Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2000
Publication Date: N/A
Interpretive Summary: Sarcocystis neurona is a single-celled parasite of companion animals. It causes a fatal neurologic syndrome in horses in the U.S., called eqine protozoal myeloencephalitis (EPM). Its full life cycle is not known. Opossums are its reservoir (definitive) host and horses are considered aberrant hosts. Scientists at the Beltsville Agricultural Research Center and the Montana State University describe the ultrastructure of Sarcocysti neurona stages (schizonts) found in tissues of horses. These results will be of interest to biologists and parasitologists.
The ultrastructure events associated with nuclear division were studied in schizonts of Sarcocystis neurona. Merozoites developed by type 2 endopolygeny in which numerous merozoites began development internally and later budded at the schizont surface. The nuclear envelope remained intact during all nuclear divisions. The schizont nucleus enlarged to form numerous lobes, each of which contained an intranuclear spindle. Many spindles were formed before merozoite formation. Spindles were conical- shaped with the apex of the spindle located immediately beneath a pore in the nuclear envelope. Most spindle microtubles were discontinous, terminating at kinetochores on chromosomes, whereas a few microtubules were continuous extending between each pole of the nuclear spindle. Two centrioles, a Golgi complex and a plastid were closely associated with each pole of the nuclear spindle. Spindle microtubules shortened during spindle edivisions that occurred in early and intermediate schizonts prior to the formation of merozoite primoridia. The length of the spindle microtubules remained the same during the last spindle during merozoite formation. Separation of the chromosomes appeared to result from enlongation of each nuclear lobe which occurred simultaneously with elongation of the inner membrane complex and subpellicular microtubules surrounding the merozoite primordia. In schizonts, the plastid was disc-shaped, variable in size, and consisted of a granular core with ribosomes and small vesicles surrounded by 2-4 membranes. Plastids in merozoites were structurally similar to those in schizonts except that some of them were spheroidial in shape.