Possible mechanisms of indirect gravity sensing by cells.

We have to distinguish between (a) direct gravisensing, in which specialized cells function as parts of a gravisensing organ and (b) indirect gravisensing, in which other cells that have no specialized gravity detectors are nevertheless affected by the inertial acceleration. In both cases, cells may detect (a) the direction of gravity ("up" versus "down"), and /or (b) the amplitude (0 - 1 g) of gravity. This chapter argues that the weight of single normal-sized cells (approximately 10 microns in diameter) is too small compared with other cellular forces to allow them the distinction between up and down. However, the weight of the surrounding medium is much larger. Cells may be able to sense certain environmental changes caused by gravity and thus may sense indirectly at least the amplitude of gravitational forces. In particular, the fluid environment of the cell can be expected at normal gravity to support microconvective currents that cease to flow at microgravity. Thus, the absence of gravity may be transduced into the accumulation of metabolites and ions from the cells and depletion of fresh nutrients. These changes, in turn, can affect the contacts of cells, their membrane potential, their cytoskeleton, and thus, ultimately, their behavior. As to ground-based simulations of microgravity, the above considerations suggest that the averaging of the vectorial force of gravity in clinorotation is inadequate for simulation because it may actually increase rather than suppress convective mixing above the normal levels.