Fig. 5. Model explaining how N signaling controls GSC niche formation in the Drosophila ovary. (A) Newly formed TFs (purple) express Dl protein and activate N (N^*) signaling in neighboring somatic cells and induce them to form cap cells (red oval) supporting two GSCs (light blue circle), while the rest of the somatic cells that are not in contact with TFs form ESCs or IGS cells (brown). PGCs are depicted as dark blue round cells, while differentiated germ cells, including cystoblasts, are yellow round cells. N signaling remains active in cap cells of the adult ovary and is required for their maintenance. (B) When N signaling is expanded to the somatic cells that do not contact TFs but are adjacent to the somatic cells destined to become cap cells, these somatic cells will also become cap cells (and possibly ESCs) and thus increase the niche size in the normal location and the GSC number. (C) When N signaling is ectopically activated in the somatic cells a few cells distant from TFs, these somatic cells differentiate into cap cells (and possibly ESCs). Thus, ectopic niches that are surrounded by IGS cells or follicle cells are formed. (D) When N signaling is active in most, if not all, somatic cells of the gonad, the somatic cells that have active N signaling generate cap cells (and possibly ESCs), forming niche cells, which are able to sustain GSC self-renewal throughout the germaria.