Hatching mechanism of the metacercaria ofPlagiorchisspecies 1 (Trematoda: Plagiorchiidae)

ABSTRACTMetacercariae ofPlagiorchisspecies 1 were observed to hatch by an active mechanism in a medium free of enzymes. A hatching opening in the bilayered cyst wall was formed by the combined action of caecal fluid extruded by the activated larva through the mouth opening and an internal pressure due to the tendency of the cyst wall to contract on hatching, resulting in an explosive expulsion of part of the metacercarial body. The cyst wall was apparently pierced at any place where the larva delivered its caecal fluid. After excystation the hatching medium contained high phosphatase and proteinase activities and was able to dissolve the inner walls of empty cyst envelopes. The phosphatase activity assayed on 4-methylumbelliferyl phosphate was optimal at pH 3–5. The proteolytic activity was demonstrable on photographic film, Azocoll, and synthetic chromogenic and fluorogenic peptides. A preference for peptides was found which are also susceptible to plasmin. The proteolytic activity was optimal at pH 3–9 and 40–45°C and, when assayed on Suc-Ala-Phe-Lys-MCA, was only due to thiol proteinase(s) according to inhibitor studies. It is suggested that the proteinase(s) represent the hatching enzyme(s) of the metacercaria, because (a) only proteolytic activity was detectable in the pH range optimal for excystment, (b) the inner cyst wall is stabilized by proteins, and (c) the inner wall is dissolved by other proteinases such as trypsin. Enzyme histochemical investigations of metacercariae showed that, in the caeca, acid phosphatase was present mainly before hatching and non-specific esterase developed after hatching. Proteolytic activity was not localized with the methods used although it was suggested that it derived from the caecal fluid. A possible relationship of the thiol proteinase(s) detected in the hatching medium to haemoglobin-digesting proteinases from the gut of schistosomes is discussed.