Onset of apoptosis in the cystic duct during metamorphosis of a Japanese lamprey, Lethenteron reissneri.

A nonparasitic lamprey in Japan, Lethenteron reissneri, stops feeding prior to the commencement of metamorphosis. Resumption of feeding cannot take place due to major alterations in the digestive system, including loss of the gall bladder (GB) and biliary tree in the liver. This degeneration of bile ducts is considered to depend on programmed cell death or apoptosis, but molecular evidence of apoptosis remains lacking. Using terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining and immunohistochemistry with an antibody against active caspase-3, we showed that epithelial cells of the cystic duct (CD) and GB became TUNEL-positive by the early metamorphosing stage. Immunohistochemical staining of active caspase-3, a key mediator in the apoptotic cascade, showed that the apoptotic signal was initiated in the region around the CD in the late larval phase. In later stages, active caspase-3-positive epithelial cells were also observed in the large intrahepatic bile duct (IHBD) and peripheral small IHBDs. At the early metamorphosing stage, bile canaliculi between hepatocytes were dilated and displayed features resembling canaliculi in cholestasis. Onset of apoptosis around the CD, which is the pathway for the storage of bile juice, and progression of apoptosis towards the large IHBD, which is the pathway for the secretion of bile juice, may lead to temporary intrahepatic cholestasis. The present study represents the first precise spatial and temporal analysis of apoptosis in epithelial cells of the biliary tract system during metamorphosis of any lamprey species.