Model describing the function of ascl1b and neurod1 in pancreatic endocrine differentiation. (A) In wt embryos, Ascl1b initiates the expression of sox4b in the endocrine precursors at 11 hpf. Then, around 12 hpf, neurod1 expression starts and maintains sox4b expression in the precursors. The differentiation of the distinct endocrine cell types can take place. (B) In neurod1 morphants, Ascl1b initiates the expression of sox4b in the endocrine precursors at 11 hpf and maintains sox4b expression until 17 hpf. From that stage, ascl1b expression turns off and sox4b expression is no longer maintained. The differentiation of the late-appearing endocrine cells is blocked. (C) In ascl1b morphants, sox4b expression is not initiated at 11 hpf. When Neurod1 starts to be expressed from 12 hpf, it initiates sox4b expression in a reduced number of endocrine precursors. hpf, hours post fertilization; wt, wild type.