Ultrastructure of Cryptosporidium muris (strain RN 66) parasitizing the murine stomach.

The ultrastructure of Cryptosporidium muris, which parasitizes the stomach of mice, was studied by transmission electron microscopy. The entire development of the parasite occurred in the microvilli of the surface mucus cells in the gastric glands. The ultrastructural features of the attachment site of C. muris to the host cell differed remarkably from those of C. parvum and its closely related species, which parasitize the intestine of various animals. The size of C. muris was greater at almost every developmental stage than that of C. parvum. These findings confirmed that C. muris and C. parvum are distinct species. The mitochondria, subpellicular microtubules, and Golgi complex were demonstrated in detail. A small invagination in the meront and intravacuolar tubules were found in Cryptosporidium. The wall of each developing oocyst in the parasitophorous vacuole was composed of three layers: the outermost layer was considered to be a true oocyst wall, whereas the middle and innermost layers were assumed to develop into the sporocyst wall. The outermost layer was fragile and disintegrated as the oocyst matured. In excystation in vitro, a suture was seen in a thick layer of the two-layered sporocyst wall of an oocyst (sporocyst wall; see Discussion) that enveloped four sporozoites. The fine structure of the attachment site of the present species to the host cell appears to reveal a unique mode of host-parasite interaction in Cryptosporidium infection.