A scanning electron microscopic study of the excystment process of Acanthamoeba castellanii.

Abstract Excystment of Acanthamoeba castellanii has been examined by scanning electron microscopy with a veiw to establishing the manner in which the emerging trophozoite escapes from the cyst wall. The excystment process was initiated by placing mature cysts in the optimal growth medium used for routine culturing of the amoebae. Ostioles, identifiable by the presence of an operculum delimited by a circular ridge, were clearly evident on the surfaces of mature cysts. The first indication of emergence visible by scanning electron microscopy was the appearance of a cytoplasmic bud pushing through an ostiole from which the operculum had been removed. The emerging cytoplasmic bud did not possess the long filamentous filopodia characteristic of the trophozoite; these did not appear until the amoeba had completely excysted. The hole through which the amoeba had emerged was clearly apparent on the surfaces of the empty cyst walls. In other respects, however, the surfaces of the empty cyst walls were indistinguishable from those of mature cysts. Highly sculptured interconnecting ridges delimiting shallow craters were clearly apparent and in some cases intact ostioles were distinguishable.