Sites of estrogen receptor and aromatase expression in the chicken embryo

Time- and season-dependent changes in the steroidogenic markers in female tree sparrow (Passer montanus)

Seasonal breeders display elevated sex steroid hormone production during reproductive seasons, resulting in significant physiological and structural alterations. One such seasonal breeder adapted to the changing environment is a Tree sparrow (Passer montanus). The study aims to investigate 24-h rhythmicity and annual variations in the expression of steroidogenic gene markers of adult female tree sparrows. Two experiments were conducted; in experiment one, birds (n = 5 birds/time points) were sampled at six time points, i.e., ZT1, ZT5, ZT9, ZT13, ZT17, and ZT21 (ZT = Zeitgeber time, ZT0 = sunrise time) during the reproductive stage; subsequently, hypothalamus and ovary were harvested for gene expression analysis. In experiment two, birds (n = 5/month) were sampled at mid-day every month for a year. Feather molt, follicular diameter, body mass, and bill coloration were recorded. The hypothalamus and ovary were harvested for gene expression studies. Blood plasma cholesterol and progesterone were also measured. The study indicates a larger follicular size during May and June. Whereas, maximum molt was observed during the post-reproductive phase. Cholesterol levels were highest prior breeding phase and higher progesterone levels paralleled larger follicular size. While higher levels of GnIh (gonadotropin-inhibitory hormone) and Dio3 (type 3 deiodinase) were observed during the non-breeding phase, elevated expression of Tshβ (thyroid stimulating hormone subunit beta), Dio2 (type 2 deiodinase), and GnRh (gonadotropin-releasing hormone) was noted during the reproductive period. The study also reveals 24-h rhythmicity in selected steroidogenic markers (StAR, Nr4a1, Er, Scp2, Cyp17a1, Foxl2, Cyp11a1, Hsd11b2, Cyp11b, Cyp19a1, and Vdac1) and seasonal variations directly influence steroidogenesis, which connects with the annual reproductive cycle.

Mini review: Asymmetric Müllerian duct development in the chicken embryo

Müllerian ducts are paired embryonic tubes that give rise to the female reproductive tract. In humans, the Müllerian ducts differentiate into the Fallopian tubes, uterus and upper portion of the vagina. In birds and reptiles, the Müllerian ducts develop into homologous structures, the oviducts. The genetic and hormonal regulation of duct development is a model for understanding sexual differentiation. In males, the ducts typically undergo regression during embryonic life, under the influence of testis-derived Anti-Müllerian Hormone, AMH. In females, a lack of AMH during embryogenesis allows the ducts to differentiate into the female reproductive tract. In the chicken embryo, a long-standing model for development and sexual differentiation, Müllerian duct development in females in asymmetric. Only the left duct forms an oviduct, coincident with ovary formation only on the left side of the body. The right duct, together with the right gonad, becomes vestigial. The mechanism of this avian asymmetry has never been fully resolved, but is thought to involve local interplay between AMH and sex steroid hormones. This mini-review re-visits the topic, highlighting questions in the field and proposing a testable model for asymmetric duct development. We argue that current molecular and imaging techniques will shed new light on this curious asymmetry. Information on asymmetric duct development in the chicken model will inform our understanding of sexual differentiation in vertebrates more broadly.

Becoming female: Ovarian differentiation from an evolutionary perspective

Differentiation of the bipotential gonadal primordium into ovaries and testes is a common process among vertebrate species. While vertebrate ovaries eventually share the same functions of producing oocytes and estrogens, ovarian differentiation relies on different morphogenetic, cellular, and molecular cues depending on species. The aim of this review is to highlight the conserved and divergent features of ovarian differentiation through an evolutionary perspective. From teleosts to mammals, each clade or species has a different story to tell. For this purpose, this review focuses on three specific aspects of ovarian differentiation: ovarian morphogenesis, the evolution of the role of estrogens on ovarian differentiation and the molecular pathways involved in granulosa cell determination and maintenance.

The Diverse Roles of 17β-Estradiol in Non-Gonadal Tissues and Its Consequential Impact on Reproduction in Laying and Broiler Breeder Hens

Estradiol-17β (E2) has long been studied as the primary estrogen involved in sexual maturation of hens. Due to the oviparous nature of avian species, ovarian production of E2 has been indicated as the key steroid responsible for activating the formation of the eggshell and internal egg components in hens. This involves the integration and coordination between ovarian follicular development, liver metabolism and bone physiology to produce the follicle, yolk and albumen, and shell, respectively. However, the ability of E2 to be synthesized by non-gonadal tissues such as the skin, heart, muscle, liver, brain, adipose tissue, pancreas, and adrenal glands demonstrates the capability of this hormone to influence a variety of physiological processes. Thus, in this review, we intend to re-establish the role of E2 within these tissues and identify direct and indirect integration between the control of reproduction, metabolism, and bone physiology. Specifically, the sources of E2 and its activity in these tissues via the estrogen receptors (ERα, ERβ, GPR30) is described. This is followed by an update on the role of E2 during sexual differentiation of the embryo and maturation of the hen. We then also consider the implications of the recent discovery of additional E2 elevations during an extended laying cycle. Next, the specific roles of E2 in yolk formation and skeletal development are outlined. Finally, the consequences of altered E2 production in mature hens and the associated disorders are discussed. While these areas of study have been previously independently considered, this comprehensive review intends to highlight the critical roles played by E2 to alter and coordinate physiological processes in preparation for the laying cycle.

Sexually dimorphic expression of sexual differentiation genes in the internal genital organs of Japanese quail embryos

Japanese quail (Coturnix japonica) is an avian model used to evaluate the reproductive and developmental toxicity of chemicals. The National Institute for Environmental Studies (NIES) of Japan established a strain of Japanese quail, NIES-L, which may be a better model because of its highly inbred characteristics. To understand sexual differentiation of the reproductive organs and the value of using NIES-L quails for avian toxicity assessment, we profiled estradiol and androgen plasma levels by enzyme-linked immunosorbent assay; the mRNA levels of estrogen receptor-α (ERα), ERβ, and androgen receptor (AR) in the gonads, Müllerian ducts, Wolffian ducts; and the mRNA levels of steroidogenic enzymes, cholesterol side chain cleavage enzyme (P450_scc), 17α-hydroxylase/C_17-20 lyase (P450_{17α, lyase}), 3β-hydroxysteroid dehydrogenase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), and aromatase (P450_arom), anti-Müllerian hormone (AMH), and AMH receptor type 2 (AMHR2) in the gonads of NIES-L Japanese quails on embryonic days 9, 12, and 15 using a real-time quantitative PCR method. The plasma estradiol concentration was higher in females than males on these embryonic days, but no sex difference was found in the plasma androgens. The mRNA levels of all examined steroidogenic enzymes were significantly higher in female than male embryos. In particular, the P450_arom mRNA levels showed a striking sex difference: P450_arom was expressed in female but not male gonads. In contrast, the AMH and AMHR2 mRNA levels in the gonads were higher in males than females. The ERα, ERβ, and AR mRNA levels increased in the left female gonad and peaked on embryonic day 15, but not in the left and right male gonads; therefore, there was a female-biased sex difference. The ERα, ERβ, and AR mRNA levels in the left Müllerian duct, but not in the right Müllerian duct, of females increased and peaked on embryonic day 15, which resulted in asymmetric mRNA levels. The Wolffian ducts expressed ERα, ERβ, and AR in both sexes, and no sex difference or asymmetry of mRNA levels was found. The information obtained from this study helps elucidate the molecular endocrinological basis of sexual dimorphism formation of reproductive organs and clarify the value of NIES-L quails for toxicity assessment.

