Neural Tube Defect-causing Teratogens Affect Tissue Mechanical Properties and Cytoskeletal Morphology in Axolotl Embryos

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. Abstract The teratogenic drugs cytochalasin B and valproic acid have been shown to alter F-actin polymerization, an effect that is crucial in forming microfilaments. Microfilaments form important cytoskeletal structures that maintain the structural integrity of the cell, cause cell motility and cell migration. Microfilament alterations are known to cause neural tube defects such as spina bifida and anencephaly (Walmod et al., 1999). We here aim to show that disruption of microfilaments by cytochalasin B and valproic acid affects the tensile properties of the tissue. Biomechanics is an interdisciplinary field that allows mechanical concepts to help us understand embryo development. This project used a novel tissue stretching device that measures the tensile properties of neural and epidermal tissue. The instrument used a pair of cantilevered wires to which the specimen was glued. This device stretched the mid-neural and-lateral tissue anterior-posterior (AP) and medio-lateral (ML) unidirectionally. The tensile properties of the tissue were determined by Resultant Young's Modulus that depends on the true stress and true strain in the tissue sample. The experiment was conducted at a strain rate of 50%. Axolotl embryos were treated with 5ug/mL and 2.5ug/mL cytochalasin B and 5mM valproic acid at stage 13 (early neurula) for an hour, washed, and allowed to develop to stage 15 before it was used in the uniaxial tissue stretcher. Changes in the F-actin filaments were analysed by phalloidin staining and viewed under a confocal microscope. The tests show that disruption of microfilaments by cytochalasin B increases the stiffness of the dorsal-tissue by as much as 101% for CB-treated tissues stretched in the AP direction and 298% when stretched in the ML direction. VA-treated neural plate tissue showed a stiffness increase of 278% when stretched in the AP direction and 319%, when stretched in the ML direction. Changes in the F-actin filaments are quantified by phalloidin staining viewed with confocal microscopy. These findings indicate that direction-dependent mechanical forces in the tissue are contributing factors in closure of the neural tube in axolotl embryos. iv Acknowledgements