The shape of a tissue or entire organ is important for its biological function. Tissue and organ shapes arise from molecular activities that control and execute cellular processes, such as oriented cell divisions, cell shape changes or cell rearrangements. However, how molecular activities control cellular processes during the shaping of organs is not well understood. This thesis spotlights two aspects of organ shaping based on Drosophila egg chambers as model tissue. One focus lies on three-dimensional imaging of cellular mechanics during development and the other aspect dissects the molecular function of the fat2 gene, that is crucial for tissue elongation in Drosophila egg chambers.:TABLE OF CONTENTS 1 SUMMARY ................................................................................................................... I 2 ZUSAMMENFASSUNG ...............................................................................................III 3 TABLE OF CONTENTS ................................................................................................V 4 LISTS ..........................................................................................................................10 5 INTRODUCTION ........................................................................................................16 6 AIMS OF THE THESIS ...............................................................................................40 7 MATERIALS AND METHODS .....................................................................................41 8 RESULTS ....................................................................................................................58 9 DISCUSSION .............................................................................................................92 10 ACKNOWLEDGEMENTS .......................................................................................103 11 REFERENCES ........................................................................................................105 12 APPENDIX ..............................................................................................................118 13 ERKLÄRUNG ..........................................................................................................123

The basement membrane is a specialized extracellular matrix (ECM) that is crucial for the development of epithelial tissues and organs. In Drosophila, the mechanical properties of the basement membrane play an important role in the proper elongation of the developing egg chamber; however, the molecular mechanisms contributing to basement membrane mechanical properties are not fully understood. Here, we systematically analyze the contributions of individual ECM components towards the molecular composition and mechanical properties of the basement membrane underlying the follicle epithelium of Drosophila egg chambers. We find that the Laminin and Collagen IV networks largely persist in the absence of the other components. Moreover, we show that Perlecan and Collagen IV, but not Laminin or Nidogen, contribute greatly towards egg chamber elongation. Similarly, Perlecan and Collagen, but not Laminin or Nidogen, contribute towards the resistance of egg chambers against osmotic stress. Finally, using atomic force microscopy we show that basement membrane stiffness mainly depends on Collagen IV. Our analysis reveals how single ECM components contribute to the mechanical properties of the basement membrane controlling tissue and organ shape.