Genetic Regulation of Avian Testis Development

As in other vertebrates, avian testes are the site of spermatogenesis and androgen production. The paired testes of birds differentiate during embryogenesis, first marked by the development of pre-Sertoli cells in the gonadal primordium and their condensation into seminiferous cords. Germ cells become enclosed in these cords and enter mitotic arrest, while steroidogenic Leydig cells subsequently differentiate around the cords. This review describes our current understanding of avian testis development at the cell biology and genetic levels. Most of this knowledge has come from studies on the chicken embryo, though other species are increasingly being examined. In chicken, testis development is governed by the Z-chromosome-linked DMRT1 gene, which directly or indirectly activates the male factors, HEMGN, SOX9 and AMH. Recent single cell RNA-seq has defined cell lineage specification during chicken testis development, while comparative studies point to deep conservation of avian testis formation. Lastly, we identify areas of future research on the genetics of avian testis development.

An evo-devo perspective of the female reproductive tract

The vertebrate female reproductive tract has undergone considerable diversification over evolution, having become physiologically adapted to different reproductive strategies. This review considers the female reproductive tract from the perspective of evolutionary developmental biology (evo-devo). Very little is known about how the evolution of this organ system has been driven at the molecular level. In most vertebrates, the female reproductive tract develops from paired embryonic tubes, the Müllerian ducts. We propose that formation of the Müllerian duct is a conserved process that has involved co-option of genes and molecular pathways involved in tubulogenesis in the adjacent mesonephric kidney and Wolffian duct. Downstream of this conservation, genetic regulatory divergence has occurred, generating diversity in duct structure. Plasticity of the Hox gene code and wnt signaling, in particular, may underlie morphological variation of the uterus in mammals, and evolution of the vagina. This developmental plasticity in Hox and Wnt activity may also apply to other vertebrates, generating the morphological diversity of female reproductive tracts evident today.

The homeobox gene TGIF1 is required for chicken ovarian cortical development and generation of the juxtacortical medulla

During early embryogenesis in amniotic vertebrates, the gonads differentiate into either ovaries or testes. The first cell lineage to differentiate gives rise to the supporting cells: Sertoli cells in males and pre-granulosa cells in females. These key cell types direct the differentiation of the other cell types in the gonad, including steroidogenic cells. The gonadal surface epithelium and the interstitial cell populations are less well studied, and little is known about their sexual differentiation programs. Here, we show the requirement of the homeobox transcription factor gene TGIF1 for ovarian development in the chicken embryo. TGIF1 is expressed in the two principal ovarian somatic cell populations: the cortex and the pre-granulosa cells of the medulla. TGIF1 expression is associated with an ovarian phenotype in estrogen-mediated sex reversal experiments. Targeted misexpression and gene knockdown indicate that TGIF1 is required, but not sufficient, for proper ovarian cortex formation. In addition, TGIF1 is identified as the first known regulator of juxtacortical medulla development. These findings provide new insights into chicken ovarian differentiation and development, specifically cortical and juxtacortical medulla formation.

Proliferative and Apoptotic Pathways in the Testis of Quail Coturnix coturnix during the Seasonal Reproductive Cycle

Simple Summary

The quail Coturnix coturnix exhibits an annual cycle of testis size, sexual steroid production, and spermatogenesis. The testicular levels of both 17β-estradiol (E₂) and androgens are higher during the reproductive period compared to the non-reproductive period, suggesting that estrogens act in synergy with the androgens for the initiation of spermatogenesis. Therefore, the present study aimed to investigate the estrogen responsive system in quail testis in relation to the reproduction seasons, with a focus on the molecular pathways activated in both active and regressive quail testes. The results indicated that estrogens participated in the activation of mitotic and meiotic events during the reproductive period by activating the ERK1/2 and Akt-1 pathways. In the non-reproductive period, when the E₂/ERα levels are low, ERK1/2 and Akt-1 pathways remain inactive and apoptotic events occur. Our results suggest that the activation or inhibition of these molecular pathways plays a crucial role in the physiological switch “on/off” of the testicular activity in male quail during the seasonal reproductive cycle.

Abstract

The quail Coturnix coturnix is a seasonal breeding species, with the annual reproductive cycle of its testes comprising an activation phase and a regression phase. Our previous results have proven that the testicular levels of both 17β-estradiol (E₂) and androgens are higher during the reproductive period compared to the non-reproductive period, which led us to hypothesize that estrogens and androgens may act synergistically to initiate spermatogenesis. The present study was, therefore, aimed to investigate the estrogen responsive system in quail testis in relation to the reproduction seasonality, with a focus on the molecular pathways elicited in both active and regressive quail testes. Western blotting and immunohistochemistry analysis revealed that the expression of ERα, which is the predominant form of estrogen receptors in quail testis, was correlated with E₂ concentration, suggesting that increased levels of E₂-induced ERα could play a key role in the resumption of spermatogenesis during the reproductive period, when both PCNA and SYCP3, the mitotic and meiotic markers, respectively, were also increased. In the reproductive period we also found the activation of the ERK1/2 and Akt-1 kinase pathways and an increase in second messengers cAMP and cGMP levels. In the non-reproductive phase, when the E2/ERα levels were low, the inactivation of ERK1/2 and Akt-1 pathways favored apoptotic events due to an increase in the levels of Bax and cytochrome C, with a consequent regression of the gonad.

Insights into Gonadal Sex Differentiation Provided by Single-Cell Transcriptomics in the Chicken Embryo

Although the genetic triggers for gonadal sex differentiation vary across species, the cell biology of gonadal development was long thought to be largely conserved. Here, we present a comprehensive analysis of gonadal sex differentiation, using single-cell sequencing in the embryonic chicken gonad during sexual differentiation. The data show that chicken embryonic-supporting cells do not derive from the coelomic epithelium, in contrast to other vertebrates studied. Instead, they derive from a DMRT1⁺/PAX2⁺/WNT4⁺/OSR1⁺ mesenchymal cell population. We find a greater complexity of gonadal cell types than previously thought, including the identification of two distinct sub-populations of Sertoli cells in developing testes and derivation of embryonic steroidogenic cells from a differentiated supporting-cell lineage. Altogether, these results indicate that, just as the genetic trigger for sex differs across vertebrate groups, cell lineage specification in the gonad may also vary substantially.

Assessing the Ability of Developmentally Precocious Estrogen Signaling to Recapitulate Ovarian Transcriptomes and Follicle Dynamics in Alligators from a Contaminated Lake

BACKGROUND

Concern has grown in recent decades over anthropogenic contaminants that interfere with the functioning of endocrine hormones. However, mechanisms connecting developmental processes to pathologies associated with endocrine-disrupting chemical (EDC) exposure are poorly understood in naturally exposed populations.

OBJECTIVES

We sought to a) characterize divergence in ovarian transcriptomic and follicular profiles between alligators originating from a historically EDC-contaminated site, Lake Apopka, and a reference site; b) test the ability of developmentally precocious estrogen exposure to recapitulate site-associated patterns of divergence; and c) test whether treatment with exogenous follicle-stimulating hormone (FSH) is capable of rescuing phenotypes associated with contaminant exposure and/or embryonic estrogen treatment.

METHODS

Alligators eggs were collected from a contaminated site and a reference site, and a subset of eggs from the reference site were treated with estradiol (E2) during embryonic development prior to gonadal differentiation. After hatching, alligators were raised under controlled laboratory settings for 5 months. Juveniles from both sites were divided and treated with exogenous FSH. Histological analyses and RNA-sequencing were conducted to characterize divergence in ovarian follicle dynamics and transcriptomes between sites, between reference and E2-treated animals, and between FSH-treated and nontreated animals.

