[The cell as a gravity-dependent biomechanic system]

In the period of 1995-1997 experimental and theoretical studies with various biomechanic objects, i.e. individual cells and cell associations, were performed under changed gravity (0.00001-5 g). Experimental investigations were conducted using clinostats and centrifuges to model effects of hypo- and hypergravity, and aboard space vehicles in real microgravity. Cell cultures in vitro including fibroblasts and osteoblasts on a solid glass or plastic substrate served as objects of the studies. Changes in value and direction of the gravity vector were found to modify the morphophysiological characteristics of cells: structural organization (spatial rearrangement of the intracell component, changes in forms, sizes and quantity of cells) and functional activity (alterations in energy expenditure and intensity of intracellular metabolism). The data suggest that there should be mechanisms of gravitational sensitivity in living systems on the cellular level. As was stated, sensitivity of unicellular free-living organisms to gravity is mostly defined by the motor activity determined by the level of general metabolism. Morphological characteristics (form, size and mass) are of secondary importance. Theoretical analysis resulted in correction of one of the principle postulates of gravitational biology stating a direct link between size (mass) and gravitational sensitivity of organism. Described were consistent patterns of growth, development, and behavior of unicellular cultures in gravitational fields. Strengthening of the force of gravity (hypergravity) leads to eventual deceleration of cell growth and diminution of biomass gain. On the other hand, the spaceflight environment (microgravity) stimulates growth mechanisms. In our opinion, behind these gravitational effects are altered levels of energy spent by cells to overcome the force of gravity. Opposite trends were observed in experiments with cell cultures in vitro. During space microgravity, fibroblast cultures on the solid substrate decreased the growth rate, and inhibited cell division and migration within the substrate. Compared to the Earth's gravity, under elevated gravity these parameters were noticeably higher. It was demonstrated that the main cause of the unfavorable effects of space microgravity on the cellular level is decay in the adherence of cells to the substrate. Explored were also the most probable mechanisms of the effects of changed gravity on the cell as a biomechanic structure. Specialized and non-specialized graviceptors of various types of cells were crypt-analyzed and classified. In future, investigations should be angled for elucidation of the role of intracellular components in perception and implementation of the gravitational stimulus, and description of quantitative characteristics of energy exchange and metabolism in cells as a function of gravity force and direction.