Fig. 3. Histological analysis of tissue-specific knock-out gonads at E18.5. (A-C) Control females. In contrast to control males (D-F), Sox9 knock-out animals show somewhat irregular and poorly differentiated sex cords (G, and arrow in H). Next to quiescent gonocytes (arrows in I), several gonocytes in meiotic prophase could be detected (asterisks), suggesting differentiation of these primordial germ cells along the female pathway (compare with C). (J-L) Mice homozygous for the Sox8/Sox8 knock-out allele, but without the Sox9 tissue-specific knock-out showed normal sex cord formation and no sign of meiosis. By contrast, embryos heterozygous for the Sox8 knock-out and homozygous for the Sox9 knock-out (M-O) often showed complete sex reversal (compare with female development in A-C). See Table 1 for the frequencies of the sex reversal phenotype.