|M.Sc Student||Ella Rosenzweig|
|Subject||The Role of Pax-3 in Somitogenesis|
|Department||Department of Biology||Supervisor||Mr. Ram Reshef|
|Full Thesis text|
During vertebrate gastrulation, the paraxial mesoderm is laid down as two stripes laterally flanking the neural tube and the notochord. While the primitive streak continuously adds new material to the posterior end of the pre-segmented mesoderm (PSM), the cells at the anterior PSM condense and then epithelialize to form epithelial somites. After somitogenesis, the paraxial mesoderm differentiates dorso-ventrally. In the ventral region, the somite de-epithelializes and forms the mesenchymal sclerotome which expresses Pax-1 and generates the vertebral column and ribs. Dorsally, the somite remains epithelial forming the dermomyotome, which later gives rise to the myotome and the dermatome. Stimulated by Wnt-1 secreted from the ectoderm and dorsal neural tube, the transcription factor Pax-3 is expressed in the rostral PSM and later is restricted to the dermomyotome where it activates Myf-5 and MyoD, thus triggering the myogenic program in the somite.
Previous study in our laboratory showed that ectopic Wnt-1 expression adjacent to the ventral aspect of the somite caused the expansion of Pax-3 expression through the entire somite, concomitantly with prevention of the epithelial to mesenchymal transition in its ventral part. Wnt-1 also increases the expression of epithelial markers such as Paraxis and N-Cadherin. Therefore, we aimed at understanding the role of Pax-3 in the formation of the epithelial state of somites during the course of somitogenesis.
In this study we demonstrate that ectopic introduction of Pax-3 via electroporation into the primitive streak of stage IV embryos disrupts segmentation and leads to abnormal somite formation. In addition, Pax-3 over-expression results in the expansion of Myf-5 and MyoD in somites and the PSM and the repression of Pax-1, and Paraxis (epithelial factor) expression. Furthermore, Pax-3 over-expressing cells organized into condensed structures which exhibited elevated N-Cadherin expression.
Electroporation of the negative construct of Pax-3, Pax-3 engrailed disrupted segmentation and somitogenesis and inhibited Paraxis expression in the PSM and the somites. In contrast to Pax-3, Pax-3 engrailed over-expression resulted in decreased Myf-5 and N-Cadherin expression in somites and the PSM and elevated Pax-1 expression.
Taken together, these results suggest that Pax-3 does not take part in forming the epithelial state during somitogenesis. However, it may play an important role in the condensation process of mesenchymal cells in the PSM by stimulating N-Cadherin expression.