Primordial germ cells (PGCs) are the embryonic precursors of adult gametes. In the mouse, they arise around E7.5 in the allantois, and migrate through the developing hindgut and midline dorsal body wall mesenchyme to colonize the gonad primordia by E11.5. PGC behavior, including proliferation, survival, and motility, is controlled by cellular signaling during migration. Steel factor is known as an essential survival factor for PGCs. It is the protein product of the Steel locus, and the ligand for the receptor tyrosine kinase c-kit, which is expressed by PGCs throughout migration. Steel factor exists in two forms, membrane-bound and the soluble, generated by alternative splicing. This thesis addresses two general questions: (1) Is PGC behavior controlled by Steel factor from the beginning of their migration? (2) Do the two different forms of Steel factor control different aspects of PGC behavior? Using the mouse reporter line Stella-GFP, in which PGCs express GFP under the control of the promoter of Stella gene, I demonstrate that PGC number is significantly reduced in Steel -/- embryos at E7.5. Similarly, in the absence of Steel factor, either by null mutation or antibody blockade, PGCs aggregate together and show dramatically decreased motility, but their directionality is maintained. These data indicate an essential role for Steel factor in PGC survival and motility. I then show that Steel factor-expressing cells surround PGCs from the time of their initial specification in the allantois, to the time of their colonization of the gonad primordia, providing a “spatio-temporal niche” that travels with PGCs to regulate their survival, proliferation and motility throughout migration. Further, I show that these functions of Steel factor in PGC behavior are distributed between the membrane-bound and soluble forms by analyzing PGC behavior in Steel-dickie mutant embryos, which make only the soluble form. Soluble Steel factor alone is sufficient for PGC survival at E7.5. However, PGCs in E7.5 Steeld/d embryos aggregate and migrate at much slower rate, indicating that membrane-bound Steel factor is required for PGC motility. Addition of excess soluble Steel factor to Steeld/d embryos rescued PGC motility, suggesting that the membrane-bound form provides a higher local concentration of Steel factor that controls the balance between germ cell motility and aggregation. Taken together, this thesis demonstrates that the two forms of Steel factor control different aspects of germ cell behavior, and the membrane-bound Steel factor, by providing higher local ligand concentration, defines a “motility zone” for PGCs. This ensures that PGCs in appropriate locations maintain survival and motility, but ectopic PGCs cease motility and then die by apoptosis.
|Commitee:||Campbell, Kenneth, Cartwright, Iain, Wells, James, Zorn, Aaron|
|School:||University of Cincinnati|
|Department:||Molecular & Developmental Biology|
|School Location:||United States -- Ohio|
|Source:||DAI-B 73/05, Dissertation Abstracts International|
|Subjects:||Molecular biology, Cellular biology, Developmental biology|
|Keywords:||Cell migration, Cell survival, Primordial germ cells, Stem cell niche|
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