RESULTS

We observed broad site-of-origin divergence in ovarian transcriptomes and reductions in ovarian follicle density between juvenile alligators from Lake Apopka and the reference site. Treating embryos from the reference site with E2 overwhelmingly recapitulated transcriptional and histological alterations observed in Lake Apopka juveniles. Ovarian phenotypes observed in Lake Apopka alligators or resulting from estrogen treatment were only partially rescued by treatment with exogenous FSH.

DISCUSSION

Recapitulation of ovarian abnormalities by precocious E2 revealed a relatively simple mechanism underlying contaminant-induced pathologies in a historical example of environmental endocrine disruption. Findings reported here support a model where the developmental timing of estrogen signaling has the potential to permanently alter ovarian organization and function. https://doi.org/10.1289/EHP6627.

CYP19A1 (aromatase) dominates female gonadal differentiation in chicken (Gallus gallus) embryos sexual differentiation

Cytochrome P450 Family 19 SubFamily A member 1 (CYP19A1) gene encodes an aromatase which regulates the sexual differentiation in vertebrates by initiating and maintaining 17β-Estradiol (E2) synthesis. Here, we described the spatiotemporal expression pattern of CYP19A1 and its functional role in the embryonic gonad development in amphoteric chickens (Gallus gallus). Results showed that CYP19A1 exhibited a sexually dimorphic expression pattern in female gonads early at embryonic day 5.5 (HH 28) and robustly expressed within the cytoplasm in ovarian medullas. Most importantly, we induced the gonadal sex reversal by ectopically delivering the aromatase inhibitor (AI) or estradiol (E2) into chicken embryos. To further explore the role of CYP19A1 in chicken embryonic sexual differentiation, we successfully developed an effective method to deliver lentiviral particles with CYP19A1 manipulation into chicken embryos via embryonic intravascular injection. The analysis of interference and overexpression of CYP19A1 provided solid evidences that CYP19A1 is both necessary and sufficient to initiate sex differentiation toward female in chicken embryos. Collectively, this work demonstrates that CYP19A1 is a crucial sex differentiation gene in the embryonic development, which provides a foundation for understanding the mechanism of sex determination and differentiation in chickens.

Photoperiod dependent expression of estrogen receptor alpha in testes of Japanese quail: Involvement of Withania somnifera in apoptosis amelioration

Light plays important function in the regulation of reproduction in vertebrates including birds. The prolonged long day length exposure causes reproductively inactive state or photorefractoriness in many avian species including Japanese quail. Withania somnifera (WS) is a medicinal plant known to have beneficial effects on stress and infertility. The study investigates the physiological effect of WS on the light-induced stress in quail mediated by estrogen receptor alpha. Quails were exposed to long day length for three months and then transferred into intermediate day length to make them photorefractory (PR) while controls under natural day length. Administration of Withania somnifera root extract (WSRE) in PR quail induces estrogen and decreases corticosterone in male Japanese quail. Immunoreactivity of ERα decreased in testis of PR quail and increased after oral administration of WSRE compared to control. Expression of ir-Caspase-3 and ir-p53 in the testis increased in PR while decreased in PR + WS. Histologically, seminiferous tubules size decreased in PR whereas increased in PR + WS quails. Scanning electron microscopic study reveals sperms in clusters with proper head and tail in control. In PR quails sperms were few and distorted while WSRE improved the sperm morphology. From the study, it is concluded that during photorefractoriness gonadal regression occurs due to testicular apoptosis which causes stress. WSRE helps to overcome stress and improve reproductive performance via increase in expression of ir-ERα during PR condition. Further, the stress ameliorating effect of WSRE in reducing apoptosis mediated by ir-Caspase-3 and ir-p53 in the testes is clearly evident in Japanese quail.

Transcriptome analysis of genes related to gonad differentiation and development in Muscovy ducks

BACKGROUND

Sex-related genes play a crucial role in gonadal differentiation into testes or ovaries. However, the genetic control of gonadal differentiation in Muscovy ducks remains unknown. Therefore, the objective of our study was to screen new candidate genes associated with ovarian and testicular development.

RESULTS

In this study, 24 males before gonadal differentiation (MB), 24 females before gonadal differentiation (FB), 24 males after gonadal differentiation (MA) and 24 females after gonadal differentiation (FA) were selected from Putian Muscovy ducks, forming 4 groups. RNA-Seq revealed 101.76 Gb of clean reads and 2800 differentially expressed genes (DEGs), including 46 in MB vs FB, 609 in MA vs FA, 1027 in FA vs FB, and 1118 in MA vs MB. A total of 146 signalling pathways were enriched by KEGG analysis, among which 20, 108, 108 and 116 signalling pathways were obtained in MB vs FB, MA vs MB, MA vs FA and FA vs FB, respectively. In further GO and KEGG analyses, a total of 21 candidate genes related to gonad differentiation and development in Muscovy ducks were screened. Among these, 9 genes were involved in the differentiation and development of the testes, and 12 genes were involved in the differentiation and development of the ovaries. In addition, RNA-Seq data revealed 2744 novel genes.

CONCLUSIONS

RNA-Seq data revealed 21 genes related to gonadal differentiation and development in Muscovy ducks. We further identified 12 genes, namely, WNT5B, HTRA3, RSPO3, BMP3, HNRNPK, NIPBL, CREB3L4, DKK3, UBE2R2, UBPL3KCMF1, ANXA2, and OSR1, involved in the differentiation and development of ovaries. Moreover, 9 genes, namely, TTN, ATP5A1, DMRT1, DMRT3, AMH, MAP3K1, PIK3R1, AGT and ADAMTSL1, were related to the differentiation and development of testes. Moreover, after gonadal differentiation, DMRT3, AMH, PIK3R1, ADAMTSL1, AGT and TTN were specifically highly expressed in males. WNT5B, ANXA2 and OSR1 were specifically highly expressed in females. These results provide valuable information for studies on the sex control of Muscovy ducks and reveal novel candidate genes for the differentiation and development of testes and ovaries.

Effect of estrogen and stress on estrogen receptor 1 in the HPG axis of immature male Gallus gallus domesticus: Involvement of anti-oxidant system

Estrogen plays a key role in the regulation of reproductive behavior and control of the neuroendocrine system in both males and females. However, excessive quantity of exogenous estrogen produces a deleterious effect on the male reproductive system. To elucidate the mechanism by which estrogen modulates its receptor alpha (ESR1) in immature chicken during stress the study has been undertaken. The experiment investigated the physiological changes in the abundance of ESR1 in brain, pituitary and testes of immature male chickens after stress like water restriction. Twenty four immature male chickens were randomly assigned into four groups. The control group was provided with food and water ad libitum, second was water restricted 9 h each day for seven days (WR), third was treated with estradiol benzoate (EB) and fourth group was treated with EB followed by water restriction during last seven days of treatment (EB + WR). EB was administered at a dose of 0.5 mg/100 g/day for 12 days. EB administration as well as WR increases both the H₂O₂ and Malondialdehyde levels indicating oxidative stress in brain as well as in testis. Plasma corticosterone significantly increased in all groups while estradiol significantly decreased after water restriction. ESR1 protein was detected by immuno-fluorescence predominantly in the pre-optic area of the hypothalamus, pituitary and testes after EB administration. EB administration increases ESR1 proteins abundantly in the Sertoli cells, Leydig cells, spermatogonia and spermatids while WR decreases it. The decline in ESR1 proteins after EB administration during stress appears to be mediated by interaction of estrogen with hypothalamo-pituitary-adrenal (HPA) axis. Therefore, the findings substantiate the fact that WR and EB treatment increase the stress and alter the anti-oxidant enzymes via its receptor ESR1 in the brain, pituitary and testis of immature chicks. Moreover, these findings highlight the effect of estradiol in male chicks causing stress which is disrupting the normal physiological feedback mechanism in hormone release and the expression of receptor ESR1 along the hypothalamo-pituitary-gonadal (HPG) axis.

