Collectively, these results indicate that escort cells are an essential component of the FSC niche. RESULTS Tools for the expression of transgenes in subpopulations of somatic cells in the germarium The germarium contains multiple somatic cell types, including terminal filament cells, cap cells, escort cells and follicle cells. Indeed, the male and female germline stem cell (GSC) niches are particularly useful examples of a type of static niche, in which a dedicated population of niche cells produces a stable microenvironment that is necessary and sufficient for maintaining the stem cell fate. A second populace of stem cells in the ovary, the follicle stem cells (FSCs), offers an opportunity to investigate the composition of a more dynamic type of niche that may be common in epithelial tissues (Margolis and Spradling, 1995; Nystul and Spradling, 2007). ovaries are composed of 14-18 ovarioles, and two mitotically active FSCs reside within a structure at the tip of each ovariole called the germarium. During normal adult oogenesis, the FSCs divide asymmetrically to self-renew and produce prefollicle cell daughters that differentiate to form a canonical polarized epithelium (Fig. 1A). Lineage-tracing studies exhibited that the two FSCs are consistently positioned on opposite sides of the germarium, at the boundary between stromal escort cells in region 2a and the follicle cells in region 2b (Fig. 1A) (Nystul and Spradling, 2007). Notably, when FSCs are lost from the tissue, a new alternative cell colonizes these same locations and acquires the FSC fate. These observations suggest that these locations are FSC niches, but it is usually unclear which cells within the germarium are contributing to this niche. Open in a separate windows Fig. 1. The germarium contains multiple types of somatic cells. (A) Diagram of the ovary. Each ovary is composed of multiple subunits called ovarioles, and each ovariole has a structure at the anterior tip called the germarium. Each germarium has terminal filament cells (TFCs, orange) and cap cells (CCs, red) at the anterior tip, escort cells (ECs, blue) surrounding germ cell cysts in the anterior half, and two follicle stem cells (FSCs, light green) that produce follicle cells (dark green) that surround germ cell cysts in the posterior half. The germarium is usually divided into regions (1, 2a, 2b and 3) that are defined by the stages of germ cell development. FSCs are usually found at the region 2a/2b border. (B-E) UAS::CD8-GFP; Bab1-Gal4 (B), UAS::CD8-GFP, 13C06 (C), 109-30; UAS::CD8-GFP (D) or UAS::CD8-GFP; Tj-Gal4 (E) stained for GFP (green) to identify cells that express Gal4, Lamin C (B,C,E, red) to identify terminal filament cells and cap cells or Fas3 (D, red) to identify follicle cells, and with DAPI (blue). (B-E) GFP channel only. Bab1-Gal4 is expressed strongly in terminal filament and cap cells. 13C06 is expressed strongly in posterior escort cells, FSCs and some early follicle cells, and weakly in anterior escort cells, and occasionally in one to two cap cells (white arrowhead). Cap cells that do not express Gal4 at detectable levels are indicated by the open arrowhead. 109-30 is expressed strongly in FSCs and all follicle cells in the germarium. Tj-Gal4 is expressed in escort Lifirafenib cells, FSCs and follicle cells. Anterior is to the left. Scale bar: 5 m. Among the many genes required for FSC self-renewal are several key members of the Hedgehog (Hh) (Kirilly et al., 2005; Vied and Kalderon, 2009; Zhang and Kalderon, 2001) and Wingless (Wg) (Song and Xie, 2003) signaling pathways. The only cell types known to express Hh and Wg in the germarium are two somatic cell types, the cap cells and terminal filament cells (Forbes et al., 1996a; Forbes et al., 1996b; Hartman et al., 2010), which reside at the anterior tip of the germarium, several cell diameters away from the FSCs. This led to the suggestion that the Hh and Wg produced by terminal filament cells and cap cells diffuse through the tissue to the region 2a/2b border, where they act in the FSC niche to promote self-renewal (Forbes et al., 1996a; Hartman et al., 2010; Song and Xie, 2003; Vied et al., 2012). However, whether the Hh and Wg ligands produced by the terminal filament cells Rabbit Polyclonal to GPR126 and cap Lifirafenib cells are actually required by FSCs or their daughters has not been tested genetically. A second common function of niche cells is to adhere to stem cells through cell adhesion complexes. Indeed, FSCs require DE-cadherin (Song and Xie, 2002), the core protein in adherens junctions, for maintenance in the tissue. In addition, FSCs are known to form adherens junctions with neighboring stromal Lifirafenib cells that are presumed to be a part of.