Retinoic Acid Signalling and the Control of Meiotic Entry in the Human Fetal Gonad

The development of mammalian fetal germ cells along oogenic or spermatogenic fate trajectories is dictated by signals from the surrounding gonadal environment. Germ cells in the fetal testis enter mitotic arrest, whilst those in the fetal ovary undergo sex-specific entry into meiosis, the initiation of which is thought to be mediated by selective exposure of fetal ovarian germ cells to mesonephros-derived retinoic acid (RA). Aspects of this model are hard to reconcile with the spatiotemporal pattern of germ cell differentiation in the human fetal ovary, however. We have therefore examined the expression of components of the RA synthesis, metabolism and signalling pathways, and their downstream effectors and inhibitors in germ cells around the time of the initiation of meiosis in the human fetal gonad. Expression of the three RA-synthesising enzymes, ALDH1A1, 2 and 3 in the fetal ovary and testis was equal to or greater than that in the mesonephros at 8–9 weeks gestation, indicating an intrinsic capacity within the gonad to synthesise RA. Using immunohistochemistry to detect RA receptors RARα, β and RXRα, we find germ cells to be the predominant target of RA signalling in the fetal human ovary, but also reveal widespread receptor nuclear localization indicative of signalling in the testis, suggesting that human fetal testicular germ cells are not efficiently shielded from RA by the action of the RA-metabolising enzyme CYP26B1. Consistent with this, expression of CYP26B1 was greater in the human fetal ovary than testis, although the sexually-dimorphic expression patterns of the germ cell-intrinsic regulators of meiotic initiation, STRA8 and NANOS2, appear conserved. Finally, we demonstrate that RA induces a two-fold increase in STRA8 expression in cultures of human fetal testis, but is not sufficient to cause widespread meiosis-associated gene expression. Together, these data indicate that while local production of RA within the fetal ovary may be important in regulating the onset of meiosis in the human fetal ovary, mechanisms other than CYP26B1-mediated metabolism of RA may exist to inhibit the entry of germ cells into meiosis in the human fetal testis.

哺乳动物胎儿生殖细胞沿卵发生或精子发生的命运轨迹发育，其进程由周围性腺微环境的信号调控。胎儿睾丸中的生殖细胞会进入有丝分裂阻滞，而胎儿卵巢中的生殖细胞则以性别特异性方式进入减数分裂，目前认为减数分裂的启动由胎儿卵巢生殖细胞选择性暴露于中肾来源的视黄酸（retinoic acid, RA）所介导。然而，该模型的部分内容难以与人类胎儿卵巢中生殖细胞分化的时空模式相契合。为此，我们针对人类胎儿性腺减数分裂启动前后的生殖细胞，检测了视黄酸合成、代谢及信号通路组分，及其下游效应因子与抑制因子的表达情况。在妊娠8-9周时，胎儿卵巢与睾丸内三种视黄酸合成酶——醛脱氢酶1A1（ALDH1A1）、ALDH1A2与ALDH1A3的表达水平不低于甚至高于中肾，表明性腺自身具备合成视黄酸的内在能力。我们采用免疫组化（immunohistochemistry）检测视黄酸受体RARα、RARβ与视黄酸X受体α（RXRα）的表达，发现人类胎儿卵巢内的生殖细胞是视黄酸信号的主要靶标；同时也观察到睾丸内受体广泛的核定位现象，提示睾丸中存在视黄酸信号传导，这表明人类胎儿睾丸生殖细胞并未通过视黄酸代谢酶细胞色素P450 26B1（CYP26B1）的作用被有效隔绝于视黄酸之外。与此一致的是，人类胎儿卵巢内CYP26B1的表达水平高于睾丸；尽管如此，减数分裂启动的生殖细胞内在调控因子减数分裂启动蛋白8（STRA8）与Nanos同源物2（NANOS2）的性别二态性表达模式似乎在人类中仍保守存在。最后，我们证实视黄酸可使人类胎儿睾丸培养物中STRA8的表达水平提升两倍，但不足以引发广泛的减数分裂相关基因表达。综上，本研究数据表明：尽管人类胎儿卵巢内局部合成的视黄酸可能对调控卵巢生殖细胞减数分裂的启动具有重要作用，但在人类胎儿睾丸中，除CYP26B1介导的视黄酸代谢外，还存在其他机制可抑制生殖细胞进入减数分裂。