Secondary and tertiary progenitor cell lines of Entamoeba invadens (s-SRL and t-SRL lines) are unipotent. In controlled OCB cultures both cell lines proliferate asymmetrically giving rise to a minor cell fraction of MAS cells and a dominant cell fraction of MAT cells. While MAT cells re-enter mitotic cycle MAS cells switch into a developmental endopolyploid cell cycle (encystment/excystment cycle) that includes genome amplification, nuclear reprogramming and totipotency recovery. Each unipotent MAS cell gives rise to eight totipotent amoebulae that are the cells with the most differentiating potential. MAS cells form in cultures ATD cysts also observed in polyxenic isolates of E. histolytica and improperly called “spontaneous cysts”. In contrast, axenically grown E. histolytica proliferates by symmetric cell division and complete self-renewal; it gives rise to identical daughter cells. None of these cells enter the developmental endopolyploid cell cycle. Axenic stress hinders “spontaneous” encystment. However, we can distinguish in axenic cultures between minor and dominant subpopulations. While the dominant subpopulation cycled normally, cells belonging to the minor subpopulation undergo defective mitosis. They lose duplication fidelity and form multipolar spindles. As a result, the minor cell fraction becomes polyploid and multi-nucleated. Endoreplication by defective mitosis is reversible and has nothing to do with the process of encystment.
Vladimir F. Niculescu
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