Ultrastructural Comparison of Cysts and Zoites of Toxoplasma gondii, Sarcocystis muris, and Hammondia hammondi in Skeletal Muscle of Mice

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Abstract
Cysts of H. hammondi and T. gondii in skeletal muscle of mice were compared at the ultrastructural level with cysts of S. muris. Cysts of H. hammondi and T. gondii were similar, except for the size of the cystozoite, which was about 4-5 .mu.m in H. hammondi, but about 7-8 .mu.m in T. gondii. Both cysts were limited by a relatively smooth, but intensely folded, primary cyst wall, under which a zone of granular ground substance was always present. No secondary cyst walls were observed around the parasitized fiber. Both cysts differed from Sarcocystis cysts, which possessed a thick primary cyst wall, septa and metrocytes that persisted even in old cysts. Septa were not found in cysts of H. hammondi or T. gondii, probably because of the uniformly close packing of the parasites inside the cysts and the absence of metrocytes. Toxoplasma and Hammondia can be distinguished from Sarcocystis on a morphological basis, but there are no significant morphologic differences between the cysts of Hammondia and Toxoplasma. The generic distinction is based on the distribution of zoites and cysts in organs in intermediate and definitive hosts, and by the distinctive cycles.

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