Microgravity simulation as a probe for understanding early Xenopus pattern specification.

Pattern specification in early amphibians (Xenopus) was monitored in embryos subjected to gravity compensation (microgravity simulation) by constant low-speed rotation on a horizontal axis (clinostat). The useful range of clinostat speeds was determined empirically. The results were interpreted in terms of a set of models which account for the reorganization of the egg cytoplasm that follows fertilization and that correlates with the establishment of dorsal/ventral polarity. Large percentages of clinostated eggs displayed a positive result (normal axial structure morphogenesis). Consequently, normal development of amphibian eggs in the microgravity environment of space should be possible. Models which depend upon gravity-driven rearrangements for cytoplasmic organization (e.g. dorsal/ventral polarization) of the early embryo should, therefore, not be favoured. At several clinostat speeds symmetrization of the egg in accordance with the site of sperm penetration, a natural phenomenon, was altered. The results at those clinostat speeds indicate that models which employ sperm entrance as an obligatory feature of the cytoplasmic rearrangements that generate egg polarity are not applicable.