Ultrastructure of Excystment of Toxoplasma gondii Oocysts*

The excystation of sporozoites from intact Toxoplasma gondii oocysts or mechanically released sporocysts was studied by light and electron microscopy. Both intact oocysts and free sporocysts excysted in 5% bovine bile in 0.9% NaCl solution after 30-60 min incubation at 37C. Sporozoites were first activated in either intact sporocysts or oocysts within 2-12 min of incubation in bile. Sporozoites escaped from sporocysts through 4 plate-like sutures in the sporocyst wall, and from the oocyst as the oocyst wall ruptured at one or more points.