Initial extracellular forms of Plasmodium falciparum: their ultrastructure and their definition with monoclonal antibodies.

Merozoites of the erythrocytic stage of the human malaria parasite Plasmodium falciparum, when placed under appropriate conditions in a culture medium with erythrocyte extract, differentiate into early trophic forms. These forms have much the same ultrastructure as rings of the same age that have developed intracellularly and have then been freed from their host cells by immune lysis. However, these forms differ in two respects: the extracellular forms have only their single plasma membrane, whereas the forms freed from host cells have, in addition, a surrounding parasitophorous vacuole membrane; the forms that develop extracellularly have fewer ribosomes. Five monoclonal antibodies against the ring stage have been prepared and characterized. Their pattern of immunofluorescence localization differs in merozoites as compared with rings, but their pattern is identical in rings developed extracellularly and those developed intracellularly. These results and the observations on fine structure demonstrate biochemical and morphological differentiation in the extracellular forms.