Neurogenin-3 (ngn-3) expression is critical for endocrine development in the developing pancreas. We found that when ngn-3 was inhibited in an E11.5 pancreas, using either morpholino antisense or siRNA, it led to a significant decrease in endocrine differentiation after seven days in culture. Endocrine differentiation was rescued when ngn-3 inhibition was withdrawn after three days of culture, suggesting that the embryonic pancreas retains progenitor cells with the ability to differentiate into endocrine cell types when ngn-3 expression recurs. To determine whether the rescue phenomenon observed after withdrawing ngn-3 antisense treatment was the result of the original endocrine-committed cells reinitiating endocrine differentiation, or was instead due to new recruitment of later progenitor cells, we blocked ngn-3 expression for only the last four days of a seven-day culture. Here, insulin-positive differentiation was slightly reduced, but there was a normal number of glucagon-positive cells. In addition, there was an increase in SOX9-positive cells in ngn-3 inhibited, as well as in ngn-3 rescued pancreata, with a significant proportion of these SOX9-positive cells co-localized with DBA, an early ductal marker. This increased number of cells with co-localization of SOX9 and DBA could indicate an increased number of endocrine progenitor cells. (C) 2010 Elsevier Inc. All rights reserved.