CELL-INTERACTIONS AND DEVELOPMENT

The establishment of the vertebrate primary body plan is dependent upo n cell interactions occurring during early embryogenesis. Evidence for such interactions and definition of the related concept of induction have arisen from the data of experimental embryology. The amphibian em bryo provides a suitable system for the study of the molecular and cel lular mechanisms of these interactions. Inductive interactions lead to the irreversible commitment of embryonic cells into one differentiati on pathway. Inductive signals are not equivalent with respect to dorso ventral and anteroposterior embryonic axes. These variations in the si gnals which control the regionalization result in part from the presen ce of localized cytoplasmic determinants in the egg and the early embr yo. In early steps of development, the first inductive interactions le ad to the formation of the mesoderm in the equatorial region of the bl astula. In the past few years, molecular biological techniques have le d to a considerable progress in the understanding of the molecular bas is of this process. There is excellent evidence that mesoderm patterni ng is formed in response to several different signals acting together. Studies carried out with Xenopus embryos have demonstrated that mesod erm inducing signals involve peptide growth factors from the transform ing growth factor-beta (TGF-beta) and fibroblastic growth factor (FGF) families. These peptides can induce mesoderm in vitro, some of which are present in the egg. Mesoderm patterning might result from the syne rgistic action of these factors with other molecules such as the Wnt a nd noggin genes products.