============================================================================== Your Bones in Space ASTRONOMY AND SPACE SCIENCE ------------------------------------------------------------------------------ Hypogravitational Osteoporosis: A review of literature. By Lambert Titus Parker. May 19 1987. Osteoporosis: a condition characterized by an absolute decrease in the amount of bone present to a level below which it is capable of maintaining the structural integrity of the skeleton. To state the obvious, Human beings have evolved under Earth\'s gravity "1G". Our musculoskeleton system have developed to help us navigate in this gravitational field, endowed with ability to adapt as needed under various stress, strains and available energy requirement. The system consists of Bone a highly specialized and dynamic supporting tissue which provides the vertebrates its rigid infrastructure. It consists of specialized connective tissue cells called osteocytes and a matrix consisting of organic fibers held together by an organic cement which gives bone its tenacity, elasticity and its resilience. It also has an inorganic component located in the cement between the fibers consisting of calcium phosphate [85%]; Calcium carbonate [10%] ; others [5%] which give it the hardness and rigidity. Other than providing the rigid infrastructure, it protects vital organs like the brain], serves as a complex lever system, acts as a storage area for calcium which is vital for human metabolism, houses the bone marrow within its mid cavity and to top it all it is capable of changing its architecture and mass in response to outside and inner stress. It is this dynamic remodeling of bone which is of primary interest in microgravity. To feel the impact of this dynamicity it should be noted that a bone remodeling unit [a coupled phenomena of bone reabsorption and bone formation] is initiated and another finished about every ten seconds in a healthy adult. This dynamic system responds to mechanical stress or lack of it by increasing the bone mass/density or decreasing it as per the demand on the system. -eg; a person dealing with increased mechanical stress will respond with increased mass / density of the bone and a person who leads a sedentary life will have decreased mass/density of bone but the right amount to support his structure against the mechanical stresses she/she exists in. Hormones also play a major role as seen in postmenopausal females osteoporosis (lack of estrogens) in which the rate of bone reformation is usually normal with the rate of bone re-absorption increased. In Skeletal system whose mass represent a dynamic homeostasis in 1g weight- bearing,when placed in microgravity for any extended period of time requiring practically no weight bearing, the regulatory system of bone/calcium reacts by decreasing its mass. After all, why carry all that extra mass and use all that energy to maintain what is not needed? Logically the greatest loss -demineralization- occurs in the weight bearing bones of the leg [Os Calcis] and spine. Bone loss has been estimated by calcium-balance studies and excretion studies. An increased urinary excretion of calcium , hydroxyproline & phosphorus has been noted in the first 8 to 10 days of microgravity suggestive of increased bone re-absorption. Rapid increase of urinary calcium has been noted after takeoff with a plateau reached by day 30. In contrast, there was a steady increase off mean fecal calcium throughout the stay in microgravity and was not reduced until day 20 of return to 1 G while urinary calcium content usually returned to preflight level by day 10 of return to 1G. There is also significant evidence derived primarily from rodent studies that seem to suggest decreased bone formation as a factor in hypogravitational osteoporosis. Boy Frame,M.D a member of NASA\'s LifeScience Advisory Committee [LSAC] postulated that "the initial pathologic event after the astronauts enter zero gravity occurs in the bone itself, and that changes in mineral homeostasis and the calcitropic hormones are secondary to this. It appears that zero gravity in some ways stimulate bone re-absorption, possibly through altered bioelectrical fields or altered distribution of tension and pressure on bone cells themselves. It is possible that gravitational and muscular strains on the skeletal system cause friction between bone crystals which creates bioelectrical fields. This bioelectrical effect in some way may stimulate bone cells and affect bone remodeling." In the early missions, X-ray densitometry was used to measure the weight-bearing bones pre & post flight. In the later Apollo, Skylab