Piwi positive cells that line the vasculature epithelium, underlie whole body regeneration in a basal chordate.

The colonial tunicate Botrylloides leachi can regenerate functional adults from minute vasculature fragments, in a poorly understood phenomenon termed Whole Body Regeneration (WBR). Using Piwi expression (Bl-Piwi), blood cell labeling and electron microscopy, we show that WBR develops through activation, mobilization and expansion of 'dormant' cells which normally line the internal vasculature epithelium of blood vessels. Following a mechanical insult, these cells express Bl-Piwi de novo, change morphology and invade niches of the vasculature lumen, where they proliferate and differentiate, regenerating a functional organism. Mitomycin C treatments and siRNA knockdown of Bl-Piwi result in deficient cells incapable of expanding or differentiating and to subsequent regeneration arrest. Last, we find similar transient mobilization of Piwi(+) cells recurring every week, as part of normal colony development, and also during acute environmental stress. This recurrent activation of Piwi(+) cells in response to developmental, physiological and environmental insults may have enabled the adaptation of colonial tunicates to the imposed varied conditions in the marine, shallow water environment. Copyright 2010 Elsevier Inc. All rights reserved.