| Abstract |
Many animals have the extraordinary ability to replace lost body parts, even so we humans do not. One critical but poorly understood aspect of this phenomenon is how wounds tailor the regeneration response to the particular target structure that needs to be regrown. In my thesis work I have attempted to address this problem in the champions of regeneration, the planarian flatworms. If one of these animals is cut into tiny pieces, each of the pieces will regenerate a head at the anterior end and tail at the posterior end. For over a century investigators have searched for the intrinsic polarity cue underlying this regeneration polarity, but until now its mechanistic basis is not known. The explicit goal of my thesis work was to identify this cue. The general approach that I have taken toward identification of the intrinsic polarity is to systematically compare two different planarian species with subtle variations in the establishment of regeneration polarity, Schmidtea mediterranea and Girardia tigrina. First, I demonstrate through systematic comparison of different amputation paradigms that regeneration polarity is dependent not only on species, but also on piece length, body size and anteroposterior axis position. Second, given that these findings are consistent with a gradient- based intrinsic polarity cue as prevalent hypothesis in the field, I tested whether the recently identified tail-to-head gradient of canonical Wnt (cWnt) signalling could be mechanistic basis of regeneration polarity. As precondition for doing so, I developed new approaches to measure and manipulate cWnt signalling in planaria. The data acquired with these tools suggest that the cWnt gradient may contribute to the observed position-dependence of regeneration polarity but is overall not the (only) intrinsic polarity cue. Third, I present my initial efforts to test whether the longitudinal muscle fibres (LMFs) in which notum is exclusively activated are an intrinsic polarity cue. My results suggest that “bundles” of short, intrinsically polarised LMFs running along the AP axis may express notum when they are cut anterior to their nucleus and moreover that misregulation of such a mechanism may underlie the species-dependence of regeneration polarity. Overall, the work presented in this thesis offers new insight into the cellular and conceptual basis of planarian regeneration polarity and, in doing so, the more general question of how regenerative organisms “sense” precisely what body part is missing and therefore needs to be regrown. Furthermore, it puts forward new hypotheses that through additional experimentation may explain lead to elucidation of the underlying molecular mechanisms. |