Determinants of Early Amphibian Development

Abstract

The animal-vegetal organization of the amphibian egg may originate from the axis of organelles and cytoskeletal elements established in the oocyte as it divides from the oogonium. Along this axis, cytoplasmic materials are localized during oogenesis: yolk platelets, for example, are translocated toward the vegetal pole, increasing their amount and size in that region. In the first cell cycle after fertilization, the egg cortex rotates 30° relative to the cytoplasmic core, modifying animal-vegetal organization. The direction of this rotation, biased by the point of sperm entry, defines the site of development of anatomical structures of the dorsal midline of the embryo. As its immediate effect, rotation activates the cytoplasm of a subregion of the vegetal hemisphere, causing cells cleaved from this subregion to be more effective than other vegetal parts in inducing marginal zone cells to initiate gastrulation movements. The most strongly induced part of the marginal zone begins gastrulation first (the dorsal lip of the blastopore) and proceeds through a series of cell interactions leading to its determination as the anterior dorsal mesoderm of the embryo. If these cell movements are inhibited in the gastrula stage, or if vegetal induction is inhibited in the blastula stage, or if cortical rotation is inhibited in the first cell cycle after fertilization, the embryo always fails to develop dorsal structures of the anterior end of its body axis; the more inhibition, the more posterior is the level of truncation, until a radial ventralized embryo develops, derived from the animal-vegetal organization of the oocyte.