|Ph.D Student||Arraf Alaa|
|Subject||Roles of the Coelomic Epithelium in the Generation and|
Morphogenesis of Ventral Embryonic
|Department||Department of Medicine||Supervisor||Professor Thomas Schultheiss|
|Full Thesis text|
The vertebrate body plan is bilaterally organized with respect to a central axis located in the body midline. The dorsal midline, characterized by the notochord, is the first midline structure to form, and secretes multiple factors including the Bone Morphogenetic Protein (BMP) antagonists Noggin and Chordin, as well as Sonic hedgehog (Shh), which participate in specifying the midline of the neural plate and surrounding mesodermal tissues. However, there are other midline structures that form later in development in more ventral regions, including the aorta, the mesenteries that connect the gut tube and its derivatives to the body wall, the urinary bladder, the uterus, and the external genitalia. While dorsal midline structures have been extensively studied, formation of ventral midline structures, and how they become aligned with the dorsal midline, is a fundamental aspect of vertebrate development that is poorly understood.
In vertebrates, the Coelomic epithelium (CE) is derived from the lateral plate (LP) mesoderm. Although in recent years increasing evidence indicates that the CE may be an important part of ventral body structure embryogenesis, the roles of the CE in directing formation and placement of ventral midline structure has remained elusive.
In this research we use the CE and the Dorsal Mesentery (DM) (which, despite its name, is a ventral tissue) as a model system for investigating this morphogenetic question. We document formation of the DM by way of epithelial-to-mesenchymal transition (EMT) of the CE, and show that the CE undergoes intrinsic morphological changes throughout its ingression towards the midline, from a densely packed, columnar morphology to a flatter arrangement. We functionally prove that positioning of the DM is a dynamic process, requiring symmetric rates of medial ingression on the two sides of the LP. If this symmetry is perturbed, this causes misalignment of the DM and disturbances during initial stages of lung morphogenesis.
We characterize two signaling pathways, BMP and Hedgehog (Hh), as regulators of CE ingression towards the midline. Bilaterally unbalanced BMP or Hh signaling in the two CE’s causes asymmetric convergence of the CE’s and mis-positioning of the DM. Our results indicate that BMP at least partly controls symmetrical EMT from the CE, while Hh signaling from the adjacent endoderm modulates the shape of CE cells and thus coordinate CE tissue morphogenesis during its convergence towards the midline. We identify Sec5 (ExoC2), a component of the exocyst complex, and RhoU (Wrch-1), a small GTPase, as hedgehog-regulated genes that modulate epithelial cell morphology and CE convergence, at least in part through their effects on cellular N-Cadherin distribution and maintenance of apical actin-containing elements, respectively. Since the dorsal midline is a source of BMP antagonists and Shh signaling, these results suggest a mechanism for aligning the dorsal and ventral embryonic midlines.
This research presents an integrated, understanding of how embryonic form is generated. Our results provide insight into the generation of the bilaterally symmetric vertebrate body plan, and establish a mechanism for positioning ventral midline structures. This research may shed light on the genesis of ventral midline developmental defects.