Comprehensive analysis of miRNAs, lncRNAs, and mRNAs reveals potential players of sexually dimorphic and left-right asymmetry in chicken gonad during gonadal differentiation

Despite thousands of sex-biased genes being found in chickens, the genetic control of sexually dimorphic and left-right asymmetry during gonadal differentiation is not yet completely understood. This study aimed to identify microRNAs (miRNAs), long noncoding RNAs (lncRNAs), messenger RNAs (mRNAs), and signaling pathways during gonadal differentiation in chick embryos (day 6/stage 29). The left and right gonads were collected for RNA sequencing. Sex-biased, side-biased miRNAs, lncRNAs, mRNAs, and shared differentially expressed miRNAs (DEmiRNA)–differentially expressed mRNAs (DEmRNA)–differentially expressed lncRNAs (DElncRNA) interaction networks were performed. A total of 8 DEmiRNAs, 183 DElncRNAs, and 123 DEmRNAs were identified for the sex-biased genes, and 7 DEmiRNAs, 189 DElncRNAs, and 183 DEmRNAs for the side-biased genes. The results of quantitative real-time PCR were generally consistent with the RNA-sequencing results. The study suggested that miRNAs and lncRNAs regulation were novel gene-specific dosage compensation mechanism and they could contribute to left-right asymmetry of chicken, but sex-biased and side-biased miRNAs, lncRNAs, and mRNAs were independent of each other. The competing endogenous RNA (ceRNA) networks showed that 17 target pairs including miR-7b (CYP19A1, FSHR, GREB1, STK31, CORIN, and TDRD9), miR-211 (FSHR, GREB1, STK31, CORIN, and TDRD9), miR-204 (FSHR, GREB1, CORIN, and TDRD9), and miR-302b-5p (CYP19A1 and TDRD9) may play crucial roles in ovarian development. These analyses provide new clues to uncover molecular mechanisms and signaling networks of ovarian development.

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.

In ovo metabolism of estradiol to estrone sulfate in chicken eggs: Implications for how yolk estradiol influences embryonic development

The steroid 17β-estradiol (herein "estradiol") is a potent regulator of sexual differentiation that exerts wide-ranging effects on the developing brain and other tissues. The developing gonads are an important source of estradiol but most, if not all, vertebrate embryos are also exposed to maternally derived estradiol during development. In birds, this maternally derived estradiol is present in the egg at the time of oviposition but very little is known about how this source of estradiol influences development. A critical aspect of understanding yolk estradiol effects is deciphering how steroid metabolism may regulate embryonic exposure to yolk estradiol. In this study, we examine the metabolic fate of estradiol during the first five days of incubation in chicken (Gallus gallus) eggs. Using tritiated estradiol to trace the movement and metabolism of estradiol, we demonstrate that estradiol is metabolized to estrone, which is subsequently conjugated to estrone sulfate as the primary metabolite. Estrone sulfate then accumulates in the albumen by day five of incubation. Overall, these findings have important implications for how yolk estradiol may influence development and alter offspring phenotype. Mechanisms through which estradiol, as well as estrone sulfate, might elicit effects are discussed.

Reproductive management of poultry

Based on data from the UN's Food and Agricultural Organization, about 120 million metric tons of poultry meat were produced globally in 2016. In addition, about 82 million metric tons of eggs were produced. One of the bases for this production is the reproductive efficiency of today's poultry. This, in turn, is due to their inherent reproductive physiology, intensive genetic selection and advances in husbandry/management. The system of reproduction in males in largely similar to that in mammals except that there is no descent of testes. In females, there are marked differences with there being a single ovary and oviduct; the latter being the name of the differentiated entire Müllerian duct. Moreover, females produce eggs with a yolky oocyte surrounded by albumen, membranes and shell. Among the most successful reproductive management techniques are optimizing photoperiod, light intensity and nutrition. Widespread employment of these has allowed maximizing production. Laying hens can be re-cycled toward the end egg production. Other aspects of reproductive management in poultry include the following: artificial insemination (almost exclusively employed in turkeys) and approaches to reduce broodiness together with cage free (colony), conventional, enriched and free-range systems.

Steroid receptors and their regulation in avian extraembryonic membranes provide a novel substrate for hormone mediated maternal effects

Exposure of the vertebrate embryo to maternal hormones can have long-lasting effects on its phenotype, which has been studied extensively by experimentally manipulating maternal steroids, mostly androgens, in bird eggs. Yet, there is a severe lack of understanding of how and when these effects are actually mediated, hampering both underlying proximate and ultimate explanations. Here we report a novel finding that the embryo expresses androgen receptor (AR) and estrogen receptor (ERα) mRNA in its extraembryonic membranes (EMs) as early as before its own hormone production starts, suggesting a novel substrate for action of maternal hormones on the offspring. We also report the first experimental evidence for steroid receptor regulation in the avian embryo in response to yolk steroid levels: the level of AR is dependent on yolk androgen levels only in the EMs but not in body tissues, suggesting embryonic adaptation to maternal hormones. The results also solve the problem of uptake of lipophilic steroids from the yolk, why they affect multiple traits, and how they could mediate maternal effects without affecting embryonic sexual differentiation.

Determination of testicular estrogen receptor alpha expression of male chickens (Gallus domesticus) with age

Background and Aim:

Estrogen activity, a central component of reproductive growth, is regulated by the receptor proteins, estrogen receptor alpha (ERα), and ER beta (ERβ) in chickens as in many other species. ERα expresses predominantly in gonads. Although the expression of ERα in embryonic gonads has been studied in detail, the expression of ERα in post-hatching male gonads has not been studied adequately. Therefore, the current research was conducted to determine the post-hatching changes in the expression of ERα in the left gonads of male chickens with age.

Materials and Methods:

Shaver Brown male chickens were raised and cared for according to the management guide and sacrificed at the intervals of 1, 4, and 8 weeks of age. The total RNA was extracted from the left gonads using the Trizol method and reverse transcribed using a pair of gene-specific primers. Following polymerase chain reaction amplification, the expression of ERα was quantified relative to the expression of the reference gene GAPDH.

Results:

The results showed that ERα expression significantly increases with age at p=0.0032. However, the increment of ERα expression from week 1 to week 4 was 2.04-fold and from week 4 to week 8 was 1.39-fold, with the later age reflecting a diminishing pattern in the increment.

Conclusion:

These results differentiate the post-hatching ERα expression of the left gonads of male chickens increase with age but with a diminishing gradient that may support their reproductive functions in later stages of life.

Early embryonic modification of maternal hormones differs systematically among embryos of different laying order: A study in birds

Vertebrate embryos are exposed to maternal hormones that can profoundly affect their later phenotype. Although it is known that the embryo can metabolize these maternal hormones, the metabolic outcomes, their quantitative dynamics and timing are poorly understood. Moreover, it is unknown whether embryos can adjust their metabolic activity to, for example, hormones or other maternal signals. We studied the dynamics of maternal steroids in fertilized and unfertilized rock pigeon eggs during early incubation. Embryos of this species are naturally exposed to different amounts of maternal steroids in the egg according to their laying position, which provides a natural context to study differential embryonic regulation of the maternal signals. We used mass spectrometric analyses to map changes in the androgen and estrogen pathways of conversion. We show that the active hormones are heavily metabolized only in fertilized eggs, with a corresponding increase in supposedly less potent metabolites already within one-fourth of total incubation period. Interestingly, the rate of androgen metabolism was different between embryos in different laying positions. The results also warrant a re-interpretation of the timing of hormone mediated maternal effects and the role of the supposedly biologically inactive metabolites. Furthermore, the results also provide a potential solution as to how the embryo can prevent maternal steroids in the egg from interfering with its sexual differentiation processes as we show that the embryo can metabolize most of the maternal steroids before sexual differentiation starts.

