Fine Structure of the Malaria Parasite Plasmodium lophurae Developing Extracellularly in vitro*

SYNOPSIS Duck malaria parasites ( Plasmodium lophurae ), synchronized at the uninucleate trophozoite stage, were freed from their host erythrocytes by immune lysis and cultured extracellularly in duck erythrocyte extract medium. At 0 time, 1, 2, and 3 days, samples were taken for light and electron microscopy and for measurement of incorporation of [ 14 C]‐methionine or [ 14 C]‐proline. For 2 days the parasites developed fairly normally, progressing from large trophozoites‐early schizonts at 1 day to segmenters‐forming merozoites at 2 days. However, the 3‐day samples showed signs of deterioration: incorporation of amino acids dropped; the percentage degenerate cells rose; the progression of developmental stages slowed. At the fine structure level 2 abnormalities were observed which may indicate the limits of extracellular cultivation in vitro. Through 2 days of culture all parasites were surrounded by 2 membranes. The 3‐day samples contained some organisms with only one membrane, which may have arisen from merozoites produced extracellularly. The 2nd alteration was in the food vacuoles, which were progressively fewer, smaller, and less dense in the cultured samples and may indicate an abnormality in the extracellular parasite's feeding mechanism.