Localization of trypsin, chymotrypsin and elastase in the digestive tract of amphioxus Branchiostoma japonicum with implications to the origin of vertebrate pancreas.

The organ pancreas is characteristic of all vertebrates, but its evolutionary origin remains largely unknown. The serine proteinases trypsin, chymotrypsin and elastase are produced primarily in the pancreas in vertebrates. Here we clearly demonstrate by enzymatic activity assay, histochemical staining and in situ hybridization that trypsin, chymotrypsin and elastase are widely distributed in the digestive tract of the amphioxus Branchiostoma japonicum, especially in the hepatic caecum and mid-gut. This suggests that an extensive region of the amphioxus digestive tract including the hepatic caecum is homologous to the vertebrate exocrine pancreatic cells, providing a new angle for the study of the origin and evolution of vertebrate pancreas.