Freeze-fracture study of the Gregarine trophozoite: I the top of the epicyte folds

The fine organization of the "membrane complex" present at the tip of the epicyte folds in the sporozoan Gregarina trophozoite has been studied. Electron microscopy revealed bundles of intermediate filaments (8-10 nm) in the interspace between plasma and middle membrane. Doublets of filaments are present in the cytoplasm adhering to the inner membrane. The narrow space between the inner and middle membrane exhibits electron dense dots arranged in pairs. Freeze-fracture replicas reveal a different organization in each of the three membranes. The plasma membrane shows the P-face rich in particles and E-face with a few clusters of intramembrane particles (IMPs). Parallel ridges are present on the E-face and parallel grooves on the P-face of the intermediate membrane. The P-face of the inner membrane shows double longitudinal arrays of IMPs and the E-face has large grooves alternating with rows of ridges. The rows of particles on the P-face of the inner membrane are in register with the clusters of filaments seen in the cytoplasm and adhering to the inner membrane. Our observations suggest connections among the three membranes at the tip of the epicyte folds; moreover, an interaction occurs between the intermediate filaments and intramembrane complexes. The arrangement of the membranes is discussed in reference to the gliding of gregarines.