SPECIFYING THE DORSOANTERIOR AXIS IN FROGS - 70 YEARS SINCE SPEMANN AND MANGOLD

Many molecules have been identified to generate mesoderm and axial structures in the Xenopus embryo, but that does not guarantee that all the important molecules have been discovered. Other ligand–receptor combinations and molecules that are yet to be uncovered are probably involved in signal transduction. The absence of genetics makes it difficult to know when most of the major molecular players have been detected. A further problem presented by the absence of genetics is that as more and more molecules are found, assessing the molecular interactions and causal relationships in axis formation becomes increasingly difficult. The assays described earlier explain what a molecule can do but not necessarily what it normally does in the embryo. The assays lack the precision of genetic mutants because the time and place of expression relative to normal events are not well controlled. © 1995 Academic Press Inc.