Anterior Regeneration Of Schmidtea Mediterranea

Planarians can regenerate; significantly this includes the regeneration of a head with a functional brain upon decapitation. In this thesis classical cutting experiments were revisited in the planarian species S. mediterranea. A schedule of events that follow the same temporal dynamics between anterior blastemas originating from amputations from different AlP locations is uncovered. This schedule is delayed at the early stages of regeneration at more posterior levels. To investigate the molecular mechanisms underlying this delay, the roles of Hh and canonical Wnt signalling, previously implicated in planarian anteroposterior (A/P) specification, were explored. Anterior regeneration rates are dependent on canonical Wnt signalling, whilst the gradation of anterior regeneration rates over the A/P axis is dependent on Hh signalling In contrast, brain primordia differentiation occurs independently of Hedgehog (Hh) and canonical Wnt signalling and also exhibits a temporal anterior to posterior graded appearance. A combination of cell cycle inhibitor treatments, double amputation strategies and ribonucleic acid (RNA) mediated interference (RNAi) experiments indicate that these brain primordia are formed by dividing neoblasts. These cells are past S-phase of the cell cycle and are specified to form brain primordial fate in the vicinity of the anterior facing wound within the first 16 hours of regeneration, even if this region will eventually form tail structures. The data presented here is consistent with these cells being either at G2 or M of the cell cycle at the time of amputation, or even possibly post-mitotic neoblast progeny that are still able to respond to an anterior wound to become brain fated cells. In addition the blastema/post-blastema positions that brain primordia differentiate at are dependent on existing canonical Wnt signalling. In conclusion, ectopic Hh signalling results in a transition from anterior to posterior polarity, whilst ectopic canonical Wnt signalling leads to posterior polarity from the onset of anterior regeneration. Later, two-tailed Smed-ptc(RNAi) and Smed-APC-1(RNAi) animals regenerate previously unreported brain structures. Smed-ptc(RNAi) animals develop centrally positioned outgrowths with cephalic identity whilst Smed-APC-1 (RNAi) animals develop peri-pharyngeal brain primordia-like structures. In Smed-ptc(RNAi) animals ectopic Hh signalling at anterior facing wounds specify posterior organisers which instigate a ~catenin-1 activity gradient through which an outgrowth with cephalic identity is regenerated. In contrast Smed-APC- 1(RNAi) animals have globally high levels of ~catenin-1 activity which prevent an outgrowth forming. Canonical Wnt signalling is shown to instruct AlP polarity of new and existing tissue, intact Smed-hh(RNAi) and Smed-ptc(RNAi) animals do not display AlP disruption. This implicates a regeneration specific role of Hh signalling during AlP polarity establishment. These results elaborate on the current models of both AlP axis and brain regeneration and have broad implications for understanding regenerative processes and the molecular networks that control them. Here two modes of brain tissue regeneration are proposed; early brain primordia are controlled by mechanisms that are independent of canonical Wnt signalling whilst later low ~catenin-1 activity levels are required for the regeneration of elaborated brains.