[Giardia intestinalis: transmission electron microscopy study of in vitro excystation]

The in vitro excystation process in Giardia intestinalis was studied by transmission electron microscopy (TEM). Untreated cysts served as controls. The excystation process was monitored by examination of organisms after the in vitro induction and at several times during the incubation phase. The control cyts had a thick wall, made of microfibrils, that appeared not to contain any weak areas. The peritrophic space extended between the cyst wall and the organism peripherally, the space was delimited by a thin cytoplasmic layer, "the outer cytoplasmic envelope" that subtended the cyst wall. During the in vitro incubation, the trophozoite cytoplasm retracted from the wall; thus, the peritrophic space became progressively larger. The outer cytoplasmic envelope detached from the cyst wall, then broke up forming numerous small vesicles lodged between the wall and the organism. The tight arrangement of the wall microfibirils was lost. Electron-dense vacuoles appeared in the peripheral cytoplasm of the trophozoite. The organism emerged through the posterior end of the cyst, leaving behind the empty husk. Emergence was followed by cell division. The possible interrelationships of biochemical and mechanical factors affecting the process of excystation are discussed in light of the present TEM findings.