Ultrastructural study of asexual development of Cryptosporidium parvum in a human intestinal cell line.

The lack of a well-defined in vitro model of Cryptosporidium infection has severely hampered research on the biology of parasitic invasion of the host cell and on intracellular development of the parasite. In vitro infection of the differentiated human enterocyte cell line HT29.74 was studied by electron microscopy to detect changes in parasite and host cell morphology. Cryptosporidium oocysts obtained from AIDS patients were applied to a monolayer of cloned differentiated HT29.74 cells. Parasites and infected cells were evaluated by transmission electron microscopy at 20 min, 1 h, 6 h, 24 h and 7 days. Sporozoite invagination within the epithelial cell microvilli and subsequent penetration was evident at 1 h. At 6 h, the development of a dense band and feeder layer was visible. Development of the trophozoite into a schizont occurred over 24 h. Micronemes and dense granules were clearly visible within sporozoites and merozoites. Organization of vacuoles within the cytoplasm of the host cell was evident below the dense band. A sexual Cryptosporidium development in vitro was morphologically no different from initial development in vivo. In vitro infection of HT29.74 cells provides an excellent model to study parasite-host cell interaction and asexual parasite development.