Do Blastocystis hominis colony forms undergo programmed cell death?

Ultrastructural observations were made on colony forms of the protozoan par asite, Blastocystis hominis. Cross-sections of entire colonies were observe d by TEM. Cells within a colony were heterogeneous in morphology, consistin g of vacuolar, amoeboid, multi-vacuolar and unusual forms. Dying cells appe ared to be in the process of fragmenting into numerous membrane-bound vesic les, giving rise to empty spaces within the colony. Interestingly, older co lonies appeared to show cell fragmentation which resulted in larger, membra ne-bound structures. Numerous cytoplasmic inclusions were present in the ce ntral vacuole of some cells, with many containing mitochondria. Amoeboid fo rms were observed to harbour small membrane-bound vesicles in endosome-like compartments. Other unusual features included margination of chromatin mat erial and distinct blebbing of nuclei. These ultrastructural features sugge st that B. hominis colony forms perhaps undergo a form of programmed cell d eath.