Sexual differentiation and early embryonic/fetal gonad development is a tightly regulated process controlled by numerous endocrine and molecular signals. These signals ensure appropriate structural organization and subsequent development of gonads and accessory organs. Substantial differences exist in adult reproductive characteristics in Meishan (MS) and White Composite (WC) pig breeds. This study compared the timing of embryonic sexual differentiation in MS and WC pigs. Embryos/fetuses were evaluated on 26, 28, 30, 35, 40 and 50 days postcoitum (dpc). Gonadal differentiation was based on morphological criteria and on localization of GATA4, Mullerian-inhibiting substance (MIS) and 17alpha-hydroxylase/17,20-lyase cytochrome P450 (P450(c17)). The timing of testicular cord formation and functional differentiation of Sertoli and Leydig cells were similar between breeds. Levels of GATA4, MIS and P450(c17) proteins increased with advancing gestation, with greater levels of MIS and P450(c17) in testes of MS compared with WC embryos. Organization of ovarian medullary cords and formation of egg nests was evident at similar ages in both breeds; however, a greater number of MS compared with WC embryos exhibited signs of ovarian differentiation at 30 dpc. In summary, despite breed differences in MIS and P450(c17) levels in the testis, which may be related to Sertoli and Leydig cell function, the timing of testicular differentiation did not differ between breeds and is unlikely to impact reproductive performance in adult boars. In contrast, female MS embryos exhibited advanced ovarian differentiation compared with WC embryos which may be related to the earlier reproductive maturity observed in this breed.

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