Induction of Mesoderm in Amphibians

Earlier you know that an event that precedes neural ectoderm induction in amphibians is the induction of mesoderm. Until 32 cell stage in amphibian blastulae there are only two cell sorts. In animal hemisphere there are small ectodermal cells and in the vegetal hemisphere there are large endodermal cells. A third cell type from which mesoderm develops is made in the equatorial region of the embryo throughout early blastula stage.

Fate map of Xenopus embryo reveals that equatorial cells in contact with both of the animal and vegetal cells in the equator of blastula are precursors of mesoderm. How is the fate of the equatorial cells as prospective mesoderm cells determined? Either the ooplasmic determinants like intrinsic factors or cell interactions as extrinsic factors may play a role in such type of a determination. To test one of the two probabilities, the blastulae were divided into explants of animal, equatorial and vegetal cells and cultured independently. Animal cells that are developed into ectoderm cells, vegetal cells into endoderm cells and the equatorial cells into mesoderm cells. Though, if the animal cells were recombined with vegetal cells and cultured, they formed mesoderm cells also suggesting the probability that mesoderm formation is not exclusively dependent on ooplasmic determinants. The equatorial cells develop into mesoderm cells possibly since the adjacent vegetal cells secrete some inducing substance which influences the adjacent cells of the equatorial region inducing them to become mesodermal cells. In the general blastula, the animal hemisphere cells differentiate only into ectoderm since they are widely separated from the vegetal cells by the blastocoel and thus are too far from the source of inducer.