Ultrastructure of Cryptosporidium parvum Found in the Small Intestine of Immunosuppressed Mice

Abstract

The ultrastructure of various stages of Cryptosporidium parvum was observed by transmission electron microscopy. C. parvum infection was activated in the small intestine of Korean laboratory mice (fCR) by immunosuppression with prednisolone for 7 weeks. The oocyst discharge was confirmed by modified Kinyon's acid fast stain of fecal specimens. Various endogenous stages of parasites, i. e., trophozoites, meronts, merozoites, and macrogametocytes, were observed in the middle part of the small intestine, as an extracytoplasmic but intracellular parasite of host mucosal epithelial cells. In trophozoites, a large nucleus with a prominent nucleolus was seen, and as they developed into meronts, endoplasmic reticulum appeared prominently in the cytoplasm. Two kinds of meronts, type I and type II, with eight and four merozoites respectively, were found. New merozoites were produced by nuclear division and external budding of the residual body of the meronts. The merozoites were lined with two unit membranes, unlike C. muris that has three membranes, and a nucleus was located near the posterior end. Mature merozoites had conoids, rhoptries and numerous micronemes; the characteristic structures of coccidian parasites. Macrogametocytes were largely vacuolated and "wall-forming body I" was recognized. Other sexual stages were difficult to recognize from our specimens. The present study confirmed that the Cryptosporidium found in the small intestine of Korean laboratory mice has a characteristic ultrastructure consistent with C. parvum.