Emili Saló and Kiyokazu Agata
Barcelona and Kyoto
ABSTRACT The term planarian is derived from the Latin planus (meaning flat) and refers to worms of the order Tricladida (phylum Platyhelminthes) whose body is flattened along the dorso-ventral axis. Planarians are best known for their ability to regenerate complete animals from tiny fragments of tissue, and for their capacity to continually remodel both the size and form of their body. Both features are dependent on the presence of pluripotent adult stem cells called “neoblasts”. This complex cell population is able to proliferate and give rise to all differentiated cell types. Furthermore, although the mechanisms remain poorly understood, it confers adult planarians with a degree of plasticity similar to that of embryos. Alongside these fascinating properties, freshwater planarians are easy to culture and handle in the laboratory. They are therefore an ideal bilaterian model system in which to study the molecular and cellular mechanisms that control stem-cell proliferation and differentiation, pattern formation, organogenesis, growth, regeneration, senescence, and even rejuvenation.