(A) Schematic representation of hypoxia-induced mouse model of ROP. (B) S1P₁, S1P₂, and S1P₃ mRNA expression at P12 (“Hypoxia,” day 0) was determined by quantitative RT-PCR analysis (n = 3). S1P₂R expression was upregulated by 3-fold at P13 (24 hours of hypoxia; *P < 0.035) compared with P12 and was further increased by 5-fold at P17 (5 days of hypoxia; *P < 0.035). S1P₁ expression was upregulated by 3-fold at P14 (2 days of hypoxia; ^#P < 0.01). S1P₃ expression was induced by 3-fold at P16 (4 days of hypoxia; **P < 0.03). (C) Ang-2 and VEGF mRNA in the course of hypoxia (n = 3). Ang-2 expression was 14-fold higher at P16 (4 days of hypoxia; *P < 0.015). VEGF expression was induced by 3-fold at P16 (4 days of hypoxia; ^#P < 0.0015). (D) Immunohistochemical localization of S1P₂R in P17 hypoxic retinal cross sections. S1P₂R was detected in the GCL, showing a vessel-like distribution pattern (arrow) and in the INL (arrowhead). Scale bar: 100 μm. At a higher magnification (right panel), S1P₂R immunodetection revealed a strong signal in the INL (arrowhead) as well as in vascular tufts (VT; arrow); scale bar: 10 μm. Nuclear counterstaining was with methyl green.