merozoite

Merozoite is the motile, asexual life stage of certain apicomplexan parasites, most notably Plasmodium species, that initiates the next round of red blood cell infection. Merozoites form by schizogony within schizonts, and they are released into the bloodstream where they rapidly invade erythrocytes. In malaria, merozoites originate from both hepatic schizonts (liver stage) and intraerythrocytic schizonts, with liver-derived merozoites entering the blood to invade red cells. After invasion, merozoites develop into ring-stage trophozoites and mature through further cycles to eventually produce new merozoites.

Morphology and invasion machinery: Merozoites are elongated, densely packed with organelles suited for host cell entry, including an apical complex containing rhoptries, micronemes, and dense granules. The parasite’s invasion relies on surface ligands such as MSP1 and other MSP family proteins, as well as EBA and PfRH ligands, which interact with receptors on red blood cells (for example glycophorin A and other RBC surface proteins). The final steps involve reorientation, formation of a moving junction through AMA1 and rhoptry neck proteins, and a parasite actin-myosin motor that drives entry into the host cell.

Lifecycle impact and significance: Merozoite invasion and subsequent rupture of infected erythrocytes produce the clinical parasitemia seen in malaria. The process is a major focus of vaccine and drug development, as blocking merozoite attachment or invasion could interrupt disease transmission. Similar merozoite stages are found in other apicomplexans, such as Babesia, where merozoites invade red blood cells by analogous mechanisms.