During embryonic life, the development of a proper mass of mature pancreatic tissue is thought to require the proliferation of precursor cells, followed by their differentiation into endocrine or acinar cells. We investigated whether perturbing the proliferation of precursor cells in vitro could modify the final mass of endocrine tissue that develops. For that purpose, we used activators or inhibitors of signals mediated by receptor tyrosine kinases. We demonstrated that when embryonic day 13.5 rat pancreatic epithelium is cultured in the presence of PD98059, an inhibitor of the mitogen-activated protein (MAP) kinase, epithelial cell proliferation is decreased, whereas endocrine cell differentiation is activated. On the other hand, in the presence of epidermal growth factor (EGF), an activator of the MAP kinase pathway, the mass of tissue that develops is increased, whereas the absolute number of endocrine cells that develops is decreased. Under this last condition, a large number of epithelial cells proliferate but remain undifferentiated. In a second step, when EGF is removed from the pool of immature pancreatic epithelial cells, the cells differentiate en masse into insulin-expressing cells. The total number of insulin-expressing cells that develop can thus be increased by first activating the proliferation of immature epithelial cells with growth factors, thus allowing an increase in the pool of precursor cells, and next allowing their differentiation into endocrine cells by removing the growth factor. This strategy suggests a possible tissue engineering approach to expanding β-cells.