The encysted form of the fertilized macrogamete, or zygote, in coccidian Sporozoea in which sporogonic multiplication occurs; results in the formation of sporozoites, infectious agents for the next stage of the sporozoan life cycle. [G. oon, egg, + kystis, bladder]
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n.
a spherical structure, 50-60 µm in diameter, that develops from the zygote (see ookinete) of the malarial parasite (Plasmodium) on the outer wall of the mosquito's stomach. The oocyst steadily grows in size and its contents divide repeatedly to form sporozoite, which are released into the body cavity of the mosquito when the oocyst bursts.
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oo·cyst (oґo-sist) [oo- + cyst] the encysted or encapsulated zygote in the life cycle of sporozoan protozoa, which by the process of sporogony develops into a sporozoite or a sporocyst containing sporozoites.Medical dictionary. 2011.