Rhoptry secretion of membranous whorls by Plasmodium falciparum merozoites.

Multilamellar membranous whorls were localized, by electron microscopy, in elements of the rhoptry-microneme complex from glutaraldehyde-tannic acid (TA)-fixed merozoites of the human malarial parasite, Plasmodium falciparum. These multilaminate structures, which have a dark line-to-dark line periodicity of approximately 5 nm, were also found in the nuclear envelope and closely apposed to the external surface of merozoites. Segmented schizonts, which contain intracellular merozoites, often showed membranous whorls within their parasitophorous vacuoles and closely apposed to the external surface of the parasitophorous vacuole membrane. Whorls were not found in trophozoites, immature schizonts, and uninfected erythrocytes. Most rhoptries in merozoites fixed in glutaraldehyde-TA were electron-lucent whereas rhoptries fixed in glutaraldehyde alone were electron-dense. Some merozoites fixed in glutaraldehyde-TA had both an electron-dense and an electron-lucent rhoptry. These findings suggest that TA induces the premature extrusion of rhoptry materials. Our findings support previous suggestions in the literature that phospholipid materials secreted from merozoite rhoptries are involved in merozoite interaction with host erythrocytes.