Penetration of erythrocytes by merozoites of mammalian and avian malarial parasites

Entry of merozoites of Plasmodium berghei yoeli and of P. gallinaceum into erythrocytes has been elucidated by electron microscopy. Merozoites approac h host cells with the conoid leading the way. At the point of contact betwe en the anterior pole of the parasite and the host cell a focal depression o f the red cell membrane forms, which deepens as the merozoite advances. The continuity of the host cell membrane is not disrupted. With deeper invagin ation of the red cell membrane the resulting cavity conforms to the shape o f the merozoite. The site of initial parasite contact forms a relatively co nstricted orifice through which the posterior portion of the parasite passe s. Later the edges of this orifice fuse and the parasite now lies in a vacu ole inside the host cell. At this stage the merozoite undergoes a transform ation and dedifferentiation. The pellicle loses the characteristic thick in ner membrane. The parasite becomes rounded and the conoid and paired organe lles are no longer discernible, Trophozoite development is initiated. This sequence of events unequivocally establishes the origin of the parasite mem brane envelope and the origin of the space separating the two membranes. Th e outer membrane is derived from the plasmalemma of the host red cell; the inner is the covering of the parasite.