Immunoelectron microscopic demonstration of the exocytosis of dense granule contents into the secondary parasitophorous vacuole of Sarcocystis muris (Protozoa, Apicomplexa).

Merozoites of the parasitic protozoon Sarcocystis muris (Apicomplexa) possess three types of characteristic organelles with electron dense contents named rhoptries, micronemes, and dense granules, which are supposed to be involved in the parasite-host cell interactions during and after invasion. Dense granules were purified from a merozoite homogenate by centrifugation on a sucrose density gradient. It was shown by SDS polyacrylamide gel electrophoresis that they contain a major protein of 21 kDa. Polyclonal antibodies raised against this protein were applied to ultrathin frozen and Lowicryl-K4M-embedded sections of the parasite before and after host cell invasion. Dense granules were distinctly labeled by immunogold before and after invasion. After host cell invasion the parasite is enclosed in a secondary parasitophorous vacuole which contains an electron-dense material. This deposition was heavily labeled by anti 21 kDa antibodies which clearly demonstrated that the dense granule contents is released into the secondary parasitophorous vacuole.