Epithelial cells surround every organ of the human body and about 85% of cancers arise from epithelial cells. These cells are highly polarized having distinct apical and basolateral sides; they are tightly connected to each other through cell-cell junctions. Epithelial morphogenesis is important for organogenesis and pivotal for carcinogenesis, although the mechanism is poorly understood. Drosophila FE represents a genetically tractable model system that enables to address this aspect in vivo. In Drosophila, FCs form the FE that surrounds the egg chamber, and this epithelium shares molecular and morphological features with vertebrate epithelia. Hence, this system provides a powerful model to study the mechanisms underlying the biological process relevant to the maintenance of mono-layered epithelium.

In this study, the underlying mechanism by which the FE is maintained has been investigated by studying α–spec mutant FCs. α–spec gene encodes the subunit that forms the building blocks of the SBMS. The scope of the study has been further extended to the analysis of the SBMS function in a mono-layered epithelium by studying the consequences of SBMS loss on the proliferation, polarization, and differentiation of FCs, as well as on the architecture of the FE. Since α-Spec is the major component of both apical and lateral Spectrin cytoskeletons, the study was mainly focused on α-Spec. The study showed that α-Spec and β-Spec are essential in maintaining a mono-layered FE, but, contrary to previous reports, Spectrins are not required to control proliferation. Furthermore, loss of Spectrin show differentiation and polarity defects only in the ectopic layers of stratified epithelia, similar to loss of Integrin in FCs. The results identify α-Spec and Integrins as novel regulators of apical constriction-independent cell elongation, as α- Spec and Integrin cells fail to columnarize. Also, the findings show that an increase and a decrease in the activity of Rho1-Myosin-II pathway enhances and decreases multi- layering of α-spec mutant FCs, respectively. Similarly, higher Myosin-II activity enhances the multi-layering phenotype of FCs that lack Integrins.

Furthermore, the role of α-Spec in the proper localization of cell junctions has been established. Reduction of AJ components enhances multi-layering of FE in α-spec mutant FCs. Finally, wildtype egg chambers were found to have a lower tension at the terminal region and higher tension at the central region, while this trend was lost in egg chambers containing α-spec and integrins mutant FCs. This suggests that the stiffness gradient might play an important role in the maintenance of monolayer FE. Additionally, the results presented in this thesis show that the reduction of ECM component level (Collagen and Laminin) can rescue multi-layered FE phenotype in mutant FCs.

In conclusion, this work identifies a primary role for α-Spec in controlling cellular shape, perhaps by modulating actomyosin, and the proper localization of cellular junction. Thus, it is possible that proper cell-cell interactions, adequate force balance and precise spindle orientation are the keys in the maintenance of a mono-layered epithelium, especially upon mechanical stress induced by morphogenesis.