Description, measurement, and automatic classification of the Plasmodium berghei oocyst morphology during early differentiation

After colonization of the mosquito midgut by the malaria parasite, Plasmodium differentiates from an invasive, motile ookinete to a multiplicative, sessile oocyst. Despite their importance in establishing the infection and increasing its population, the morphological transformation associated with these changes in function has been scarcely explored. Oocyst differentiation begins with the formation of a spherical protrusion near the center of the crescent-shaped ookinete. As this protuberance grows, it engulfs the content of the two distal ends, thus rounding the cell. In this work, scrutinized observations of the overall changes in shape, coupled with the migration of the malaria pigment granules and the nucleus into the protuberance, revealed that the movement of the cell content happens in an anteroposterior manner. The resulting data, formalized as morphometric measurements, led to the identification of 5 transitional stages and to the development of a computer training algorithm that automatically classifies them. Since cell differentiation has been associated with redox fluctuations, the classification algorithm was tested with parasites stained with a glutathione-specific fluorescent probe, revealing a redox modulation during differentiation.