Compact exocrine pancreas of ammocoetes of the southern hemisphere lamprey Mordacia mordax contains a trypsin inhibitor: Putative evolutionary considerations

The localization of exocrine pancreatic (zymogen) cells in the epithelium of the anterior intestine of the larvae of holarctic lampreys (Petromyzontidae) is considered the most primitive arrangement of these cells in vertebrates (Barrington, '72). In contrast, the zymogen cells are located in prominent forward-projecting diverticular extensions of the anterior intestine in larvae of the two monogeneric southern hemisphere lamprey families (Geotriidae and Mordaciidae). During the present study, tryptic inhibitory activity was not found in the two diverticula of Geotria australis, whereas a trypsin inhibitor of the Kazal type was found in the diverticulum of Mordacia mordax, eluting at the same chromatographic position as the inhibitor of rat pancreas. It is suggested that a trypsin inhibitor is not required in G. australis, because its wide duct-like diverticula would facilitate a relatively steady dispersal of trypsin into the anterior intestine. In contrast, a trypsin inhibitor would be of value to M. mordax since the highly folded epithelial lining and narrow central canal of its diverticulum would presumably lead to some trypsin being retained in the diverticulum. Such a protective mechanism against unbridled proteolysis would be complemented by the relatively very low level of trypsin activity found in the diverticulum of M. mordax. Consideration of our data in the context of the proposed scheme for the phylogeny of lamprey taxa suggests that a trypsin inhibitor has been evolved in the larval exocrine pancreatic tissue of one of the lamprey families, i.e., Mordaciidae, independent of that developed in the exocrine pancreas of gnathostomes