Epithelial morphogenesis of the Caenorhabditis elegans pharynx
Rasmussen, Jeffrey Philip
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The assembly of cells into functional organs requires the coordination of cell shape and polarity with organ architecture. Although defects in cell shape and polarity can lead to human disease, the in vivo regulation of these processes during organ formation is poorly understood. In this thesis, I used the Caenorhabditis elegans pharynx as an in vivo model to study organogenesis. Similar to many human organs, the pharynx is a tube formed from a monolayer of polarized epithelial cells that surround a central lumen. The pharyngeal lumen develops midway through embryogenesis when cells polarize along a central axis to form an epithelial cyst. Using live cell imaging, I found that cyst development is preceded by the grouping of pharyngeal precursors into a bilaterally symmetric intermediate. The organization of cells into this bilateral intermediate requires the POP-1/TCF cell fate pathway and the pharyngeal organ identity factor PHA-4/FoxA. Cyst formation begins when cells within the bilateral structure undergo a mesenchymal to epithelial transition and localize apical polarity determinants along their inner surfaces. These inner surfaces then constrict, creating the central lumen. I found that laminin, an integral basement membrane component, is required to define the axis of pharyngeal polarity. In laminin mutants, cells develop inverted polarity and constrict their outer surfaces, resulting in multiple pharyngeal lumens. After cyst formation, pharyngeal cells adopt highly reproducible morphologies. A previous ultrastructural study found that two cells, called pm8 and vpi1, develop into toroidally shaped single-cell tubes. By combining live cell microscopy with genetic analysis, I showed that pm8 and vpi1 form toroids via a novel mechanism: They wrap around the midline and then self-fuse, thereby creating an intracellular lumen. The self-fusion of pm8 and vpi1 into toroids requires fusogens and activation of the Notch signaling pathway in pm8. Thus, organogenesis of the pharynx requires sequential action of global, tissue-level signals, such as laminin, and local, cell-specific signals, such as Notch.