Trypsinogen expression during the development of the exocrine pancreas in winter flounder (Pleuronectes americanus)

Histological, biochemical and molecular techniques were used to describe the functional development of the pancreas in winter flounder (Pleuronectes americanus) with specific reference to the expression of three trypsinogen genes. The pancreas was identified shortly following hatch, appearing as a compact structure situated dorsal and slightly posterior to the liver. As the larval fish approached metamorphosis, the pancreas became diffuse, spreading throughout the mesentery surrounding the stomach, the upper intestine and the pyloric caecae. Trypsin 2 expression was detected from 5 days post-hatch (dph). Two other related trypsinogen genes isolated from the pyloric caecae (Trypsin 1) and the intestine (Trypsin 3) showed contrasting results. Trypsin 1 showed very low levels of expression and only in late larval stages and metamorphosis. Trypsin 3 showed expression only after 20 dph. In order to determine tissue-specific expression of the three trypsinogen genes, the RNA from seven gastrointestinal-associated tissues was examined. Trypsin 1 and Trypsin 2 expression was most notably associated with the pyloric caecae, cardiac stomach, pyloric stomach and the rectum, although some variation in expression level between tissues was observed. Trypsin 3 expression had a narrower tissue distribution and was only associated with the pyloric caecae and the rectum. The tissue expression patterns observed here are likely due in part to the diffuse nature of the pancreas. Trypsin-like activity was evident from hatch and continued at significant levels through to at least 25 dph.