Gonadal and Endocrine Analysis of a Gynandromorphic Chicken

Birds have a ZZ male and ZW female sex chromosome system. The relative roles of genetics and hormones in regulating avian sexual development have been revealed by studies on gynandromorphs. Gynandromorphs are rare bilateral sex chimeras, male on one side of the body and female on the other. We examined a naturally occurring gynandromorphic chicken that was externally male on the right side of the body and female on the left. The bird was diploid but with a mix of ZZ and ZW cells that correlated with the asymmetric sexual phenotype. The male side was 96% ZZ, and the female side was 77% ZZ and 23% ZW. The gonads of this bird at sexual maturity were largely testicular. The right gonad was a testis, with SOX9+ Sertoli cells, DMRT1+ germ cells, and active spermatogenesis. The left gonad was primarily testicular, but with some peripheral aromatase-expressing follicles. The bird had low levels of serum estradiol and high levels of testosterone, as expected for a male. Despite the low percentage of ZW cells on that side, the left side had female sex-linked feathering, smaller muscle mass, smaller leg and spur, and smaller wattle than the male side. This indicates that these sexually dimorphic structures must be at least partly independent of sex steroid effects. Even a small percentage of ZW cells appears sufficient to support female sexual differentiation. Given the lack of chromosome-wide dosage compensation in birds, various sexually dimorphic features may arise due to Z-gene dosage differences between the sexes.

Gene expression profiling in embryonic chicken ovary during asymmetric development

The reproductive system in female birds arises as bilateral asymmetrical anlagen, excluding the birds of prey. Earlier, histological and messenger RNA (mRNA) expression profile studies of several genes related to gonadal sex differentiation in chicken embryos tried to elucidate the query of this asymmetry in a scattered manner. To understand the matter precisely, we have focused on mRNA expression of a cohort of genes (FSHR, CYP19A1, caspase 3, caspase 8) in second half of the embryonic days (E10-E18). The established role of leptin in development of the embryo and its expression in the embryonic ovary also drove us to check leptin receptor (LEPR) expression in the ovary. Increased expression of FSHR and CYP19A1 in the left ovary compared with that in the right ovary was identified (P < 0.05), promoting preferential left ovarian development and functionality. Significant high expression (P < 0.05) of the apoptotic genes in the right ovary were also involved here. Leptin probably has no direct influence on ovarian asymmetry as no significant variation in gonadal mRNA expression of LEPR was observed within the same experimental days. We propose that asymmetric expression of this cohort of genes (FSHR, CYP19A1, caspase 3, caspase 8) leads to the development of dimorphic gonads during embryogenesis.

Sex Reversal and Comparative Data Undermine the W Chromosome and Support Z-linked DMRT1 as the Regulator of Gonadal Sex Differentiation in Birds

The exact genetic mechanism regulating avian gonadal sex differentiation has not been completely resolved. The most likely scenario involves a dosage mechanism, whereby the Z-linked DMRT1 gene triggers testis development. However, the possibility still exists that the female-specific W chromosome may harbor an ovarian determining factor. In this study, we provide evidence that the universal gene regulating gonadal sex differentiation in birds is Z-linked DMRT1 and not a W-linked (ovarian) factor. Three candidate W-linked ovarian determinants are HINTW, female-expressed transcript 1 (FET1), and female-associated factor (FAF). To test the association of these genes with ovarian differentiation in the chicken, we examined their expression following experimentally induced female-to-male sex reversal using the aromatase inhibitor fadrozole (FAD). Administration of FAD on day 3 of embryogenesis induced a significant loss of aromatase enzyme activity in female gonads and masculinization. However, expression levels of HINTW, FAF, and FET1 were unaltered after experimental masculinization. Furthermore, comparative analysis showed that FAF and FET1 expression could not be detected in zebra finch gonads. Additionally, an antibody raised against the predicted HINTW protein failed to detect it endogenously. These data do not support a universal role for these genes or for the W sex chromosome in ovarian development in birds. We found that DMRT1 (but not the recently identified Z-linked HEMGN gene) is male upregulated in embryonic zebra finch and emu gonads, as in the chicken. As chicken, zebra finch, and emu exemplify the major evolutionary clades of birds, we propose that Z-linked DMRT1, and not the W sex chromosome, regulates gonadal sex differentiation in birds.

Effects of selective and combined activation of estrogen receptor α and β on reproductive organ development and sexual behaviour in Japanese quail (Coturnix japonica)

Excess estrogen exposure of avian embryos perturbs reproductive organ development in both sexes and demasculinizes the reproductive behaviors of adult males. We have previously shown that these characteristic effects on the reproductive organs also can be induced by exposure of Japanese quail (Coturnix japonica) embryos to selective agonists of estrogen receptor alpha (ERα). In contrast, the male copulatory behavior is only weakly affected by developmental exposure to an ERα agonist. To further elucidate the respective roles of ERα and ERβ in estrogen-induced disruption of sexual differentiation, we exposed Japanese quail embryos in ovo to the selective ERα agonist 16α-lactone-estradiol (16αLE₂), the selective ERβ agonist WAY-200070, or both substances in combination. The ERα agonist feminized the testes in male embryos and reduced cloacal gland size in adult males. Furthermore, anomalous retention and malformations of the Müllerian ducts/oviducts were seen in embryos and juveniles of both sexes. The ERβ agonist did not induce any of these effects and did not influence the action of the ERα agonist. Male copulatory behavior was not affected by embryonic exposure to either the ERα- or the ERβ-selective agonist but was slightly suppressed by treatment with the two compounds combined. Our results suggest that the reproductive organs become sexually differentiated consequent to activation of ERα by endogenous estrogens; excessive activation of ERα, but not ERβ, during embryonic development may disrupt this process. Our results also suggest that the demasculinizing effect of estrogens on male copulatory behavior is only partly mediated by ERα and ERβ, and may rather involve other estrogen-responsive pathways.

Gene Expression Profiling Reveals Potential Players of Left-Right Asymmetry in Female Chicken Gonads

Most female birds develop only a left ovary, whereas males develop bilateral testes. The mechanism underlying this process is still not completely understood. Here, we provide a comprehensive transcriptional analysis of female chicken gonads and identify novel candidate side-biased genes. RNA-Seq analysis was carried out on total RNA harvested from the left and right gonads on embryonic day 6 (E6), E12, and post-hatching day 1 (D1). By comparing the gene expression profiles between the left and right gonads, 347 differentially expressed genes (DEGs) were obtained on E6, 3730 were obtained on E12, and 2787 were obtained on D1. Side-specific genes were primarily derived from the autosome rather than the sex chromosome. Gene ontology and pathway analysis showed that the DEGs were most enriched in the Piwi-interactiing RNA (piRNA) metabolic process, germ plasm, chromatoid body, P granule, neuroactive ligand-receptor interaction, microbial metabolism in diverse environments, and methane metabolism. A total of 111 DEGs, five gene ontology (GO) terms, and three pathways were significantly different between the left and right gonads among all the development stages. We also present the gene number and the percentage within eight development-dependent expression patterns of DEGs in the left and right gonads of female chicken.

