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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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Immunohistochemical localisation of retinoid receptor expression in the human fetal gonad.In the second trimester human fetal testis (A) RARα staining was detected in germ cell (GC) and peritubular myoid (PTM) nuclei. Two populations of Sertoli cells (SC; immunopositive and immunonegative) could also be detected. In the fetal ovary at the same developmental stage (B), RARα expression was detected in the nuclei and cytoplasm of germ cells in nests, and in the nuclei of pregranulosa cells (PG) interspersed between germ cells. Mesenchymal somatic cells in streams (CS) displayed variable staining. RARβ expression was widespread in the fetal testis (C) with all major cell populations displaying intense nuclear staining. In contrast, variable RARβ expression was detected in the germ cells of the fetal ovary (D); with some displaying intensely stained nuclei or both nuclear and cytoplasmic staining (solid arrows) and others showing little or no staining (dashed arrows). Pregranulosa cells were immunonegative, as were somatic cells in streams, although some displayed nuclear staining for RARβ (arrowheads). Peritubular myoid and Sertoli cell nuclei in the testes displayed intense staining for RARβ (E), with weaker expression in detected in germ cells. A population of immunonegative IC was also detected. The distribution of immunostaining for RXRα in the fetal ovary (F) was comparable to that of RARβ, with expression restricted to germ cells in nests (GCn) and absent in somatic cell streams and pregranulosa cells. The widespread nuclear localization of RA receptors in testis suggests cells of all types (including germ cells) are exposed to RA signals. Magnification: 400× (A, B), 1000× (C–F).
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pone-0020249-g003: Immunohistochemical localisation of retinoid receptor expression in the human fetal gonad.In the second trimester human fetal testis (A) RARα staining was detected in germ cell (GC) and peritubular myoid (PTM) nuclei. Two populations of Sertoli cells (SC; immunopositive and immunonegative) could also be detected. In the fetal ovary at the same developmental stage (B), RARα expression was detected in the nuclei and cytoplasm of germ cells in nests, and in the nuclei of pregranulosa cells (PG) interspersed between germ cells. Mesenchymal somatic cells in streams (CS) displayed variable staining. RARβ expression was widespread in the fetal testis (C) with all major cell populations displaying intense nuclear staining. In contrast, variable RARβ expression was detected in the germ cells of the fetal ovary (D); with some displaying intensely stained nuclei or both nuclear and cytoplasmic staining (solid arrows) and others showing little or no staining (dashed arrows). Pregranulosa cells were immunonegative, as were somatic cells in streams, although some displayed nuclear staining for RARβ (arrowheads). Peritubular myoid and Sertoli cell nuclei in the testes displayed intense staining for RARβ (E), with weaker expression in detected in germ cells. A population of immunonegative IC was also detected. The distribution of immunostaining for RXRα in the fetal ovary (F) was comparable to that of RARβ, with expression restricted to germ cells in nests (GCn) and absent in somatic cell streams and pregranulosa cells. The widespread nuclear localization of RA receptors in testis suggests cells of all types (including germ cells) are exposed to RA signals. Magnification: 400× (A, B), 1000× (C–F).

Mentions: To establish the cellular targets of retinoid signalling in the human fetal testis and ovary, we performed immunohistochemistry using specific antibodies raised against RARα, RARβ and RXRα on sections of second trimester human fetal ovaries and testes (Figure 3). RARα expression was widely distributed in the second trimester human fetal testis (Figure 3A). Expression was detected in germ cells, which displayed either nuclear or both nuclear and cytoplasmic staining. Sertoli cells were predominantly immunopositive, and displayed strong nuclear staining, although a sub-population of these cells could be identified which did not express RARα. Peritubular myoid (PTM) cells were also mostly immunopositive, with the nucleus the predominant site of receptor localisation in this cell type. Interstitial cells were mostly immunopositive, although a subpopulation which showed no staining was also detectable. RARα expression in the human fetal ovary at a comparable developmental stage was present in germ cells in syncitial clusters (also known as germ cell nests; Figure 3B), and localised to both the cytoplasm and nuclei of these cells. Pregranulosa cells interspersed between germ cells also displayed strong nuclear staining for RARα. Expression of RARα in streams of mesenchymal cells between germ cell nests was variable, with some cells displaying nuclear staining and some being immunonegative.


