During tissue maintenance and regeneration, stem cells (SCs) are mobilised and migrate towards sites of tissue turnover or repair. However, owing to the inaccessible nature of most in vivo SC populations, very little is known about the molecular factors that regulate SC mobilisation. Otto Guedelhoefer and Alejandro Sánchez Alvarado tackle this problem by studying SC mobilisation during tissue homeostasis and regeneration in the planarian flatworm Schmidtea mediterranea (p. 3510). They show that planarian SCs migrate minimally in the absence of wounding; in partially irradiated animals, SCs do not migrate from the areas that were shielded from radiation to populate irradiated areas. Importantly, the researchers report, amputation increases SC dispersal and induces directional recruitment of SCs to sites of tissue repair, suggesting that factors capable of directing SCs are activated upon amputation. These findings highlight that, in planaria, SC depletion alone is not sufficient to trigger SC migration and that loss of tissue integrity is required to promote the directional migration of SCs.

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