In vitro estradiol synthesis and secretion by tadpole ovaries of different developmental stages

Effects of the herbicide ametryn on development and thyroidogenesis of bullfrog tadpoles (Aquarana catesbeiana) under different temperatures

Thyroid hormones (TH) are essential for the metamorphosis of amphibians and their production can be influenced by environmental stressors, such as temperature fluctuations, and exposure to aquatic pollutants, such as herbicides. In the present study we evaluated the influence of different temperatures (25 and 32 °C) on the effects of the herbicide ametryn (AMT, 0 - control, 10, 50 and 200 ng.L^-1) for 16 days on thyroidogenesis of bullfrog tadpoles. Higher temperature and AMT exposure caused a delay in the development of tadpoles, despite no differences were noted in weight gain and total length of the animals. Levels of triiodothyronine (T3) and thyroxine (T4) were not altered neither by AMT nor by temperature, but the highest temperature caused a decrease in total area and number of follicles in the thyroid gland. Transcript levels of thyroid hormone receptors alpha and beta (TRα and TRβ) and iodothyronine deiodinase 3 (DIO3) were lower at 32 °C, which is consistent with developmental delay at the higher temperature. Tadpoles exposed to 200 ng.L^-1 of AMT at 25 °C also presented delayed development, which was consistent with lower TRα and DIO3 transcript levels. Lower levels of estradiol were noted in tadpoles exposed to AMT at the higher temperature, being also possibly related to a developmental delay. This study demonstrates that higher temperature and AMT exposure impair thyroidgenesis in bullfrog tadpoles, disrupting metamorphosis.

Dynamics of Estradiol Level during Metamorphosis in the Daubed Shanny (Leptoclinus maculatus, Fries, 1838) from Spitsbergen Island

The estradiol content of muscles during metamorphosis in the daubed shanny (Leptoclinus maculatus) from different fjords of Spitsbergen Island, Svalbard, have been studied using mass spectrometry. The level of estradiol at the L4 larval stage of development is significantly lower than at the L2 stage. Values of this parameter in adults are significantly higher than in post-larvae (stage L5). Variations in the estradiol level during metamorphosis can be associated with the growth stimulation and regulation of the process.

Gonadal sex differentiation, development up to sexual maturity and steroidogenesis in the skipper frog, Euphlyctis cyanophlyctis

Gonadal sex differentiation, development up to sexual maturity and steroidogenesis were studied in the Indian skipper frog, Euphlyctis cyanophlyctis. In stage 25 tadpoles, gonads contained a few yolk laden germ cells and somatic cells. Ovarian differentiation occurred at stage 27 with the initiation of meiosis. Interestingly, meiosis preceded the formation of a central lumen that was discernible at stage 28. Folliculogenesis in the developing ovary was observed at stage 29. Vitellogenesis was observed in the 3 months old frogs and the females attained sexual maturity around 4 months. Testicular differentiation occurred indirectly through an ovarian phase. In some animals, from stage 37 onwards, oocyte degeneration was observed that was completed around metamorphic climax. Concurrently, large numbers of mesonephric cells were invading the gonads. Around metamorphosis, reorganization of the germ and somatic cells into testicular cords was observed. Following metamorphosis, the formation of seminiferous tubules was observed in the 2 weeks old males. Meiosis in the developing testes was observed in 1.5 months old males. In 3 months old males, the testes contained all stages spermatogenesis including spermatozoa. Steroidogenesis in the developing gonads was studied by immunohistochemical localization of 3β-HSD enzyme. At stage 26, a few immunoreactive cells were seen in the kidneys (interrenal cells). However, during and after differentiation, gonads failed to show positive immunoreaction. In the developing ovary at stage 37, follicular cells surrounding the oocytes were positive for 3β-HSD immunoreactivity. In the ovaries of 3 months old females, follicular cells surrounding the vitellogenic oocytes and stromal cells were positive for 3β-HSD immunoreaction. E. cyanophlyctis exhibits undifferentiated type of gonadal differentiation, in which gonadal differentiation precedes steroidogenesis.

The mechanism of sex determination in vertebrates-are sex steroids the key-factor?