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)

Immunohistochemical localisation of retinoid receptor expression in the human fetal gonad.In the second trimester human fetal testis (A) RARα staining was detected in germ cell (GC) and peritubular myoid (PTM) nuclei. Two populations of Sertoli cells (SC; immunopositive and immunonegative) could also be detected. In the fetal ovary at the same developmental stage (B), RARα expression was detected in the nuclei and cytoplasm of germ cells in nests, and in the nuclei of pregranulosa cells (PG) interspersed between germ cells. Mesenchymal somatic cells in streams (CS) displayed variable staining. RARβ expression was widespread in the fetal testis (C) with all major cell populations displaying intense nuclear staining. In contrast, variable RARβ expression was detected in the germ cells of the fetal ovary (D); with some displaying intensely stained nuclei or both nuclear and cytoplasmic staining (solid arrows) and others showing little or no staining (dashed arrows). Pregranulosa cells were immunonegative, as were somatic cells in streams, although some displayed nuclear staining for RARβ (arrowheads). Peritubular myoid and Sertoli cell nuclei in the testes displayed intense staining for RARβ (E), with weaker expression in detected in germ cells. A population of immunonegative IC was also detected. The distribution of immunostaining for RXRα in the fetal ovary (F) was comparable to that of RARβ, with expression restricted to germ cells in nests (GCn) and absent in somatic cell streams and pregranulosa cells. The widespread nuclear localization of RA receptors in testis suggests cells of all types (including germ cells) are exposed to RA signals. Magnification: 400× (A, B), 1000× (C–F).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3108594&req=5

pone-0020249-g003: Immunohistochemical localisation of retinoid receptor expression in the human fetal gonad.In the second trimester human fetal testis (A) RARα staining was detected in germ cell (GC) and peritubular myoid (PTM) nuclei. Two populations of Sertoli cells (SC; immunopositive and immunonegative) could also be detected. In the fetal ovary at the same developmental stage (B), RARα expression was detected in the nuclei and cytoplasm of germ cells in nests, and in the nuclei of pregranulosa cells (PG) interspersed between germ cells. Mesenchymal somatic cells in streams (CS) displayed variable staining. RARβ expression was widespread in the fetal testis (C) with all major cell populations displaying intense nuclear staining. In contrast, variable RARβ expression was detected in the germ cells of the fetal ovary (D); with some displaying intensely stained nuclei or both nuclear and cytoplasmic staining (solid arrows) and others showing little or no staining (dashed arrows). Pregranulosa cells were immunonegative, as were somatic cells in streams, although some displayed nuclear staining for RARβ (arrowheads). Peritubular myoid and Sertoli cell nuclei in the testes displayed intense staining for RARβ (E), with weaker expression in detected in germ cells. A population of immunonegative IC was also detected. The distribution of immunostaining for RXRα in the fetal ovary (F) was comparable to that of RARβ, with expression restricted to germ cells in nests (GCn) and absent in somatic cell streams and pregranulosa cells. The widespread nuclear localization of RA receptors in testis suggests cells of all types (including germ cells) are exposed to RA signals. Magnification: 400× (A, B), 1000× (C–F).
Mentions: To establish the cellular targets of retinoid signalling in the human fetal testis and ovary, we performed immunohistochemistry using specific antibodies raised against RARα, RARβ and RXRα on sections of second trimester human fetal ovaries and testes (Figure 3). RARα expression was widely distributed in the second trimester human fetal testis (Figure 3A). Expression was detected in germ cells, which displayed either nuclear or both nuclear and cytoplasmic staining. Sertoli cells were predominantly immunopositive, and displayed strong nuclear staining, although a sub-population of these cells could be identified which did not express RARα. Peritubular myoid (PTM) cells were also mostly immunopositive, with the nucleus the predominant site of receptor localisation in this cell type. Interstitial cells were mostly immunopositive, although a subpopulation which showed no staining was also detectable. RARα expression in the human fetal ovary at a comparable developmental stage was present in germ cells in syncitial clusters (also known as germ cell nests; Figure 3B), and localised to both the cytoplasm and nuclei of these cells. Pregranulosa cells interspersed between germ cells also displayed strong nuclear staining for RARα. Expression of RARα in streams of mesenchymal cells between germ cell nests was variable, with some cells displaying nuclear staining and some being immunonegative.

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