detected not only in neoblasts but also in recent neoblast
progeny that are committed to differentiate [4,10].
Furthermore, the RNAi phenotype of at least one
planarian piwi homolog indicates that neoblasts can
persist and proliferate for many days following RNAi
treatment, and that during this time their progeny can
still migrate to wound sites and become incorporated
into tissues, but they fail to differentiate properly [9].
Therefore, the primary function of piwi in these animals
may not be in stem-cell self-renewal but rather in
potentiating the proper differentiation of neoblast
progeny. The neoblast depletion RNAi phenotype
observed might thus be due not to a specific failure of
neoblast self-renewal but rather to exhaustion of the
neoblast pool, resulting from an exceedingly high
demand for differentiated cells as normal homeostasis
fails.