Morphological changes of malaria parasites during transmission from the human host to the mosquito vector. The intraerythrocytic gametocyte stages mature in the human host and are taken up by the blood-feeding female mosquito. By entering the mosquito midgut, the gametocytes become activated and round up, before emerging from the enveloping host erythrocyte. Proteases (P) are involved in these processes. During gametogenesis, the female macrogametocyte transforms into a macrogamete, while the activated male microgametocyte forms eight microgametes. Within approximately twenty-minute post-activation, the motile microgamete fertilizes the macrogamete and the resulting zygote transforms within a day into the infective ookinete. Two classes of sexual stage proteins are expressed in association with the parasite surface. A first class of proteins (shown in green) is expressed in the parasitophorous vacuole of the developing gametocyte, where some of them assemble to form adhesive multiprotein complexes. The proteins are later exposed on the surface of the newly emerged gametes, but expression ceases during fertilization. Expression of a second class of surface-associated proteins (shown in pink) is repressed in the gametocyte stage, but repression is released during fertilization (R) and protein expression persists to the ookinete stage.
