|Ph.D Student||Livshits Anton|
|Subject||Structural Organization and Body-Axis Polarity in|
Regeneration of Fused Hydra Tissues
|Department||Department of Physics||Supervisors||PROF. Erez Braun|
|PROF. Kinneret Keren|
|Full Thesis text|
Morphogenesis is a dynamical process leading to the patterning of functional tissues during development, resulting in a stereotypical body plan of a viable animal. One of the major events in this patterning process, is the emergence of a body axis, defining a polarity of the body plan. This thesis focuses on polarity organization during Hydra regeneration. In particular, we develop a methodology to study the plasticity of axis polarization along the regeneration of Hydra by fusing excised tissue rings. The main idea is to utilize the flexibility of Hydra as a model system, to define a wide range of frustrating initial conditions, as a mean for exposing the dynamics underlying the formation of a polar body axis in regenerated Hydra.
It is known (and shown again here) that tissue segments excised from the gastric region of an adult Hydra preserve the original polarity direction of the parent animal. The origin and stability of this memory remain largely elusive. To study this issue, we combined classic grafting techniques with modern microscopy tools to generate composite tissues made by fusion of two rings excised from different adult animals and follow their regeneration dynamics. Ring doublets tissues are combined in a variety of configurations which span the various possibilities in terms of the polarity of the original ring as well as their position along the axis of the parent animal. We follow the regeneration of these tissue composites using live time-lapse microscopy. Frustrating conditions emerge in the initial configuration of fused ring doublets with regards to two aspects; first, when the two tissue rings are fused with opposite polarities, biochemical and structural constraints due to the presence of the second adhered tissue may arise. Second, tissue segments excised from different places along the axis of the donor animal, exhibit position-dependent regeneration kinetics and head-formation potential and thus compete on resources.
We use various combinations of fused ring doublets that differ in the relative polarity orientations of the two rings and in their original positions along the donor’s axis. These experiments reveal that under certain frustrating conditions, the original polarity that is otherwise stably inherited from the parent animal, may become labile and can be even reversed under certain conditions. Polarity reversal, with a head regenerating from an originally foot-facing edge of the tissue, is observed both in ring doublets formed in a head-to-head (H2H) configuration and in a head-to-foot (H2F) configuration. This is a highly remarkable result; head formation on the side of the original foot is not compatible with the current picture and models that consider morphogenetic fields alone.
Importantly, the experiments described in this thesis also expose the importance of mechanical processes in establishing polarity in the regenerated animal. Overall the work presented in this thesis promotes a broader vista of morphogenesis, beyond patterning based solely on morphogenetic fields. Our methodology can be further developed to introduce other constraints and frustrating initial conditions that will eventually help in developing a holistic framework for morphogenesis that integrates biochemical signaling and mechanical processes.