Figure 4.

Expression profiles of critical factors in the embryonic testes and a hypothetical model for the sexual differentiation of mouse germ cells. (A) Expression patterns for Cyp26b1 (blue), Stra8 (red), and Nanos2 (green) in E12.5–E15.5 wild-type, Cyp26b1-null, and Nanos2-null testes. In wild-type mice, Cyp26b1 is increased until E13.5 and gradually reduces thereafter (Fig. 2H; Bowles et al. 2006), whereas Nanos2 expression starts at around E13.5 and Stra8 is not induced. In Cyp26b1-null mice, Stra8 is up-regulated at E13.5 but is not induced until E14.5 in Nanos2-null mice, possibly due to the effects of Cyp26b1 (Fig. 2G). (B, left) RA signaling triggers meiotic initiation via the induction of Stra8 expression in female germ cells (pink). (Middle) In embryonic testes, Cyp26b1 inhibits RA signaling, which suppresses the entry of male gonocytes into meiosis and may provide the required conditions for the expression of Nanos2 until E13.5 (blue). (Right) After Cyp26b1 expression is decreased, Nanos2 inhibits meiotic initiation via suppression of Stra8 expression and induces male-type differentiation, which is indicated by the presence of the Dnmt3L and TDRD1 (blue).