Abstract
Planarians have gained a well-deserved reputation as an excellent model organism for research on the biology of adult stem cells and their role in regeneration. Though less widely recognized, these animals also offer many advantages for investigating mechanisms and functions of programmed cell death in self-renewing tissues. Apoptosis complements stem cell division during physiological cell turnover and constitutes a prominent feature of the tissue remodeling process that restores anatomical scale and proportion during regeneration. One technical advantage to studying apoptosis in planarians is the availability of a whole-mount TUNEL assay for visualizing dying cells throughout the animal. Here, we provide a detailed protocol for this assay that is likely to benefit researchers investigating planarian cell death in either physiological or pathological contexts.