Extracellular (axenic) development in vitro of the erythrocytic cycle of Plasmodium falciparum.

Merozoites of the erythrocytic stage of Plasmodium falciparum were suspended in erythrocyte sonicate medium with ATP and pyruvate and mixed with Matrigel to form a soft gel. The gel was overlaid with complete medium; this was replaced with fresh medium at 12, 24, and 36 hr. At these times and also at 45 hr rhodamine 123 was added to some cultures and gels were sampled. Viable extracellular forms showing rhodamine fluorescence were seen: rings at 12 hr, trophozoites and early schizonts with pigment at 36 hr, and late schizonts with developing merozoites at 45 hr. These merozoites were shown to be infective to erythrocytes added to the cultures at 45 hr. Electron micrographs of 36-hr trophozoites show the organisms to have only their single plasma membrane; no parasitophorous membrane is evident. We conclude that the complex process of entry, the intactness of the host erythrocyte, and the parasitophorous membrane are not essential to the development of a merozoite through its complete asexual cycle.