Overexpression of Anti-Müllerian Hormone Disrupts Gonadal Sex Differentiation, Blocks Sex Hormone Synthesis, and Supports Cell Autonomous Sex Development in the Chicken

The primary role of Anti-Müllerian hormone (AMH) during mammalian development is the regression of Müllerian ducts in males. This highly conserved function is retained in birds and is supported by the high levels of AMH expression in developing testes. Mammalian AMH expression is regulated by a combination of transcription factors, the most important being Sry-type high-mobility-group box transcription factor-9 (SOX9). In the chicken embryo, however, AMH mRNA expression precedes that of SOX9, leading to the view that AMH may play a more central role in avian testicular development. To define its role in chicken gonadal development, AMH was overexpressed using the RCASBP viral vector. AMH caused the gonads of both sexes to develop as small and undeveloped structures at both embryonic and adult stages. Molecular analysis revealed that although female gonads developed testis-like cords, gonads lacked Sertoli cells and were incapable of steroidogenesis. A similar gonadal phenotype was also observed in males, with a complete loss of both Sertoli cells, disrupted SOX9 expression and gonadal steroidogenesis. At sexual maturity both sexes showed a female external phenotype but retained sexually dimorphic body weights that matched their genetic sexes. These data suggest that AMH does not operate as an early testis activator in the chicken but can affect downstream events, such as sex steroid hormone production. In addition, this study provides a unique opportunity to assess chicken sexual development in an environment of sex hormone deficiency, demonstrating the importance of both hormonal signaling and direct cell autonomous factors for somatic sex identity in birds.

Transgenic Chickens Overexpressing Aromatase Have High Estrogen Levels but Maintain a Predominantly Male Phenotype

Estrogens play a key role in sexual differentiation of both the gonads and external traits in birds. The production of estrogen occurs via a well-characterized steroidogenic pathway, which is a multistep process involving several enzymes, including cytochrome P450 aromatase. In chicken embryos, the aromatase gene (CYP19A1) is expressed female-specifically from the time of gonadal sex differentiation. Ectopic overexpression of aromatase in male chicken embryos induces gonadal sex reversal, and male embryos treated with estradiol become feminized; however, this is not permanent. To test whether a continuous supply of estrogen in adult chickens could induce stable male to female sex reversal, 2 transgenic male chickens overexpressing aromatase were generated using the Tol2/transposase system. These birds had robust ectopic aromatase expression, which resulted in the production of high serum levels of estradiol. Transgenic males had female-like wattle and comb growth and feathering, but they retained male weights, displayed leg spurs, and developed testes. Despite the small sample size, this data strongly suggests that high levels of circulating estrogen are insufficient to maintain a female gonadal phenotype in adult birds. Previous observations of gynandromorph birds and embryos with mixed sex chimeric gonads have highlighted the role of cell autonomous sex identity in chickens. This might imply that in the study described here, direct genetic effects of the male chromosomes largely prevailed over the hormonal profile of the aromatase transgenic birds. This data therefore support the emerging view of at least partial cell autonomous sex development in birds. However, a larger study will confirm this intriguing observation.

Anti-Müllerian Hormone Is Required for Chicken Embryonic Urogenital System Growth but Not Sexual Differentiation

In mammals, the primary role of anti-Müllerian hormone (AMH) during development is the regression of Müllerian ducts in males. These structures otherwise develop into fallopian tubes, oviducts, and upper vagina, as in females. This highly conserved function is retained in birds and is supported by the high levels of AMH expression in developing testes. In mammals, AMH expression is controlled partly by the transcription factor, SOX9. However, in the chicken, AMH mRNA expression precedes that of SOX9 , leading to the view that AMH may lie upstream of SOX9 and play a more central role in avian testicular development. To help define the role of AMH in chicken gonad development, we suppressed AMH expression in chicken embryos using RNA interference. In males, AMH knockdown did not affect the expression of key testis pathway genes, and testis cords developed normally. However, a reduction in the size of the mesonephros and gonads was observed, a phenotype that was evident in both sexes. This growth defect occurred as a result of the reduced proliferative capacity of the cells of these tissues, and male gonads also had a significant reduction in germ cell numbers. These data suggest that although AMH does not directly contribute to testicular or ovarian differentiation, it is required in a sex-independent manner for proper cell proliferation and urogenital system growth.

Identification of candidate gonadal sex differentiation genes in the chicken embryo using RNA-seq

BACKGROUND

Despite some advances in recent years, the genetic control of gonadal sex differentiation during embryogenesis is still not completely understood. To identify new candidate genes involved in ovary and testis development, RNA-seq was used to define the transcriptome of embryonic chicken gonads at the onset of sexual differentiation (day 6.0/stage 29).

RESULTS

RNA-seq revealed more than 1000 genes that were transcribed in a sex-biased manner at this early stage of gonadal differentiation. Comparison with undifferentiated gonads revealed that sex biased expression was derived primarily from autosomal rather than sex-linked genes. Gene ontology and pathway analysis indicated that many of these genes encoded proteins involved in extracellular matrix function and cytoskeletal remodelling, as well as tubulogenesis. Several of these genes are novel candidate regulators of gonadal sex differentiation, based on sex-biased expression profiles that are altered following experimental sex reversal. We further characterised three female-biased (ovarian) genes; calpain-5 (CAPN5), G-protein coupled receptor 56 (GPR56), and FGFR3 (fibroblast growth factor receptor 3). Protein expression of these candidates in the developing ovaries suggests that they play an important role in this tissue.

CONCLUSIONS

This study provides insight into the earliest steps of vertebrate gonad sex differentiation, and identifies novel candidate genes for ovarian and testicular development.

Differential expression of estrogen receptor alpha in the embryonic adrenal-kidney-gonadal complex of the oviparous lizard, Calotes versicolor (Daud.)

Estrogen signalling is critical for ovarian differentiation in reptiles with temperature-dependent sex determination (TSD). To elucidate the involvement of estrogen in this process, adrenal-kidney-gonadal (AKG) expression of estrogen receptor (ERα) was studied at female-producing temperature (FPT) in the developing embryos of the lizard, Calotes versicolor which exhibits a distinct pattern of TSD. The eggs of this lizard were incubated at 31.5±0.5°C (100% FPT). The torso of embryos containing adrenal-kidney-gonadal complex (AKG) was collected during different stages of development and subjected to Western blotting and immunohistochemistry analysis. The ERα antibody recognized two protein bands with apparent molecular weight ∼55 and ∼45kDa in the total protein extracts of embryonic AKG complex of C. versicolor. The observed results suggest the occurrence of isoforms of ERα. The differential expression of two different protein isoforms may reveal their distinct role in cell proliferation during gonadal differentiation. This is the first report to reveal two isoforms of the ERα in a reptile during development. Immunohistochemical studies reveal a weak, but specific, cytoplasmic ERα immunostaining exclusively in the AKG during late thermo-sensitive period suggesting the responsiveness of AKG to estrogens before gonadal differentiation at FPT. Further, cytoplasmic as well as nuclear expression of ERα in the medulla and in oogonia of the cortex (faint activity) at gonadal differentiation stage suggests that the onset of gonadal estrogen activity coincides with sexual differentiation of gonad. Intensity and pattern of the immunoreactions of ERα in the medullary region at FPT suggest endogenous production of estrogen which may act in a paracrine fashion to induce neighboring cells into ovarian differentiation pathway.

