Transmission electron microscopy of sporozoites adhered to MDBK cells. Cryptosporidium parvum sporozoites were incubated with MDBK cells. A. Ten-minute incubation (arrows denote bound sporozoites), ×13,200. B. Thirty-minute incubation. Note firm adhesion with host cell and beginning development of an electron-dense layer (arrow), ×32,000. C. Sixty-minute incubation. Note enlarged anterior invagination (long arrow) and beginning development of an electron-dense collar (short arrows). Also note extension of host plasma membrane surrounding bound sporozoite (arrowhead), ×100,000.