The Drosophila ovarian terminal filament imports molecules needed for lipid droplets, the fusome, and functional germ cells

Insect ovarioles begin with a protruding stack of flat cells known as the terminal filament (TF) located in species such as Drosophila just upstream from germline stem cells (GSCs) in the germarium. Terminal filaments function during development, but an adult role has not been clearly established. TF-specific disruption of exocyst vesicle trafficking genes indicated that each TF stack continuously imports at least 0.75μ ³ of lipophilic molecules per day, and transfers them through the cellular stack into the GSC region. When lipid import between the TF and germ cells is cut off, LDs accumulate in TF cells, stem cell function and spectrosome size decreases, cyst formation and fusome structure is altered and follicle development becomes abnormal. Knockdown of organic anion transporter Oatp30B in the TF blocked the differentiation of GSC daughters, generating a tumor. The Oatp30B human ortholog SLCO5A1 has prognostic value in ovarian cancer. In sum, the terminal filament is essential to import ovarian lipids required for normal germ cell development and oocyte production.