The Florey lecture, 1988. From egg to embryo: the initiation of cell differentiation in Amphibia.

Some of the principles by which different cell types first arise at the beginning of animal development are illustrated by muscle cell formation in Amphibia. If the nucleus of a differentiated muscle cell is transplanted to an enucleated egg, some of the resulting embryos develop into tadpoles with a wide range of normally differentiated cells. These experiments show that genes undergo major changes in activity as a response to components of egg cytoplasm. Two fundamental mechanisms account for the regional activation of genes in early embryos. One involves the effect of localized 'determinants' in egg cytoplasm, and the other concerns cell interactions or embryonic induction. Both these mechanisms seem to be responsible for muscle cell formation in amphibian development. The old problem of embryonic induction has recently become accessible to analysis at the molecular level, especially in the case of the mesoderm or muscle-forming induction. This has been greatly facilitated by using a sensitive and quantitative assay to detect the first transcripts of muscle genes a few hours after the start of induction. The role of early events and of interactions among like cells during response to induction is discussed. In analysing specific gene activation following induction, DNA injection into fertilized eggs has shown that a very small part of the cardiac actin gene promoter is sufficient to enable it to respond to induction. Although the experimental work summarized here has been done on amphibian embryos, which are more suitable than other embryos for embryological manipulation, the conclusions reached are believed to be generally applicable to the development of other organisms.