Schistosoma mansoni: eggshell formation is regulated by pH and calcium

The protein precursors of the schistosome eggshell are synthesized and packaged into secretory vesicles in the vitelline cells. These vesicles appear to contain an emulsion of eggshell precursor material. Evidence is presented to show that these secretory vesicles are acidic as in other systems and that this acidity stabilizes the emulsion and prevents the eggshell cross-linking reactions from occurring. Alkalinizing treatments trigger eggshell formation within the secretory vesicles as shown by (1) the induction of autofluorescence and (2) by electron microscopy which shows that the eggshell precursors have aggregated within the secretory vesicles into spherical particles bearing microspines. These aggregates formed in the secretory vesicles were isolated and shown to have the same protease resistance and amino acid composition as authentic eggshell. The calcium ionophore A23187 induces scattered autofluorescence in intact female worms which electron micrographs show to be due to exocytosis of eggshell material. Based on these observations we propose a model for the formation of schistosome eggshell and suggest that it may apply to all trematodes in which the eggshell precursors are present as stable emulsions in the secretory vesicles of the vitelline cells.