DMRT1 is required for Müllerian duct formation in the chicken embryo

DMRT1 is a conserved transcription factor with a central role in gonadal sex differentiation. In all vertebrates studied, DMRT1 plays an essential function in testis development and/or maintenance. No studies have reported a role for DMRT1 outside the gonads. Here, we show that DMRT1 is expressed in the paired Müllerian ducts in the chicken embryo, where it is required for duct formation. DMRT1 mRNA and protein are expressed in the early forming Müllerian ridge, and in cells undergoing an epithelial to mesenchyme transition during duct morphogenesis. RNAi-mediated knockdown of DMRT1 in ovo causes a greatly reduced mesenchymal layer, which blocks caudal extension of the duct luminal epithelium. Critical markers of Müllerian duct formation in mammals, Pax2 in the duct epithelium and Wnt4 in the mesenchyme, are conserved in chicken and their expression disrupted in DMRT1 knockdown ducts. We conclude that DMRT1 is required for the early steps of Müllerian duct development. DMRT1 regulates Müllerian ridge and mesenchyme formation and its loss blocks caudal extension of the duct. While DMRT1 plays an important role during testis development and maintenance in many vertebrate species, this is the first report showing a requirement for DMRT1 in Müllerian duct development.

Novel genes and hormonal regulation for gonadal development during embryogenesis in chickens

Asymmetrical gonadal morphogenesis is well known in female chickens in contrast to males where both gonads develop symmetrically. However, only a few genes have been reported to determine differential morphology between female and male gonads in chicken and their mechanisms of action are unclear. Therefore, we focused on three genes (TOM1L1, TTR, and ZEB1) that are related to cellular proliferation and embryonic development based on previous study indicating up- or down-regulated transcripts in the asymmetric female gonads between embryonic day 6 (E6) and E9 by microarray analyses. To define the validity of the gene expression pattern discovered, q-PCR and in situ hybridization analyses were performed. In the left female gonad between E6 and E9 the expression of TOM1L1, TTR and ZEB1 increased at E9. On the other hand, TOM1L1 and TTR increased significantly in both male gonads between E6 and E9. In addition, recombinant FSH and LH stimulated proliferation of gonadal cells and influenced expression of selected genes in chickens. This suggests that hormonal regulation is involved in growth and development in the embryonic gonad of chickens. Collectively, the results show differential gene expression between the left and right gonads in chicken embryos and that of is regulated by gonadotropin. These results provide novel insights into candidate genes regulating gonad development and differentiation.

Microanatomical Study of Embryonic Gonadal Development in Japanese Quail (Coturnix japonica)

Gonadal development of quail embryos was examined histologically using histological and histochemical methods. In the present study, quail embryos were studied at various stages of incubation period based on phases of gonadogenesis. Germ cell migration was observed on day 3-4 but gonadal differentiation and gonadal function were observed on day 6-8 and day 11-14, respectively. During germ cell migration, quail primordial germ cells (qPGCs) were successfully detected in both left and right genital ridges as well as the dorsal mesentery by lectin histochemistry. Unexpectedly, qPGCs-like cells were found next to the neural tube by Mallory-AZAN stain. During gonadal differentiation, embryonic sex can be distinguished histologically since day 8 of incubation. Embryonic testis exhibited a thin cortex, whereas embryonic ovary exhibited a thick cortex. Testicular cord formation was found in the medulla of embryonic testes while the lacunae and fat-laden cells were found in the medulla of embryonic ovary during gonadal function. This is the first report on a comparison of phases of gonadogenesis and histochemical study of quail embryonic gonads in both sexes.

Synergistic effect of fadrozole and insulin-like growth factor-I on female-to-male sex reversal and body weight of broiler chicks

The aim of this study was to investigate the effects of Fadrozole hydrochloride and recombinant human insulin-like growth factor I (rhIGF-I) on female-to-male sex reversal, hatching traits, and body weight of broiler chickens. On the third day of incubation, fertile eggs were randomly assigned to five experimental groups comprising (i) Fadrozole (0.1 mg/egg), (ii) rhIGF-I (100 ng/egg), (iii) Fadrozole (0.1 mg/egg) + rhIGF-I (100 ng/egg), (iv) vehicle injection (10 mM acetic acid and 0.1% BSA), and (v) non-injected eggs. Eggs in the rhIGF-I-injected groups showed the mode of hatching time at the 480th hour of incubation, 12 hours earlier compared to the other groups, with no statistically significant difference in mortality and hatchability. On Day 1 and 42 of production, 90% of genetically female chicks were masculinized using Fadrozole treatment, while 100% female-to-male phenotypic sex reversal was observed in the Fadrozole+rhIGF-I group. Fadrozole equalized the body weight of both genders, although rhIGF-I was effective on the body weight of male chicks only. Interestingly, combined rhIGF-I and Fadrozole could increase the body weight in both sexes compared to the individual injections (P<0.05). These findings revealed that (i) IGF-I-treated chicken embryos were shown to be an effective option for overcoming the very long chicken deprivation period, (ii) the simultaneous treatment with Fadrozole and IGF-I could maximize the female-to-male sex reversal chance, (iii) the increase in the body weight of masculinized chickens via Fadrozole could be equal to their genetically male counterparts, and (iv) the IGF-I effectiveness, specifically along with the application of aromatase inhibitors in female chicks, indicates that estrogen synthesis could be a stumbling block for the IGF-I action mechanism in female embryos.

Development of retroviral vectors for tissue-restricted expression in chicken embryonic gonads

The chicken embryo has long been a useful model organism for studying development, including sex determination and gonadal differentiation. However, manipulating gene expression specifically in the embryonic avian gonad has been difficult. The viral vector RCASBP can be readily used for embryo-wide transgene expression; however global mis-expression using this method can cause deleterious off-target effects and embryo-lethality. In an attempt to develop vectors for the over-expression of sequences in chicken embryonic urogenital tissues, the viral vector RCANBP was engineered to contain predicted promoter sequences of gonadal-expressed genes. Several promoters were analysed and it was found that although the SF1 promoter produced a tissue-restricted expression pattern that was highest in the mesonephros and liver, it was also higher in the gonads compared to the rest of the body. The location of EGFP expression from the SF1 promoter overlapped with several key gonad-expressed sex development genes; however expression was generally low-level and was not seen in all gonadal cells. To further validate this sequence the key testis determinant DMRT1 was over-expressed in female embryos, which due to insufficient levels had no effect on gonad development. The female gene aromatase was then over-expressed in male embryos, which disrupted the testis pathway as demonstrated by a reduction in AMH protein. Taken together, although these data showed that the SF1 promoter can be used for functional studies in ovo, a stronger promoter sequence would likely be required for the functional analysis of gonad genes that require high-level expression.

Localization of apoptotic and proliferating cells and mRNA expression of caspases and Bcl-2 in gonads of chicken embryos

The aim of the present study was to analyze participation of apoptosis and proliferation in gonadal development in the chicken embryo by: (1) localization of apoptotic (TUNEL) and proliferating (PCNA immunoassay) cells in male and female gonads and (2) examination of mRNA expression (RT-PCR) of caspase-3, caspase-6 and Bcl-2 in the ovary and testis during the second half of embryogenesis and in newly hatched chickens. Apoptotic cells were found in gonads of both sexes. At E18 the percentage of apoptotic cells (the apoptotic index, AI) in the ovarian medulla and the testis was lower (p<0.05) than in the ovarian cortex. In the ovarian medulla, the AI at E18 was lower (p<0.05) than on E12. In the testis, the AI was significantly lower (p<0.05) at E18 than at E15 and 1D. The percentage of proliferating cells (the proliferation index: PI) within the ovary significantly increased from E15 to 1D in the cortex, while proliferating cells in the medulla were detected only at E15. In the testis, the PI gradually increased from E12 to 1D. The mRNA expression of caspase-3 and -6 as well as Bcl-2 was detected in male and female gonads at days 12 (E12), 15 (E15) and 18 (E18) of embryogenesis and the day after hatching (1D). The expression of all analyzed genes on E12 was significantly higher (p<0.05) in female than in male gonads. This difference was also observed at E15 and E18, but only for the caspase-6. The results obtained showed tissue- and sex-dependent differences in the number of apoptotic and proliferating cells as well as mRNA expression of caspase-3, -6 and Bcl-2 genes in the gonads of chicken embryos. Significant increase in the number of proliferating cells in the ovarian cortex and lack of these cells in the ovarian medulla (stages E12, E18, 1D) simultaneous with decrease in the intensity of apoptosis only in the medulla indicates that proliferation is the dominant process involved in the cortical development, which constitutes the majority of the functional structure of the fully developed ovary. No pronounced changes in the expression of apoptosis-related genes found during embryogenesis suggest that they cannot be considered as important indicators of gonad development. The molecular mechanisms of the regulation of balance between apoptosis and proliferation in developing avian gonads need to be further investigated.

