ULTRASTRUCTURE OF BLASTOCYSTIS-HOMINIS CYSTS

Blastocystis hominis cysts were concentrated from human faeces by repe ated washing in distilled water and centrifugation on Ficoll-Paque. Th is procedure gave a concentrated suspension of cysts without yielding any non-cystic forms. The cysts were examined by both transmission (TE M) and scanning electron microscopy (SEM). Cysts were surrounded by a fibrillar layer that appeared porous in SEM. Many naked cysts without the fibrillar layer were also visible in SEM. They were variable in sh ape but mostly circular to oval. The diameter of the cyst without the fibrillar layer was 3.5 mu m in both TEM and SEM. The nuclear structur e was typical of Blastocystis and exhibited multiple mitochondria with poorly developed cristae. Glycogen was present in small to large clum ps in the cytoplasm. The cyst wall was 5-100 nm thick and was bounded internally by an electron-dense plasmalemma.