Initial Extracellular Development in Vitro of Merozoites of Plasmodium falciparum1

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Abstract
Late schizonts from continuous cultures of P. falciparum were concentrated over Percoll, inoculated to various experimental media at the rate of .apprx. 20 .times. 106/0.5 ml of medium, and incubated in a candle jar at 37.degree. for 1 day. Controls in standard culture medium showed a heavy invasion with young rings in the previously uninfected red cells introduced with the inoculum of schizonts. In a medium of high K content containing a 33% extract of human erythrocytes, this invasion was inhibited and many free merozoites were present. If this same medium was supplemented with both ATP, as the dipotassium salt at 1.6 mM, and sodium pyruvate at 3.6 mM, there appeared large numbers of extracellular forms resembling young rings. Examination of these by EM shows that they are indeed merozoites that have begun to differentiate extracellularly. The trigger for differentiation of merozoites may not depend on the process of entry into a red cell but rather on specific factors within the red cell.