ULTRASTRUCTURAL-CHANGES DURING IN-VITRO ENCYSTMENT OF BLASTOCYSTIS-HOMINIS

The morphological changes occurring in Blastocystis hominis at differe nt time points following in vitro encystment were studied by electron microscopy. The following stages of the parasite were sequentially see n: (a) the amoebic form, which was irregular in shape, with a majority of the organelles being concentrated at the condensed cytoplasmic reg ion; (b) the pre-cystic form, which was rounded and had an electron-de nse material forming a homogeneous wall around the central body; and ( c) the cystic form, which had a very prominent, thick osmiophilic elec tron-dense wall, within which there were many inclusions and possibly reproductive granules. The amoebic form appeared to be an intermediate stage between the vacuolar form and the pre-cystic form, as this stag e allowed the parasite to ingest bacteria to enhance encystment. The p re-cystic stage had previously been shown in experimental infection to be infective. The role of the cystic stage in producing infection is currently being investigated.