Determination, induction and pattern formation in early amphibian embryos.

Determination (inducing) factors, the extracellular matrix, signaling pathways, transcription factors and genes interact in pattern formation and neural induction. Genes can either be activated or repressed. The animalvegetal and dorso-ventral polarities are determined in very early developmental stages. Factors of the TGF-β superfamily in a graded distribution are involved in the determination of endoderm, mesoderm and ectoderm. The differentiation of mesoderm also depends on the animal ectoderm. Neural inducing factors have been partially purified.