In many vertebrate species, sex is determined at fertilization of zygotes by sex chromosome composition, knows as genotypic sex determination (GSD). But in some species-fish, amphibians and reptiles-sex is determined by environmental factors; in particular by temperature-dependent sex determination (TSD). However, little is known about the mechanisms involved in TSD and GSD. How does TSD differ from GSD? As is well known, genes that activated downstream of sex-determining genes are conserved throughout all classes of vertebrates. What is the main factor that determines sex, then? Sex steroids can reverse sex of several species of vertebrate; estrogens induce the male-to-female sex-reversal, whereas androgens do the female-to-male sex-reversal. For such sex-reversal, a functioning sex-determining gene is not required. However, in R. rugosa CYP19 (P450 aromatase) is expressed at high levels in indifferent gonads before phenotypic sex determination, and the gene is also active in the bipotential gonad of females before sex determination. Thus, we may predict that an unknown factor, a common transcription factor locates on the X and/or W chromosome, intervenes directly or indirectly in the transcriptional up-regulation of the CYP19 gene for feminization in species of vertebrates with both TSD and GSD. Similarly, an unknown factor on the Z and/or Y chromosome probably intervenes directly or indirectly in the regulation of androgen biosynthesis for masculinization. In both cases, a sex-determining gene is not always necessary for sex determination. Taken together, sex steroids may be the key-factor for sex determination in some species of vertebrates.

Sex determination in amphibians

The heterogametic sex is male in all mammals, whereas it is female in almost all birds. By contrast, there are two heterogametic types (XX/XY and ZZ/ZW) for genetic sex determination in amphibians. Though the original heterogametic sex was female in amphibians, the two heterogametic types were probably interchangeable, suggesting that sex chromosomes evolved several times in this lineage. Indeed, the frog Rana rugosa has the XX/XY and ZZ/ZW sex-determining systems within a single species, depending on the local population in Japan. The XY and ZW geographic forms with differentiated sex chromosomes probably have a common origin as undifferentiated sex chromosomes resulted from the hybridization between the primary populations of West Japan and Kanto forms. It is clear that the sex chromosomes are still undergoing evolution in this species group. Regardless of the presence of a sex-determining gene in amphibians, the gonadal sex of some species can be changed by sex steroids. Namely, sex steroids can induce the sex reversal, with estrogens inducing the male-to-female sex reversal, whereas androgens have the opposite effect. In R. rugosa, gonadal activity of CYP19 (P450 aromatase) is correlated with the feminization of gonads. Of particular interest is that high levels of CYP19 expression are observed in indifferent gonads at time before sex determination. Increases in the expression of CYP19 in female gonads and CYP17 (P450 17alpha-hydroxylase/C17-20 lyase) in male gonads suggest that the former plays an important role in phenotypic female determination, whereas the latter is needed for male determination. Thus, steroids could be the key factor for sex determination in R. rugosa. In addition to the role of sex steroids in gonadal sex determination in this species, Foxl2 and Sox3 are capable of promoting CYP19 expression. Since both the genes are autosomal, another factor up-regulating CYP19 expression must be recruited. The factor, which may be located on the X or W chromosome, intervenes directly or indirectly, in the transcriptional regulation of the CYP19 gene for feminization in amphibians. A factor up-regulating CYP17 expression remains to be identified.

Sexually dimorphic expression of steroidogenic factor 1 (SF-1) in developing gonads of the American bullfrog, Rana catesbeiana

Genetic sex determination leads to gonadal differentiation and ultimately the differences between the sexes in steroid hormone secretion. Gonadal steroidogenesis is critical for the development of a sexually dimorphic phenotype and adult reproductive function. Control of gonadal development and steroidogenesis is under the regulation, at least in part, of steroidogenic factor-1 (SF-1). We have begun to characterize SF-1 expression in an amphibian to determine the role of this protein in development and reproduction. We have detected a putative SF-1 protein from several tissues in the American bullfrog, Rana catesbeiana, that co-migrates with mouse SF-1 on a Western blot. Our results show that bullfrog SF-1 protein is expressed in steroidogenic and other reproductive tissues in a manner similar to that reported for other species, with high expression in the brain, pituitary, gonad, liver, and interrenal, but little or no expression in non-reproductive tissues such as skin and intestine. Using a quantitative Western blot analysis system, we documented changes in SF-1 protein in the gonads of developing tadpoles. Our results indicate that there is sexually dimorphic expression of SF-1 protein that becomes evident at the time of sexual differentiation of the gonads. In males, the expression of SF-1 decreases following testicular formation and in females the expression increases with the formation of ovaries. This is the first study to investigate changes in SF-1 during development at the protein level. The expression is similar to that reported for changes in SF-1 mRNA expression in chickens and alligators, however, opposite to that seen in mammals and turtles. These results indicate that SF-1 may play a pivotal role in development of the reproductive system in amphibians as it does in other vertebrate groups.

