Three regions of the 32-cell embryo of Xenopus laevis essential for formation of a complete tadpole.

To examine the conditions of cell composition necessary for formation of a complete tadpole, defect embryos of 35 series were prepared by removing particular cells from 16- and 32-cell embryos of Xenopus laevis. These defect embryos were cultured and their development was examined. Formation of a complete tadpole did not require a special cell in the embryo, but did require cell combinations as follows: (1) A set of dorsal cells with a sufficient capacity to initiate axial structures. (2) Dorsal-most and ventral-most cells or dorsal-most and ventrolateral cells of the vegetal hemisphere. (3) The animal-most and vegetal-most tiers and either one of the upper-equatorial or the under-equatorial tiers. On the basis of these conditions, the minimal set of cells required for formation of a complete tadpole was determined. The minimal set included one dorsovegetal, two ventrovegetal, and eight animal cells in a lateral hemisphere of the 32-cell embryo. Forty percent of defect embryos with this minimal set developed into about quarter-sized but otherwise normal tadpoles. These tadpoles are the smallest complete tadpoles which have been experimentally obtained. The results of these experiments suggest that formation of a complete tadpole requires at the least cells in three different regions of the 32-cell embryo, that is, the animal, dorsovegetal, and ventrovegetal regions. The mechanism of pattern formation at the blastula stage is discussed, in consideration of the "three signal model" of mesoderm formation.