Epidermal turnover in the planarian Schmidtea mediterranea involves basal cell extrusion and intestinal digestion

Planarian flatworms undergo continuous internal turnover, wherein old cells are replaced by the division progeny of adult pluripotent stem cells (neoblasts). How cell turnover is carried out at the organismal level remains an intriguing question in planarians and other systems. While previous studies have predominantly focused on neoblast proliferation, little is known about the processes that mediate cell loss during tissue homeostasis. Here, we use the planarian epidermis as a model to study the mechanisms of cell removal. We established a covalent dye-labeling assay and image analysis pipeline to quantify the cell turnover rate in the planarian epidermis. Our findings indicate that the ventral epidermis is highly dynamic and epidermal cells undergo internalization via basal extrusion, followed by a relocation toward the intestine and ultimately digestion by intestinal phagocytes. Overall, our study reveals a complex homeostatic process of cell clearance that may generally allow planarians to catabolize their own cells. [Display omitted] •Planarian epidermal turnover rates differ between dorsal and ventral•The ventral epidermis is a highly dynamic tissue with a half-life of 4.5 days•Epidermal cells are eliminated via basal extrusion, not outward shedding•Internalized epidermal cells are taken up and digested by gut phagocytes Although planarians are known to continuously renew all cell types, how they remove old cells is poorly understood. By developing imaging assays to quantify and observe cell turnover in the epidermis, the authors find that epidermal cells undergo basal extrusion and that the internalized cells are digested by intestinal phagocytes.