Ultrastructure of schizonts and merozoites of Sarcocystis neurona

The ultrastructure of Sarcocystis neurona schizonts and merozoites was studied in specimens derived from cell culture and from the brains of infected mice. Schizonts and merozoites were located in the host cell cytoplasm without a parasitophorous vacuole at any stage of development. Merozoites divided by endopolygeny. Fully formed merozoites had a pellicle, numerous polysomes and ribosomes, smooth and rough endoplasmic reticulum, 22 subpellicular microtubules, 9-16 dense granules, 25-75 micronemes, a plastid, a Golgi complex, 1-3 mitochondria, a conoid, 2 apical rings, 2 polar rings, 0-6 lipid bodies, a nucleus and nucleolus, but no rhoptries. Most micronemes were located anterior to the nucleus including 1-6 micronemes in the conoid. Merozoites were either slender (7.3 microm x 1.7 microm) or stumpy (7.7 microm x 3.1 microm). Dense granules appeared to arise from the maturation face of the Golgi complex. The ultrastructure of in vitro derived schizonts and merozoites were similar to in vivo derived organisms.