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\begin{abstract}
Epithelial spreading plays a pivotal role in the development of organisms especially those such as zebrafish which require the epithelial enveloping layer (EVL) to spread to cover the substantial yolk surface during gastrulation. Epiboly requires the transition of the epithelium with cuboidal cells to form a thin, flat squamous epithelial sheet. During this transition, the cells show tissue-scale mechanosensation with mechanisms such as direct mechanical control over the axis of cell division. 

Cytoskeletal intermediate filaments play a crucial role in vertebrate cells, not only facilitating mechanical stability but also helping facilitate the mechanosensitive response of the cell. Mechanosenstivity displayed by intermediate filaments is due not just to their interesting physical properties but also to their interactions with other cytoskeletal elements such as actin and microtubules. Keratin is the predominant intermediate filament expressed in the EVL. It expresses concomitantly with the gastrulation movements of the developing embryo. Our work focuses on understanding the role and dynamics of the keratin cytoskeletal network in modulating the physical aspects of EVL spreading. We demonstrated with the combination of physical characterisation and manipulations of the EVL, utilising a variety of biophysical tools and microscopy, the mechanistic role of keratin in tissue spreading.

Generating novel genetic morphants and mutants, we probe the effect that the loss of the keratin network has on the physiology of the epithelium and the developing embryo. We show that the changing organisation of the keratin network is important for changing EVL physical properties as the stress imposed on the EVL increases during epiboly. By modelling the epithelium, we study how the mechanical heterogeneity in an epithelium can feed back into a mechanical loop to the maturation of the keratin network and hence affect the mechanics of the epithelium. However, unlike what would be predicted by the effect of intermediate filaments in acting as a security belt and increasing the resistance of the epithelium, we observe that loss of keratin leads to a delay in the EVL movement. Using both local aspirations of the YSL and EVL ablations, we demonstrate the mechanistic facilitation of actin mechanosensation in a keratin-dependent manner. 

Furthermore, using chemical inhibitors of microtubule polymerisation, we provide insight into the mechanisms underlying the organisation and distribution of keratin. Interestingly, the phenotype observed upon this loss of microtubules shows that keratins interact with the nucleus through microtubular interactions. Together with these diverse observations, we describe the mechanosensory feedback between resilience and that is critical for uniform and robust spreading of the epithelium. 
%\todo[inline]{
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\end{abstract}

% The Acknowledgement chapter is optional and can be removed.
\begin{acknowledgements}
 To Carl-Philipp Heisenberg and Edouard Hannezo, this work could have only been done under your critical guidance, 'Thank you!'. It was my aim as I started my PhD to perform interdisciplinary work that combines the different scales of biomechanics to understand developmentally critical processes. Your support while letting me explore and discover different directions of the project was an incredibly fulfilling experience. I also appreciate the critical feedback that helped me formulate my untempered raw conceptual intuitions into falsifiable hypotheses and empirically grounded scientific conclusions. I cannot give enough credit to the preceptive discussions we have had \textit{over coffee} over the years, and I look forward to carrying the insights I have gained about breaking down complex interconnected mechanisms into their most parsimonious, explanatory core. I would also like to thank Xavier Trepat and Jiri Friml for being a part of my thesis committee and providing me with invaluable feedback and support through our meetings. These discussions and feedback were critical in shaping and furthering my PhD project. \par
A special and deeply heartfelt thanks goes to Roland and Preeti, whose mentorship and guidance during the earliest stages of my doctoral journey gave me both the direction and the confidence to explore uncharted territories. Their patience, encouragement, and belief in me shaped the foundation of my research and my growth as a scientist.\par
I am profoundly grateful to all of my lab members, who have been a constant source of support, inspiration, and friendship throughout my PhD. Shayan, Silvia, Karla, Conny, Daniel, and Feyza—thank you for teaching me not only about the intricacies of science, but also about resilience, collaboration, and life itself. I carry forward many of your lessons both inside and outside the lab. To Diana, Ste, Nikhil, Carolina, Tushna, Viraj, David and other postdocs in the lab, your generosity with your time, your insightful discussions, and your willingness to challenge my thinking made me a sharper, more critical researcher.\par

