Epithelial layers of cells are a fundamental building block of organs and organisms. More than simple flat sheets, epithelia form the branches of our lungs, the villi of our guts, and the lining of every vein and artery in our bodies. Epithelia most often exist as part of complex, three-dimensional organs that include multiple epithelial layers or other tissue types. To properly function, epithelia must coordinate their growth and development with neighboring tissues.
The relative growth rates of multiple tissue layers can determine the shape of an organ, from the curve of the retina to the folds of the cerebral cortex. Understanding these important developmental processes requires understanding how epithelial growth regulates, and is regulated by, other tissues. This growth can occur not only by cell proliferation, but also by changes in cell shape or cross-sectional area – an important, but relatively understudied, contributor to epithelial growth.
In Chapter 1, I introduce the system in which I study epithelial growth regulation: the Drosophila wing imaginal disc (wing disc), which develops into the adult wing and part of the thorax. The wing disc is composed of two epithelial sheets which grow in synchrony, the disc proper and the peripodial epithelium. The disc proper grows primarily through cell proliferation; the peripodial epithelium grows primarily through cell shape changes. I briefly review the existing knowledge on how these two layers interact, and introduce signaling pathways that are especially salient to my studies of growth regulation.
I found that the two layers of the wing disc grow in synchrony through a “leader/follower” mechanism, in which growth of the disc proper sets the pace for growth of the peripodial epithelium. I determined that several signaling pathways that are critical for growth of the disc proper are surprisingly dispensable for growth of the peripodial epithelium. In contrast, the adaptive growth of the peripodial epithelium absolutely requires the Hippo pathway, a signaling pathway that can respond to physical forces like tissue stretching. I propose that the Hippo pathway can sense mechanical forces caused by growth of the disc proper, which allows the peripodial epithelium to grow in response. This work is described in Chapter 2.
The disc proper is extraordinarily well-studied as a model of growth; however, much remains unknown about how patterned gene expression within the peripodial epithelium drives its development and morphogenesis. In Chapter 3, I delve into an existing single-cell RNA sequencing dataset generated by previous members of the laboratory, Melanie Worley and Nicholas Everetts, to identify patterns of differential gene expression and candidate regulators of cell shape changes within the peripodial epithelium. I tested whether a number of these candidate genes were required for cell shape changes, but the key “shape factor” remains unknown. Also in Chapter 3, I investigate the extent of physical contacts between the layers, as these could be a critical avenue for inter-layer communication.
Overall, my work presents a paradigm for synchronizing growth between tissue layers, determines a role for the Hippo pathway in growth regulation during normal development, and contributes to foundational knowledge on the biology of an important model system.
