Fig. 7. Schematic showing two distinct critical time windows of SRY action for the initiation and maintenance of Sox9 expression in developing XX gonads. The horizontal bar represents the bipotential period (blue), and the early (purple) and late (red) ovarian differentiation phases of developing XX gonads. The initiation and maintenance patterns of endogenous Sox9 expression in Sry-induced XX Tg gonads at each developmental stage are shown in the upper part (XX Tg). The timings of Sry and Sox9 expression and testis cord formation in XY wild-type gonads are shown in the lower part (XY). In XX Tg gonads at 12-14 ts, artificial Sry induction cell-autonomously induces initial Sox9 activation and glycogenesis in pre-Sertoli cells. Such SRY/SOX9 expression leads to high-FGF9/low-WNT4 expression patterns in the gonadal area, which results in the maintenance of Sox9 expression and the testis-specific induction of early morphogenic events [CE (coelomic epithelial cell) proliferation and mesonephric cell migration; blue arrows]. In XX Tg gonads at 16-21 ts, the delayed Sry induction is capable of promoting initial Sox9 activation and glycogenesis in pre-Sertoli cells. However, a lack of SRY action before 16 ts results in the female-specific high Wnt4 expression that leads to failure of the maintenance of SOX9/FGF9 expression, resulting in the ovarian development at later stages (purple arrows). Beyond 22 ts, neither testis formation nor transient Sox9 activation was detected in Sry-induced XX gonads (red arrows).