This journey would not have been the same without Laura, Xin, Prajwal, and Roksi, with whom I’ve had the privilege of sharing my own mirrored experiences. Being able to guide and learn from you at the same time has been one of the most rewarding aspects of my PhD, and I am grateful for the camaraderie, laughter, and solidarity we shared along the way. Finally, a very special thanks to Koni and Irene, whose kindness, patience, and constant willingness to lend a hand were indispensable. Whether it was helping me navigate the practicalities of lab life or stepping in when my molecular biology experiments refused to cooperate. My labs have been my training grounds—not just for science, but for finding my voice, learning to express my ideas, and standing before an audience that constantly pushed me to think deeper and clearer. For those I may have missed in these lines—please know that your presence and support have been invaluable. The fact that these words come at the very end of my thesis reflects less on the depth of my gratitude and more on how difficult it is to capture the breadth of my appreciation.\par
To my dearest friends—Sarath, Patricia, Laura, Divyansh, and Djordje—you carried me through some of the most challenging times, from lockdowns and cyberattacks to moments of personal struggle and health issues. Your unwavering love, encouragement, and laughter gave me strength when I needed it most.
\par
I am deeply grateful to Verena and everyone at the fish facility for their unwavering dedication and care over the years. Their hard work in maintaining the health of the fish and the smooth operation of the facility has been essential to the success of my research. I especially want to thank Gabby, Terza, Mo, Cris, Yann, Nasser, and the other members of the IOF facility for their constant support, technical expertise, and readiness to step in whenever help was needed. Their efforts have provided me with a stable and reliable environment in which to carry out my experiments. I also greatly appreciate their work in maintaining the microscopes, which has ensured that they were always in excellent condition and available when I needed them. Without their skill, attention to detail, and genuine commitment to supporting researchers, much of my work would not have been possible. I look forward to learning more from you in the future. \par
I would also like to thank my parents and my sister Jivika, who have always been supportive of my dreams and aspirations. Himani who has been a pillar of support and understanding throughout this journey. Her encouragement during the most challenging times, her ability to celebrate the small victories with me, and her steady belief in my abilities have meant more than words can capture. She has been my anchor, reminding me to keep perspective, and my inspiration.
\par
 I would also like to thank the LSF and Cryo facility at ISTA, which have been helpful in my experiments.
I would also like to acknowledge FWF and JKU Nanocell for providing funding for my PhD research. EMBO and FWF for providing funding for travel grants to attend conferences.
\end{acknowledgements}
\begin{aboutauthor}

Suyash Naik completed an Integrated BS-MS degree at the Indian Institute of Science Education and Research, Pune with completing the requirements for his MS thesis at Laboratory of Molecular Physio-Medicine (Lp2M), CNRS Côte d'Azur University. He joined IST Austria to pursue a Ph.D. co-supervised by Carl-Phillip Heisenberg and Edouard Hannzeo in September 2018. His main research interests focus on understanding developmentally critical processes through the study of biomechanics across different scales. While at ISTA, he has worked on  uncovering the mechanics of intermediate filaments during early developmental process such as zebrafish epithelial spreading. His work during his undergraduate studies revolved around the biomechanics of a marine invertebrate organism, Hydra, and has been published in the Journal of Experimental Biology. During his PhD he has contributed to one publication in Developmental Cell and presented his work as a lead author in several international conferences including invited talks at European Intermediate Filaments and Gordon Conference. Alongside his PHD, he is actively involved in outreach and mentorship, being an organiser for the online seminar series Connecting intermediate filaments and has led PhD Balance, a volunteer-run organisation that helps academia provide spaces to grow together and talk about mental health.
\end{aboutauthor}

\begin{publicationlist}


\bibentry{Naik2025.02.14.638262} % Exemplary full citation.

This publication has been incorporated in this thesis as Chapter 3, Section 1.
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