Organisation of Xenopus egg cytoplasm: response to simulated microgravity.

The cytoplasm of Xenopus fertilised eggs appears to be organised into three major compartments based primarily on the uneven distribution of yolk platelets. There is a shift of these yolk compartments during the first cell cycle that is thought to be involved in the dorsal/ventral morphogenesis of the embryo. The involvement of gravity in Xenopus cytoplasmic organisation and in compartment shifts was addressed by examining, cytologically, the yolk compartments in embryos that developed under the simulated microgravity conditions of the horizontal clinostat. The cytoplasmic organisation into yolk compartments was found to be maintained, and the asymmetric movements of compartments still occurred in eggs that developed on the clinostat. It is suggested that the organisation of Xenopus egg cytoplasm into discrete compartments relies on forces other than those involving gravity (i.e., not density differences), and that the compartment shifts that take place during the first cell cycle are active movements. The variation in compartment size and composition observed from batch to batch of eggs, and to a lesser extent from egg to egg, during this study was addressed.