The stem cell system of the basal flatworm Macrostomum lignano

The scope of this review is to introduce the free-living flatworm Macrostomum lignano as an excellent model organism to address questions of platyhelminth stem cell biology. First, we sketch the historical origin of flatworm stem cell research. Second, we introduce M. lignano, and summarize the main advantages that we think it has over the classical planarian model. Third, we give a short summary of the simple culture techniques. Fourth, we give a detailed overview over its morphology and embryology as far as it is relevant for stem cell biology. Fifth, we summarize our main findings on stem cell biology, with respect to the identification of neoblasts, their distribution and number. We describe the ultrastructure of neoblasts, their dynamics and gene expression. Sixth, we outline ways to study sex allocation by means of stem cell labeling and manipulation. Last, we highlight the regeneration capacity of this species and link it to the stem cell system. We conclude that M. lignano is a highly suitable model organism to gain knowledge about flatworm stem cells and to provide insight into stem cell systems of higher organisms, including humans.