Expression of Pyy in the early developing mouse embryo. (A, B) Pyy expression is seen in small lateral region of the DE at as early as 2 somite stage (indicated by arrow). (C, D) At the 4 somite stage, the expression domains in the lateral region are expanded, and the second expression domain which is in the medial ventral foregut can to be observed (arrowhead). (E-J) The lateral expression domains expanded and extended anteriorly to the medial ventral foregut. Strong expression was observed in the lateral and ventral foregut in the 6–8 somite stages. Representative sections are shown in the right panel. (K-N) In the early organogenesis stage, the Pyy expression remained in the posterior foregut extending to the midgut junction.