Date of Award
Doctor of Philosophy (PhD)
Underlying the dramatic tissue movements of development—the bending, folding, squeezing, pushing, pulling, mass movements, and individual movements—are processes of cell migration. Throughout our lives, cell migration plays a role in the maintenance and regeneration of our tissues, and even in the development and progression of disease. To carry out the complex tasks of development and tissue morphogenesis, cells must coordinate their behaviors and migrate collectively. Insights into these collective behaviors have come from elegant studies of gastrulation movements in model organisms such as flies, frogs, and fish, uncovering conserved cellular and molecular mechanisms. However, the extent to which these mechanisms are refined, reiterated, and combined in the complex tissue environments of late development and adulthood is not well understood, highlighting the need for new models. Here, I develop a model of collective cell movements in late mammalian development by studying embryonic eyelid closure in mice, a process in which an epithelium locally reshapes, expands, and moves over another epithelium. Using a combination of lineage tracing and genetic ablation facilitated by ultrasound-guided lentiviral injection, I establish that the migratory cells of the eyelid front are derived from the epidermis rather than the periderm, and that the periderm is not required in the process as previously hypothesized. Quantitative analyses of cell proliferation, including inhibition of cell divisions in vivo, reveal that closure is primarily driven by cell motility rather than by proliferation. Optimizing conditions for the culture and live imaging of skin explants ex vivo, I reveal that cells of the eyelid front elongate perpendicularly to the axis of closure, extend mediolateral protrusions, and intercalate along this axis. Laser ablation and quantitative analyses of tissue deformation reveal that it is this intercalation, and not assembly and constriction of a supracellular actin cable, that drives eyelid closure. This mechanism is a novel mode of epithelial fusion in which forces generated by cell intercalation are leveraged to tow the surrounding tissue. Functional analyses in vivo show that this mechanism requires alpha- 5 beta -1 integrin/fibronectin and myosin II-dependent cell motility, is potentially organized by non-canonical Wnts/PCP, and is supported by a concomitant reduction in cadherins through localized Wnt/beta -catenin signaling. These studies establish eyelid closure as a model in which well-described mechanisms of collective cell movement are integrated into a complex morphogenetic process, set the stage for future study of the interplay between canonical and non-canonical Wnts/PCP in regulating cell motility and intercalation, and present an opportunity to uncover novel regulators of collective migration.
Heller, Evan, "Forces Generated by Cell Intercalculation Tow Epidermal Sheets in Mammalian Tissue Morphogenesis" (2014). Student Theses and Dissertations. 334.