Results
Morphology, distribution, and differentiation of stem cells in Isodiametra pulchra
In order to describe the stem cell system of acoels, we first addressed the morphology of Isodiametra pulchra (Figures. 1A, B) neoblasts. They are small in size and possess a high nuclear to cytoplasmic ratio with only a thin rim of cytoplasm (Figures. 1C, D). The nucleus consists of mostly uncondensed chromatin with few smaller clumps of condensed chromatin (Figure. 1C). When animals were macerated into a single cell suspension after a 30 min BrdU pulse, only cells with a neoblast morphology were labelled (n = 198) (Figure. 1D). On ultrastructural level, all cells that incorporated BrdU were small in size and possessed a thin rim of cytoplasm (Figures. 1E). These data suggest that neoblasts were the only dividing cells.