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Retinoic Acid signalling and the control of meiotic entry in the human fetal gonad.

Childs AJ, Cowan G, Kinnell HL, Anderson RA, Saunders PT - PLoS ONE (2011)

Bottom Line: The development of mammalian fetal germ cells along oogenic or spermatogenic fate trajectories is dictated by signals from the surrounding gonadal environment.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.

View Article: PubMed Central - PubMed

Affiliation: Medical Research Council Human Reproductive Sciences Unit, The Queen's Medical Research Institute, University of Edinburgh, Edinburgh, United Kingdom. a.childs@ed.ac.uk

ABSTRACT
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.

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Expression of genes encoding retinaldehyde dehydrogenase enzymes in the human fetal gonad.qRT-PCR analysis reveals developmentally regulated expression of ALDH1A1 (A) in the human fetal testis, with transcript levels increasing significantly between 8–9 weeks gestation and 14–16/17–20 weeks gestation (ANOVA; a,b,c; p<0.05, n = 5–6 per group). Expression was not significantly different between gonads of different sexes at the same developmental stage, not between ovaries at different gestational ages. ALDH1A2 expression (B) is also developmentally-regulated in the human fetal testis, with transcript levels in the first trimester (8–9 weeks gestation) testis significantly higher than those in the early second trimester (14–16 weeks) testis (a vs b, p<0.05) and the late second trimester (17–20 weeks) testis (a vs c, p<0.01). Expression in the testis at 8–9 weeks gestation was also significantly higher than that in the fetal ovary at the same developmental stage (a vs d, p<0.05). ALDH1A2 transcript levels were also higher in the 8–9 week human fetal testis than in mesonephroi from age-matched fetuses (a vs e, p<0.01), which contrasts with the mesonephric-specific expression of Aldh1a2 in the mouse at a comparable developmental stage. No differences in the expression of ALDH1A3 (C) were detected between samples of different gestational ages of the same sex, or between the gonads of different sexes at the same developmental stage. 8–9, 14–16 and 17–20 denote the gestational age (in weeks) of specimens, meso: 8–9 week mesonephroi (pooled male and female). Values denote mean ± s.e.m..
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pone-0020249-g001: Expression of genes encoding retinaldehyde dehydrogenase enzymes in the human fetal gonad.qRT-PCR analysis reveals developmentally regulated expression of ALDH1A1 (A) in the human fetal testis, with transcript levels increasing significantly between 8–9 weeks gestation and 14–16/17–20 weeks gestation (ANOVA; a,b,c; p<0.05, n = 5–6 per group). Expression was not significantly different between gonads of different sexes at the same developmental stage, not between ovaries at different gestational ages. ALDH1A2 expression (B) is also developmentally-regulated in the human fetal testis, with transcript levels in the first trimester (8–9 weeks gestation) testis significantly higher than those in the early second trimester (14–16 weeks) testis (a vs b, p<0.05) and the late second trimester (17–20 weeks) testis (a vs c, p<0.01). Expression in the testis at 8–9 weeks gestation was also significantly higher than that in the fetal ovary at the same developmental stage (a vs d, p<0.05). ALDH1A2 transcript levels were also higher in the 8–9 week human fetal testis than in mesonephroi from age-matched fetuses (a vs e, p<0.01), which contrasts with the mesonephric-specific expression of Aldh1a2 in the mouse at a comparable developmental stage. No differences in the expression of ALDH1A3 (C) were detected between samples of different gestational ages of the same sex, or between the gonads of different sexes at the same developmental stage. 8–9, 14–16 and 17–20 denote the gestational age (in weeks) of specimens, meso: 8–9 week mesonephroi (pooled male and female). Values denote mean ± s.e.m..

Mentions: In the human fetal testis, ALDH1A1 expression increased significantly across the gestational range examined (p<0.05; Figure 1A). In contrast, there was a trend towards decreasing expression of ALDH1A1in the fetal ovary over the same period (Figure 1A) although this did not reach significance. We found no significant differences in expression of ALDH1A1 between gonads of different sexes of the same gestational age, or between fetal gonads and mesonephroi from 8–9 week fetuses. ALDH1A2 expression in the fetal testis was significantly higher at 8–9 weeks gestation than at 14–16 weeks (p<0.05) or 17–20 weeks gestation (p<0.01. Figure 1B). Furthermore, at 8–9 weeks gestation we detected a sexual dimorphism in ALDH1A2 expression, with transcript levels significantly higher in the testis than the ovary at this developmental stage (p<0.05). Notably, ALDH1A2 transcript levels were also higher in the 8–9 week human fetal testis than in mesonephroi from the same fetuses (p<0.01), indicating that in humans the gonad, rather than the mesonephros, may be the predominant site of RA synthesis. No significant differences in the expression of ALDH1A3 were detected either within or between sexes at any developmental stage (Figure 1C) although, as with ALDH1A1 and ALDH1A2, expression in both ovary and testis was not lower than in mesonephros. Together, these data support the hypothesis [18] that the human fetal gonad has an intrinsic capacity to produce retinoic acid.


