Examples of regeneration have been documented throughout the animal kingdom, yet there is considerable variation in regenerative capacity from one species to the next (Gurley and Sánchez Alvarado, 2008
). Human organs also differ markedly in their ability to repair damage caused by physical injury or disease. Thus, one important challenge in the field of regenerative medicine is to define the cellular mechanisms of naturally occurring regenerative phenomena, with the ultimate goal of developing treatments that stimulate these mechanisms in a clinical setting.
Previous mechanistic studies of metazoan regeneration have generally focused on the question of how new tissue is created in pathological contexts. Although proliferation of differentiated cells may be sufficient in some cases (Dor et al., 2004
), the recruitment of stem or progenitor cell populations has emerged as a central and evolutionarily conserved regeneration paradigm (Gurley and Sánchez Alvarado, 2008
). Adult stem cells may be formed de novo
in response to injury via dedifferentiation, or they may be maintained under physiological conditions for tissue homeostasis (Gurley and Sánchez Alvarado, 2008
). In either case, they serve a critical purpose in regeneration by fueling the production of various differentiated cell types that are necessary for restoring a functional anatomy.
The freshwater planarian S. mediterranea
has emerged as a powerful model organism for studies of stem cell function in tissue renewal and repair. Planarians turn over most if not all of their somatic tissues throughout life (Pellettieri and Sánchez Alvarado, 2007
) and have the remarkable capacity to regenerate complete individuals from small body fragments (Reddien and Sánchez Alvarado, 2004
). Each of these attributes depends on a large population of adult stem cells termed neoblasts. In response to amputation, neoblasts increase their rate of division (Baguñà, 1976b
; Saló and Baguñà, 1984
) and migrate to the wound site (Dubois, 1949
; Eisenhoffer et al., 2008
; Newmark and Sánchez Alvarado, 2000
), where they give rise to a mass of new tissue called the blastema. Cells in the blastema then differentiate over a period of several days to replace missing body structures (Reddien and Sánchez Alvarado, 2004
An equally important, but much less well understood aspect of regeneration involves the remodeling of preexisting tissues. First described by T.H. Morgan in 1901
and termed ‘morphallaxis’ (Morgan, 1901
), this phenomenon restores anatomical scale and proportion and allows for the integration of new and old tissues (Reddien and Sánchez Alvarado, 2004
). In planarians, a similar remodeling process also occurs when animals are starved for a period of several months, resulting in ‘degrowth’, or an up to ~20-fold reduction in overall size (Baguñà and Romero, 1981
; Reddien and Sánchez Alvarado, 2004
; Romero and Baguñà, 1991
). In contrast to the relatively well-characterized neoblast proliferation response that underlies blastema formation, the cellular mechanisms responsible for remodeling preexisting tissues have remained largely unknown.
Here we report the development of a TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) assay allowing the visualization and quantification of apoptotic cells in whole-mounted planarians. Using this assay, we show that dynamic spatial and temporal changes in apoptosis are key features of tissue remodeling after injury and in response to prolonged starvation. We also show that planarian cell death is regulated by a homolog of BCL2
, an evolutionarily conserved antiapoptotic gene that inhibits caspase activation and physical dismantling of dying cells in animals ranging from C. elegans
to humans (Cotter, 2009
; Danial and Korsmeyer, 2004
). Our findings argue that the deletion of differentiated cells through apoptosis functions in concert with stem cell division to regulate the scale and proportion of adult tissues during regeneration.