Subcellular components of the amphibian egg: insights provided by gravitational studies

Most cytoplasmic regions of fertilized amphibian eggs move with respect to the gravity vector in experimentally gravity oriented eggs. The pattern and extent of this movement varies among different batches of eggs. This variation in apparent cytoplasmic viscosity (or, conversely, cytoplasmic mobility) can be correlated with variations in subsequent morphogenesis of experimental, gravitationally manipulated eggs. Therefore, the proper interpretation of gravity experiments with amphibian eggs requires that one understand the subcellular basis for this variation on cytoplasmic mobility. Variation in the packing of the major cytoplasmic organelle, the yolk platelets, or the organization and amount of cytoskeletal components may explain the variation in cytoplasmic mobility. Evidence is presented that the variation in yolk volume density (fraction of total cytoplasmic volume occupied by yolk platelets) does not account for the variation in cytoplasmic mobility in Xenopus laevis eggs. Experimental evidence from cold-shocked inverted eggs indicates that microtubules may be involved in determining cytoplasmic mobility. However, quantitative evidence that the microtubule levels and state of the microtubules (polymerized vs. non-polymerized) in the whole Xenopus laevis egg does not correlate directly with cytoplasmic mobility is presented. The apparent conflict these data represent regarding the role of the cytoskeleton in determining cytoplasmic mobility is discussed.