Although weightlessness is known to affect living cells, the manner by which this occurs is unknown. Some reaction-diffusion processes have been theoretically predicted as being gravitydependent. Microtubules, a major constituent of the cellular cytoskeleton, self-organize in vitro by way of reaction-diffusion processes. To investigate how self-organization depends on gravity, microtubules were assembled under low gravity conditions produced during space flight. Contrary to the samples formed on an in-flight 1 ؋ g centrifuge, the samples prepared in microgravity showed almost no self-organization and were locally disordered.

Microtubules are believed to be the principal organizers of the cell interior. Cells respond to a variety of stimuli by modifying the spatial distribution of the microtubules. These effects are central to cell division and morphogenesis, and embryo development. During embryo development, macroscopic patterns are frequently observed. Here we report that microtubular solutions spontaneously form alternating white and dark stripes about 1 mm wide and 1 cm long. Small-angle neutron scattering measurements show that in each segment the microtubules are aligned obliquely to the direction of the stripe, and that the white and dark stripes differ in having mutually orthogonal orientations. The formation of these structures requires an initial reservoir of organic phosphate. Phosphorus NMR measurements show that the process is accompanied by the energy-liberating conversion of organic to inorganic phosphate. These observations, together with similarities to the dissipative spatial structure formed by the Belousov-Zhabotinski reaction, provide strong evidence that the observed structures are energy-dissipative in nature. Dissipative structures are thought to be critical to the appearance of complex living organisms. Our results strongly suggest that microtubules are capable of forming such structures. Microtubular dissipative structures may occur during mitosis and embryo morphogenesis.

Nonlinear chemically dissipative mechanisms have been proposed as providing a possible underlying process for some aspects of biological self-organization, pattern formation, and morphogenesis. Nonlinearities during the formation of microtubular solutions result in a chemical instability and bifurcation between pathways leading to macroscopically self-organized states of different morphology. The self-organizing process, which contains reactive and diffusive contributions, involves chemical waves and differences in microtubule concentration in the sample. Patterns of similar appearance are observed at different distance scales. This behavior is in agreement with theories of chemically dissipative systems.

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