Cell differentiation in Entamoeba and Giardia exhibit a great deal of similarity. Unfortunately, both protists were studied over the years mostly under axenic conditions, disregarding the natural micro-ecology of host intestine. Using hypoxic culture sediments with oxygen consuming bacteria (OCB niches) we discovered in E. invadens an ancient stem cell system that shows the same basal mechanisms for stemness that exist in highly developed stem cell systems too. The ancient stem cells of E. invadens (AnSC) have all standards of the stem cell definition (self renewing and quiescent cells, glycolytic metabolism, stem cell plasticity, reprogramming and induced totipotency recovery, and both induced and autonomous terminal differentiation) indicating that stem cells were developed by the anaerobe proterozoic LECA-as stages of its life cycle-then diverged into all eukaryotes. The more protist stem cell systems we can dechipher the more we will learn about the ancestral protolineage, its cell line hierarchy and basal mechanism of stemness. We assume that Giardia could be a good model system to expand protist stem cell biology. We found in Giardia literature evidence suggesting self renewal and quiescence, plasticity and other stem cell characteristics. The present study is a reinterpretation of Giardia’s life cycle in the light of current stem cell knowledge. It is hoped this reinterpretation stimulates new developments.
Keywords: LECA, Entamoeba, Giardia, life-cycle, stem cells, stem cell lines, induced and autonomous mechanisms for terminal differentiation