Histological assessment of ovarian follicles for the fresh, control, 0.5 mmol/L S1P, and 2 mmol/L S1P groups on day 10 after transplantation. A, High density of a cluster of primordial follicles (arrow) and secondary follicles (arrowhead; magnification, ×200) in the fresh group. B, Medium density of primordial follicles in (arrow) and primary follicle (arrowhead, magnification ×200) in the 0.5 mmol/L S1P group. C, Medium density of primordial follicles in (arrow) and secondary follicle (arrowhead, magnification ×200) in the 2 mmol/L S1P group. D, Low density of primordial follicles (arrow) and secondary follicles (arrowhead; magnification, ×200) in control group. Bar = 50 μm. S1P indicates sphingosine-1-phosphate.