Experimental analyses of cytoplasmic rearrangements which follow fertilization and accompany symmetrization of inverted Xenopus eggs

Cytoplasmic rearrangements which follow fertilization were monitored in inverted eggs. A set of yolk compartments was resolved by cytological analyses of both normally oriented and inverted eggs. Those compartments were characterized by their yolk platelet compositions and movement during egg inversion. In addition to the major yolk masses which contain either small, intermediate or large platelets, minor cytoplasmic compartments which line the egg cortex were also identified. During egg inversion the yolk compartments shift. Those yolk mass shifts occurred only after the inverted egg was activated (by sperm, electrical or cold shock). The direction of shift of the major yolk components, rather than the sperm entrance site (as in normal orientation eggs), determines the dorsal/ventral polarity of the inverted egg. Among different spawnings the rate of shift varied. Eggs that displayed the fastest rate of shift exhibited the highest frequency of developmental abnormalities during organogenesis. Isopycnic density gradient analysis of yolk platelets and blastula blastomeres showed that isolated yolk platelets and mid-blastula blastomeres are not of uniform buoyant density. Three major yolk platelet density bands were resolved. Large, intermediate, and small yolk platelets were found in all bands. The high density band had the largest proportion of the large yolk platelets and the low density fraction showed the largest proportion of the small yolk platelets. Interpretation of novel observations on cytoplasmic organization provided criticisms of some earlier models. A new ‘Density Compartment Model’ was developed and presented as coherent way to view the organization of the egg cytoplasm and the development of bilateral symmetry.