Identification, expression, and regulation of anti-Müllerian hormone type-II receptor in the embryonic chicken gonad

Anti-Müllerian hormone (AMH) signaling is required for proper development of the urogenital system in vertebrates. In male mammals, AMH is responsible for regressing the Müllerian ducts, which otherwise develop into the fallopian tubes, oviducts, and upper vagina of the female reproductive tract. This role is highly conserved across higher vertebrates. However, AMH is required for testis development in fish species that lack Müllerian ducts, implying that AMH signaling has broader roles in other vertebrates. AMH signals through two serine/threonine kinase receptors. The primary AMH receptor, AMH receptor type-II (AMHR2), recruits the type I receptor, which transduces the signal intracellularly. To enhance our understanding of AMH signaling and the potential role of AMH in gonadal sex differentiation, we cloned chicken AMHR2 cDNA and examined its expression profile during gonadal sex differentiation. AMHR2 is expressed in the gonads and Müllerian ducts of both sexes but is more strongly expressed in males after the onset of gonadal sex differentiation. In the testes, the AMHR2 protein colocalizes with AMH, within Sertoli cells of the testis cords. AMHR2 protein expression is up-regulated in female embryos treated with the estrogen synthesis inhibitor fadrozole. Conversely, knockdown of the key testis gene DMRT1 leads to disruption of AMHR2 expression in the developing seminiferous cords of males. These results indicate that AMHR2 is developmentally regulated during testicular differentiation in the chicken embryo. AMH signaling may be important for gonadal differentiation in addition to Müllerian duct regression in birds.

Changes in the content of sex steroid hormone receptors in the growing and regressing ovaries of Gallus domesticus during development

Sex steroids participate in the regulation of reproduction in female chickens. In this work, we determined the content of androgen receptor (AR), intracellular progesterone receptor isoforms (PR-A and PR-B), membrane progesterone receptor γ (mPRγ) and estrogen receptor α (ER-α) in the left growing and right regressing ovaries of Gallus domesticus from 13-day-old chicken embryos to 1-month-old chickens by western blot analysis. A marked difference in the morphological characteristics of the left and the right ovaries during development was observed. Results show a higher content of AR in the left ovary than in the right one in all ages. In the left ovary, the highest content of AR was observed on day 13 of embryonic development, and diminished with age. In the right ovary, AR was expressed from day 13 of embryonic development to 1-day-old, and became undetectable at 1-week and 1-month-old. In the left ovary, PR isoforms were not detected on day 13 of embryonic development, but they presented a marked expression after hatching. In the right ovary, the highest expression of both PR isoforms was found on 1-day-old, and significantly decreased with age. PR-B was the predominant isoform on 1-day and 1-month old in the left ovary, whereas PR-A was the predominant one on day 13 of embryonic development in the right ovary. Interestingly, mPRγ was detected at 1-week and 1-month-old in the left ovary meanwhile in the right ovary, it was detected from day 13 of embryonic development to 1-day-old. ER-α was only detected in the left ovary from day 13 to 1-week-old, while in 1-month-old chickens, it was expressed in both ovaries. In the left ovary, ER-α content was lower from 1-day to 1-month-old as compared with day 13 of embryonic development. Our results demonstrate a differential expression of sex steroid hormone receptors between the left growing and the right regressing ovary, and throughout chickens' age; and this is the first report about mPR expression in birds.

Sexually dimorphic and sex-independent left-right asymmetries in chicken embryonic gonads

Female birds develop asymmetric gonads: a functional ovary develops on the left, whereas the right gonad regresses. In males, however, testes develop on both sides. We examined the distribution of germ cells using Vasa/Cvh as a marker. Expression is asymmetric in both sexes: at stage 35 the left gonad contains significantly more germ cells than the right. A similar expression pattern is seen for expression of ERNI (Ens1), a gene expressed in chick embryonic stem cells while they self-renew, but downregulated upon differentiation. Other pluripotency-associated markers (PouV/Oct3/4, Nanog and Sox2) also show asymmetric expression (more expressing cells on the left) in both sexes, but this asymmetry is at least partly due to expression in stromal cells of the developing gonad, and the pattern is different for all the genes. Therefore germ cell and pluripotency-associated genes show both sex-dependent and independent left-right asymmetry and a complex pattern of expression.

Overexpression of aromatase alone is sufficient for ovarian development in genetically male chicken embryos

Estrogens play a key role in sexual differentiation of both the gonads and external traits in birds. The production of estrogen occurs via a well-characterised steroidogenic pathway, which is a multi-step process involving several enzymes, including cytochrome P450 aromatase. In chicken embryos, the aromatase gene (CYP19A1) is expressed female-specifically from the time of gonadal sex differentiation. To further explore the role of aromatase in sex determination, we ectopically delivered this enzyme using the retroviral vector RCASBP in ovo. Aromatase overexpression in male chicken embryos induced gonadal sex-reversal characterised by an enlargement of the left gonad and development of ovarian structures such as a thickened outer cortex and medulla with lacunae. In addition, the expression of key male gonad developmental genes (DMRT1, SOX9 and Anti-Müllerian hormone (AMH)) was suppressed, and the distribution of germ cells in sex-reversed males followed the female pattern. The detection of SCP3 protein in late stage sex-reversed male embryonic gonads indicated that these genetically male germ cells had entered meiosis, a process that normally only occurs in female embryonic germ cells. This work shows for the first time that the addition of aromatase into a developing male embryo is sufficient to direct ovarian development, suggesting that male gonads have the complete capacity to develop as ovaries if provided with aromatase.

Just how conserved is vertebrate sex determination?

BACKGROUND

Sex determination in vertebrate embryos has long been equated with gonadal differentiation into testes or ovaries. This view has been challenged over the years by reports of somatic sexual dimorphisms pre-dating gonadal sex differentiation. The recent finding that sex determination in birds is likely to be partly cell autonomous has again called for a broader definition of sex determination. Inherent sexual differentiation in each and every cell may apply widely among vertebrates, and may involve more than one "master sex gene" on a sex chromosome. At the gonadal level, key genes required for proper sexual differentiation are conserved among vertebrates, but their relative positions in the ovarian and testicular cascades differ.

RESULTS

We illustrate these differences by comparing key sex genes in fishes versus birds and mammals, with emphasis on DM domain genes and the SOX9-AMH pathway in the testis and the FOXL2-Aromatase pathway in the ovary. Such comparisons facilitate the identification of ancient versus derived genes involved in gonadal sex determination.

CONCLUSIONS

The data indicate that vertebrate sex-determining cascades are not as conserved as once thought.