An ultrastructural study of first- and second-generation merogony in the coccidian Sarcocystis tenella.

Sporocysts of the coccidian Sarcocystis tennella were originally isolated in the feces of a coyote. Sporocysts used for inoculation of lambs were obtained from experimentally infected dogs. At 14, 16, and 19 days postinoculation (DPI) of lambs with the sporocysts, various developmental stages of first-generation meronts were found within cells located between the endothelium and internal elastic membrane of mesenteric arteries. At 19, 21, and 25 DPI, second-generation merogony occurred in cells associated with capillaries and arterioles of kidney glomeruli and convoluted tubules. Meronts of both generations were bounded by a double pellicular membrane and were situated free in the host cell cytoplasm. Merozoites formed by endopolygeny that involved multiple intranuclear spindles of a single, large irregular nucleus. First-generation meronts measured 22.6 x 17.1 micrometers (19-29.7 x 7.5-24 micrometers) and contained 120-240 merozoites, which measured 7.1 x 1.6 micrometers (4.8-7.5 x 1.3-1.8 micrometers). Corresponding values for second-generation meronts were 13.2 x 9.2 micrometers (8.3-15 x 7-13.5 micrometers), 32-80, and 5.8 1.7 micrometers (5.6-6.2 x 1.4-2.2 micrometers).