Localization of S1P1 receptor protein expression in normal hen ovary. (A) S1P1+ cells (black arrows) in theca of a mature follicle (F) and within small blood vessels (BV; white arrows) in ovarian stroma (100×). Primordial follicles (f) have comparatively little S1P1+ expression. (B) Endothelial cells of blood vessels (BV) in the theca externa (TE) of a follicle (F) and ovarian stroma are S1P1+ (200×). (C) Surface epithelial cells (EpC) showing intense S1P1+ expression (400×). (D) An atretic follicle (af) with characteristic infiltrating S1P1+ immune cells (100×). Inset: High magnification of (D) showing an S1P1+ immune cell (1000×). (E) Well developed blood vessels (BV) in the medullary region of the ovary also contain S1P1+ endothelial cells (400×) but staining is lighter and more diffuse than in stromal blood vessels seen in (A) and (B). Inset: high magnification (800×) shows detail of smooth muscle cells (SmC) and endothelial cells (EnC). (A-C) are frozen tissues; (D) and (E) are paraffin-embedded.