UBC Theses and Dissertations
Occluding junctions regulate germline differentiation and stem cell niche homeostasis in the Drosophila melanogaster testis Fairchild, Michael John
Spermatogenesis is a reiterative process wherein sperm are continually produced from a small group of adult stem cells that are found in a unique niche. In Drosophila melanogaster this stem cell niche is made up of a cluster of cells, called the hub, which maintains both germline stem cells and somatic cyst stem cells using a combination of cell adhesion proteins and secreted signalling molecules. As each germ cell leaves the stem cell niche, it is surrounded and encapsulated by two somatic cyst cells, which are required for the germline to differentiate. To better understand how interactions between the cyst cells and germ cells control spermatogenesis I disrupted cytoskeletal and junctional proteins specifically in the somatic cyst cells and investigated the resulting changes in cellular architecture and cell-cell signalling pathways. This thesis describes the identification of a somatic permeability barrier that separates germ cells from their environment. This barrier is analogous to the mammalian blood-testis barrier and has not been previously identified in Drosophila. The permeability barrier is established by occluding septate junctions that seal together the two cyst cells that encapsulate the germline. If this barrier is disrupted the germline fails to differentiate resulting in infertility. This thesis presents data suggesting that the barrier controls differentiation by blocking the ability of germ cells to access signalling molecules that promote stem cell fate. These findings represent a novel role for occluding junctions in controlling stem cell differentiation by isolating them from their niche. While characterizing stem cell niche signalling it was observed that disrupting occluding junctions in differentiating cyst cells led to progressive growth of the stem cell niche in adult testes. This growth was due to cyst cells converting into hub cells and recruiting more stem cells over time. The results presented in this thesis suggest that the conversion of cyst cells to hub cells was due to changes in the EGFR and Notch signalling pathways that regulate somatic differentiation. This represents an unexpected role for occluding junctions in maintaining stem cell niche homeostasis.
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