Majd, Valerie E. Meyers, Jay M. McDonald. "Microgravity: the immune response and bone". Immunological Reviews. Vol. 208: 267-280.

[6] Reid, G. "Sudden infant death syndrome (SIDS): Microgravity and inadequate sensory stimulation". Medical Hypotheses. Vol. 66, Issue 5: 920-924.

[7] A.E. Ronca, B. Fritzsch, J. Alberts and L.L. Bruce, "Effects of microgravity on vestibular development and function in rats: genetics and environment". Korean J. Biol. Sci. 4(2000), pp. 215–221.

[8] J. E. Greenleaf, R. Bulbulian, E.M. Bernauer, W. L. Haskell, and T. Moore. "Exercise-training protocols for astronauts in microgravity". Journal of Applied Physiology. Dec 1989; 67: 2191 - 2204

[9] Freed, Lisa E., Gordana Vunjak-Novakovic. "Microgravity Tissue Engineering". Division of Health Sciences and Technology, Massachusetts Institute of Technology. August 1996.

[10] Nadia Rucci, Anna Rufo, Marina Alamanou, and Anna Teti. "Modeled Microgravity Stimulates Osteoclastogensis and Bone Resorption by Increasing Oseoblast RANKL/OPG Ratio". Journal of cellular Biochemistry. Vol. 100: 464-473, 2007.

[11] Mow, V.C. and C.T. Hung, Biomechanics of Articular Cartilage. Basic Biomechanics of the Musculoskeletal System, ed. M. Nordin and V.H. Frankel. 2001: Lippincott Williams & Wilkins. 467. 

[12] Greenleaf JE, Silverstein L, Bliss J, et al. Physiological responses to prolonged bedrest and fluid immersion in man. A Compendium of Research (1974–1980). NASA Technical Memorandum 81324, January 1982.