Characterization of the Follicle Stem Cell Niche in Drosophila Ovary. Pankaj Sahai-Hernandez, Todd G. Nystul. Anatomy Dept., UCSF, San Francisco, CA.

   Adult stem cells are a small group of cells within a tissue that promote homeostasis by dividing to self-renew and produce differentiated progeny. Stem cell self-renewal is thought to be enforced by a specific microenvironment, or niche, within the tissue that may promote stemness or prevent differentiation. We have studied the follicle stem cells (FSCs) in the Drosophila ovary as a model to better understand how an epithelial niche functions. First, we determined which cells produce the signaling ligands important for FSC maintenance. We used cell-type specific Gal4 drivers and RNAi to knockdown Wingless and hedgehog ligands from different regions of the germarium and assayed for defects in the FSC lineage. We find that the Wingless ligand relevant for the FSC lineage comes specifically from escort cells whereas the hedgehog ligand comes from multiple cell types in the germarium including cap cells and escort cells. Next, we characterized the shape and distribution of escort cells near the follicle epithelium to determine which escort cells form adherens junctions with FSCs. We find that FSCs form adherens junctions with multiple posterior escort cells, which have a diversity of shapes and positions. We conclude that the posterior escort cells create the niche for the FSCs by acting in aggregate to provide a stable source of signaling ligands and cell adhesion important for FSC maintenance.