Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 1998
Publication Date: N/A
Interpretive Summary: Infections by the protozoan parasite Toxoplasma are widely prevalent in livestock and animals. Humans become infected by ingesting T. gondii from uncooked meat or by ingesting food and water contaminated with resistant stages (oocysts) excreted in the feces of infected cats. Comparatively little is known about the structure of oocysts compared with other stages of the parasite. Scientists at the Beltsville Agricultural Research Cente and the Montana State University describe in detail the structure of T. gondii oocysts and their contents. These results will be useful in understanding the life cycle of Toxoplasma and will be of interest to biologists and parasitologists.
Transmission and scanning electron microscopy were used to study the ultrastructure of the oocysts, sporocysts, and sporozoites of the VEG strain of Toxoplasma gondii and to compare the ultrastructure of sporozoites with tachyzoites (from the peritoneum of mice) and bradyzoites (from brain tissue cysts in mice). Oocysts were surrounded by a thin veil of finely reticulate material. The oocyst wall consisted of 3 layers and contained a previously unknown disc-shaped micropyle that appeared as a depression in the oocyst wall. The sporocyst contained four sporozoites and a residuum of lipid and amylopectin granules. The sporocyst wall was three-layered with the inner most layer consisting of 4 curved plates held together at sutures by an interposed strip. Exposure to excysting fluid caused the interposed strip to separate from the curved plates, which curled inward releasing the sporozoites. Sporozoites had a posteriorly-located nucleus and all the organelles typical for coccidian zoites. Sporozoites, tachyzoites and bradyzoites had similar numbers of rhoptries, but differed in the numbers and sizes of micronemes, dense granules, amylopectin granules and lipid bodies.