A freeze-fracture study of the host cell-parasite interface during and after invasion of cultured cells by cystozoites of Sarcocystis muris

Abstract

The parasite-host-cell interface of Sarcocystis muris in cell culture was studied by freeze-fracture electron microscopy. Cystozoites of S. muris have an intra membrane particle (IMP) density of 1347 ± 146 IMP/μm(2) on the P face and 638 ± 109 IMP/μm(2) on the E face. After exposure to trypsin during extraction from the cysts a reversal of IMP density to 820 ± 115 μm(2) on the P face and 1277 ± 140 μm(2) on the E face occurred. Invasion of S. muris cystozoites is followed by the enclosure of the parasite in a primary parasitophorous vacuole (PV 1), limited by a membrane which partly originates by invagination and inward expansion of the plasmalemma of the host cell. A decline of local densities of IMP on the P face of the infected host cell from 2557 ± 567 IMP/μm(2) to 417 ± 217 IMP/μm(2) in the membrane of PV 1 represents a significant decrease of protein elements. S. muris gamonts became translocated to a new vacuole (PV 2) 30 to 45 minutes after invasion. IMP reappeared on both fracture faces of the vacuole membrane. The rapid increase of IMP on the PV 2 membrane coincides with the release of large amounts of dense granule contents.