Sir

In your timely, engaging and well-illustrated News Feature “The regeneration gap” (Nature 414, 388–390; 2001), you report on the often-neglected invertebrate models of regeneration, such as planarians and hydra, and call for more attention to be paid to these invertebrates as models in the new field of regenerative medicine.

However, when referring to hydra and planarians, you describe stem cells as 'dormant' or 'mobilized when needed'. Actually, the reverse is true. These cells, called neoblasts in planarians, are continuously mobilized in young and adult worms (see J. Baguñà, E. Saló & C. Auladell Development 107, 77–86; 1989). Many planarians do not regenerate or do so poorly while bearing neoblasts with normal rates of cell renewal. Therefore, neoblasts are not 'dormant' cells for regeneration, but function to replace differentiated cells lost during daily wear and tear.

This process of renewal speeds up during regeneration, while the rates to different cell lineages change to adapt to the axial pattern remodelling of the organism. Thus, terms such as 'dormant', 'reserve', or 'mobilized when needed' should be avoided when referring to stem cells in general and to planarian neoblasts in particular.