Retinoic Acid signalling and the control of meiotic entry in the human fetal gonad.

Childs AJ, Cowan G, Kinnell HL, Anderson RA, Saunders PT - PLoS ONE (2011)

Expression of genes encoding retinaldehyde dehydrogenase enzymes in the human fetal gonad.qRT-PCR analysis reveals developmentally regulated expression of ALDH1A1 (A) in the human fetal testis, with transcript levels increasing significantly between 8–9 weeks gestation and 14–16/17–20 weeks gestation (ANOVA; a,b,c; p<0.05, n = 5–6 per group). Expression was not significantly different between gonads of different sexes at the same developmental stage, not between ovaries at different gestational ages. ALDH1A2 expression (B) is also developmentally-regulated in the human fetal testis, with transcript levels in the first trimester (8–9 weeks gestation) testis significantly higher than those in the early second trimester (14–16 weeks) testis (a vs b, p<0.05) and the late second trimester (17–20 weeks) testis (a vs c, p<0.01). Expression in the testis at 8–9 weeks gestation was also significantly higher than that in the fetal ovary at the same developmental stage (a vs d, p<0.05). ALDH1A2 transcript levels were also higher in the 8–9 week human fetal testis than in mesonephroi from age-matched fetuses (a vs e, p<0.01), which contrasts with the mesonephric-specific expression of Aldh1a2 in the mouse at a comparable developmental stage. No differences in the expression of ALDH1A3 (C) were detected between samples of different gestational ages of the same sex, or between the gonads of different sexes at the same developmental stage. 8–9, 14–16 and 17–20 denote the gestational age (in weeks) of specimens, meso: 8–9 week mesonephroi (pooled male and female). Values denote mean ± s.e.m..
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pone-0020249-g001: Expression of genes encoding retinaldehyde dehydrogenase enzymes in the human fetal gonad.qRT-PCR analysis reveals developmentally regulated expression of ALDH1A1 (A) in the human fetal testis, with transcript levels increasing significantly between 8–9 weeks gestation and 14–16/17–20 weeks gestation (ANOVA; a,b,c; p<0.05, n = 5–6 per group). Expression was not significantly different between gonads of different sexes at the same developmental stage, not between ovaries at different gestational ages. ALDH1A2 expression (B) is also developmentally-regulated in the human fetal testis, with transcript levels in the first trimester (8–9 weeks gestation) testis significantly higher than those in the early second trimester (14–16 weeks) testis (a vs b, p<0.05) and the late second trimester (17–20 weeks) testis (a vs c, p<0.01). Expression in the testis at 8–9 weeks gestation was also significantly higher than that in the fetal ovary at the same developmental stage (a vs d, p<0.05). ALDH1A2 transcript levels were also higher in the 8–9 week human fetal testis than in mesonephroi from age-matched fetuses (a vs e, p<0.01), which contrasts with the mesonephric-specific expression of Aldh1a2 in the mouse at a comparable developmental stage. No differences in the expression of ALDH1A3 (C) were detected between samples of different gestational ages of the same sex, or between the gonads of different sexes at the same developmental stage. 8–9, 14–16 and 17–20 denote the gestational age (in weeks) of specimens, meso: 8–9 week mesonephroi (pooled male and female). Values denote mean ± s.e.m..
Mentions: In the human fetal testis, ALDH1A1 expression increased significantly across the gestational range examined (p<0.05; Figure 1A). In contrast, there was a trend towards decreasing expression of ALDH1A1in the fetal ovary over the same period (Figure 1A) although this did not reach significance. We found no significant differences in expression of ALDH1A1 between gonads of different sexes of the same gestational age, or between fetal gonads and mesonephroi from 8–9 week fetuses. ALDH1A2 expression in the fetal testis was significantly higher at 8–9 weeks gestation than at 14–16 weeks (p<0.05) or 17–20 weeks gestation (p<0.01. Figure 1B). Furthermore, at 8–9 weeks gestation we detected a sexual dimorphism in ALDH1A2 expression, with transcript levels significantly higher in the testis than the ovary at this developmental stage (p<0.05). Notably, ALDH1A2 transcript levels were also higher in the 8–9 week human fetal testis than in mesonephroi from the same fetuses (p<0.01), indicating that in humans the gonad, rather than the mesonephros, may be the predominant site of RA synthesis. No significant differences in the expression of ALDH1A3 were detected either within or between sexes at any developmental stage (Figure 1C) although, as with ALDH1A1 and ALDH1A2, expression in both ovary and testis was not lower than in mesonephros. Together, these data support the hypothesis [18] that the human fetal gonad has an intrinsic capacity to produce retinoic acid.

Bottom Line: The development of mammalian fetal germ cells along oogenic or spermatogenic fate trajectories is dictated by signals from the surrounding gonadal environment.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.

View Article: PubMed Central - PubMed

Affiliation: Medical Research Council Human Reproductive Sciences Unit, The Queen's Medical Research Institute, University of Edinburgh, Edinburgh, United Kingdom. a.childs@ed.ac.uk

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus