ABSTRACT
The establishment of distinct cell types within mammalian embryos likely shares many similarities to development in Drosophila, whereby specific boundaries between different groups of cells are achieved through overlapping gradients of signaling molecules that in turn activate or repress particular signal transduction pathways. These signals consequently affect the activity (both positive and negative) of transcriptional regulators (Frasch, 1999; Skeath, 1999; Wolpert, 1996). Thus initially broad domains of cell-signaling molecules (or even transcription factors) in the developing embryo can be transformed with time and cell divisions into specific subregions of gene expression and distinct cell types. Specificity results from the interplay between modest activation threshold requirements and overlapping gradients of competing regulators. In the developing mammalian embryo, recent progress in cell-fate mapping studies has provided a clearer understanding of which cells give rise to the different tissues and organs (Lawson and Hage, 1994; Lawson and Pederson, 1992). Coincident with this understanding, in situ analysis of gene expression in embryos, combined with modern molecular genetics approaches, have demonstrated the specific role of developmental transcription factors in directing cellular differentiation, tissue formation and bodyplan formation during embryogenesis.
