The ultrastructure of the cyst wall of a murine Sarcocystis.

Electron microscope studies of the cyst wall of a murine Sarcocystis-like organism of Mus musculus indicate that the sarcocyst wall is a degenerate muscle cell. The degenerate muscle cell contains nuclei, mitochondria, ribosomes, and myofibrils in disarray with the A-bands thickened. Surrounding the outside of the muscle cell there is a layer of fibrous material, and occasional fibroblasts are seen. The parasites are found within a parasitophorous vacuole in the muscle cell. There are short-finger-like projections extending into the vacuole from the muscle cell. The ground substance of the vacuole is fairly homogeneous, but somewhat more electron dense between the finger-like projections.