CELL-INTERACTIONS AND DEVELOPMENT

The establishment of the vertebrate primary body plan is dependent upon 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 embryo provides a suitable system for the study of the molecular and cellular mechanisms of these interactions. Inductive interactions lead to the irreversible commitment of embryonic cells into one differentiation pathway. Inductive signals are not equivalent with respect to dorsoventral and anteroposterior embryonic axes. These variations in the signals which control the regionalization result in part from the presence of localized cytoplasmic determinants in the egg and the early embryo. In early steps of development, the first inductive interactions lead to the formation of the mesoderm in the equatorial region of the blastula. In the past few years, molecular biological techniques have led to a considerable progress in the understanding of the molecular basis of this process. There is excellent evidence that mesoderm patterning is formed in response to several different signals acting together. Studies carried out with Xenopus embryos have demonstrated that mesoderm inducing signals involve peptide growth factors from the transforming 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 synergistic action of these factors with other molecules such as the Wnt and noggin genes products.