Basic fibroblast growth factor suppresses meiosis and promotes mitosis of ovarian germ cells in embryonic chickens

Selection of reference genes for RT-qPCR analysis in developing chicken embryonic ovary

BACKGROUND

Normalization of the expression profiling of target genes, in a tissue-specific manner and under different experimental conditions, requires stably expressed gene(s) to be used as internal reference(s). However, to study the molecular regulation of oocyte meiosis initiation during ovary development in chicken embryos, stable reference gene(s) still need to be compared and confirmed.

METHODS AND RESULTS

Six candidate genes previously used as internal references for the chicken embryo (Actb, Cvh, Dazl, Eef1a, Gapdh and Rpl15) were chosen, and their expression profiles in left ovaries dissected at five chicken embryonic days (E12.5, E15.5, E17.5, E18.5 and E20.5) were evaluated, respectively. Separately, GeNorm, NormFinder, BestKeeper and Comparative ΔCt methods were used to assess the stability of candidate reference genes, and all results were combined to give the final rank by RefFinder. All methods identified that Eef1a and Rpl15 were the two most stable internal reference genes, whereas Cvh is the most unstable one. Moreover, expression levels of three marker genes for chicken oocyte meiosis entry (Stra8, Scp3 and Dmc1) were normalized, based on Eef1a, Rpl15, or their combinations, respectively.

CONCLUSION

Our findings provide the most suitable internal reference genes (Eef1a and Rpl15), to investigate further molecular regulation of ovary development and oocyte meiosis initiation in chicken embryos.

Expression and localization of meiosis-associated protein in gonads of female rats at different stages

It was well known that a critical process of oogenesis in the female mammalian was the entry of mitotic oogonia into meiosis. Early studies from model animal mice suggested that the retinoic acid (RA) response signal protein STRA8 (stimulated by retinoic acid gene 8) and the meiosis-specific chromosomal behavior marker protein SCP3 (Synaptonemal Complex Protein 3) were two crucial molecular markers during meiosis. The expression of STRA8 and SCP3 at different stages in rat ovaries was investigated by immunohistochemistry, qRT-PCR and Western Blot. Immunohistochemistry results showed that STRA8 and SCP3 were mainly expressed in embryonic stage. And STRA8 was expressed in the cytoplasm and nucleus of the ovaries after birth. qRT-PCR and Western Blot results showed that the mRNA and protein levels of STRA8 and SCP3 were expressed in embryonic stage. The expression of STRA8 and SCP3 indicated germ cells enter meiosis in rats embryo, and STRA8 and SCP3 could serve as molecular markers for the meiosis in rats. The localization of STRA8 in the nucleus increased the possibility that STRA8 might act as transcription factor or activate transcription to function after birth.

Oestrogen in the chick embryo can induce chromosomally male ZZ left gonad epithelial cells to form an ovarian cortex that can support oogenesis

In chickens, the embryonic ovary differentiates into two distinct domains before meiosis: a steroidogenic core (the female medulla), overlain by the germ cell niche (the cortex). The differentiation of the medulla is a cell-autonomous process based on chromosomal sex identity (CASI). In order to address the extent to which cortex differentiation depends on intrinsic or extrinsic factors, we generated models of gonadal intersex by mixing ZW (female) and ZZ (male) cells in gonadal chimeras, or by altering oestrogen levels of ZW and ZZ embryos. We found that CASI does not apply to the embryonic cortex. Both ZW and ZZ cells can form the cortex and this can happen independently of the phenotypic sex of the medulla as long as oestrogen is provided. We also show that the cortex-promoting activity of oestrogen signalling is mediated via estrogen receptor alpha within the left gonad epithelium. However, the presence of a medulla with an 'intersex' or male phenotype may compromise germ cell progression into meiosis, causing cortical germ cells to remain in an immature state in the embryo.

Progesterone regulates chicken embryonic germ cell meiotic initiation independent of retinoic acid signaling

The signaling molecule retinoic acid (RA) is known to trigger germ cells to enter meiosis. However, RA may not be the only secreted inducer of meiosis. Our previous data indicate that luteinizing hormone also promotes germ cell meiotic initiation by upregulating 3βHSDII transcription. Here, using chicken embryos, we investigate the role of progesterone (P4) in regulating germ cell meiotic initiation. Progesterone treatment at embryonic Day 9.5 accelerated germ cell meiosis entry in the female chicken embryos. However, P4 treatment in vivo did no influence on testicular germ cells but triggered their meiotic initiation in the cultured testes. As treatment with an RA receptor (RAR) inhibitor did not block the stimulatory effect of P4 on germ cell meiotic initiation, this P4 stimulatory effect seems to be independent of RAR-mediated signaling. The abundance of RA metabolism-related enzymes and RAR (RARβ) mRNAs did not differ significantly between P4-treated and control individuals. The RA concentration in the ovaries remained unchanged by P4 treatment in vivo. Because no inhibition by the P4 receptor (PR) nuclear receptor antagonist mifepristone on P4 effect was observed in either in vitro or in vivo experiments, the effect of P4 on germ cell meiotic initiation is probably mediated by membrane PRs (mPR). The mPRα, mPRβ, and mPRγ mRNAs were all expressed in the embryonic ovaries. The expression of mPRα and mPRβ was higher than that of mPRγ. Immunohistochemical results showed that mPRα-positive cells were mainly scattered in the ovarian cortex area where most germ cells were distributed. The mPRβ-positive cells were widely distributed in the ovaries, and positive cells were clustered with a similar morphology to that of germ cell clusters. In conclusion, P4 may regulate embryonic germ cell meiotic initiation independent of RA signaling through the membrane PRs. This study provides a new insight into the mechanisms of germ cell meiotic initiation in the chicken.

Gonadotropins regulate ovarian germ cell mitosis/meiosis decision in the embryonic chicken

Gonadotropins are required for gametogenesis but in embryonic gonads this mechanism is not well understood. Here we use chicken embryos to investigate the mechanism that gonadotropins regulate the ovarian germ cell mitosis/meiosis decision. Treatment with follicle-stimulating hormone (FSH) delayed germ cell meiosis entry and promoted their proliferation. This action was blocked by an aromatase inhibitor. Treatment with luteinizing hormone (LH) accelerated germ cell meiosis entry and promoted transcription of 3βHSDII to increase progesterone (P4) production. In the cultured ovaries, P4 triggered meiotic initiation in germ cells. MiR181a, which acts to downregulate the NR6A1 transcript to inhibit the meiotic initiation, was upregulated by FSH and downregulated by LH. Collectively, gonadotropins regulate germ cells mitosis and meiotic initiation through steroid hormones and a miR181a-mediated pathway. In particularly, FSH delays germ cell meiosis entry and promotes cell proliferation via estrogen while LH accelerates the meiotic initiation via elevated P4 production.