Sequence analysis and expression of the P450 aromatase and estrogen receptor genes in the Xenopus ovary

Recent studies point to a key role for the estrogen synthesizing enzyme P450 aromatase (P450 arom) in ovary determination in fish, birds and reptiles. It is unclear whether estrogen synthesis is important in sex determination of Xenopus gonad. To determine whether the aromatase gene is transcribed in the gonads of Xenopus tadpoles during the sex determination, we cloned a P450 arom cDNA and examined the level of P450 arom and estrogen receptor (ER) gene expression in association with estrogen activity. cDNA clones for P450 arom were isolated from a Xenopus ovarian cDNA library. There was an open reading frame (ORF) of 1500 bp from the ATG start to TAA stop codons encoding 500 predicted amino acids. cDNAs for P450 arom have previously been cloned from various vertebrates. The homology between the Xenopus P450 aromatase and the human P450 arom was higher. The expression of the P450 arom gene was mainly limited to reproductive organs. To determine the beginning of estrogen activity in gonads of embryos, expression of the aromatase and ER gene was also examined by RQ-RT-PCR. Both Xenopus aromatase and ER mRNA was detected at stage 51 in gonads. These observations are consistent with estrogens having a key role in ovarian development in various other vertebrates.

Pattern of gonadal sex differentiation, development, and onset of steroidogenesis in the frog, Rana curtipes

Histomorphological changes and steroidogenic potential of the gonads during sexual differentiation and development were studied in Rana curtipes from tadpole stage 25 (Gosner) until maturity. In stage 25 tadpoles of smaller snout-vent length (SVL; 4-5 mm) the gonads were indifferent, containing a few somatic and germ cells, whereas in larger tadpoles (SVL > 7 mm) gonads were differentiated into ovaries with a central lumen. Onset of meiosis was seen in these ovaries. At stage 26, diplotene and first growth phase oocytes were found. With advancement in developmental stage and after metamorphosis the ovaries progressively enlarged due to increase in number and size of oocytes. Vitellogenesis began in the ovary of 4-month-old frogs. Females attained maturity 6 months after metamorphosis. The frogs showed amplexus and one frog spawned. Onset of testicular formation seen at stage 31 was associated with the degeneration of oocytes and infiltration of darkly stained somatic cells in the central region. By stage 35 all oocytes degenerated, leaving behind a large number of somatic and germ cells interspersed with each other. At stage 38, formation of seminiferous tubules enclosing spermatogonia and pre-Sertoli cells was seen. Initiation of meiosis was evident at metamorphic climax. Cysts of elongated spermatids associated with Sertoli cells were seen in 45-day-old frogs. Histochemically, delta(5)-3 beta-hydroxysteroid dehydrogenase activity was localized in the ooplasm, follicular cells, and interstitium of the ovary from stage 28 onward. The enzyme activity in the testis appeared in 45-day-old froglets. In R. curtipes gonadal differentiation is a semidifferentiated type since gonads initially differentiate into ovaries, and later, in the prospective males, the ovaries degenerate and transform into testes. The males attain maturity much earlier than the females. In R. curtipes gonadal sex differentiation precedes the onset of gonadal steroidogenesis.

Ontogenetic profile of FSH and LH in Rana esculenta

Circulating levels and pituitary content of FSH and LH were determined by specific radioimmunoassays in Rana esculenta starting a few days after hatching until the completion of metamorphosis. Both gonadotropins were found in the pituitary as well as in the blood plasma at all stages of development examined here. The plasma concentrations of FSH and LH were more or less uniform during pre- and prometamorphosis, but increased significantly at the onset of metamorphic climax. The plasma levels of FSH and LH remained high at the completion of metamorphosis. The pituitary content of FSH and LH was low in early premetamorphosis. It increased slightly through prometamorphosis and metamorphic climax, following which a highly significant increase occurred. Whereas plasma concentrations of FSH and LH were essentially similar within a single stage of development, the pituitary FSH content was severalfold higher than pituitary LH. The significance of these results is discussed in relation to the functional maturation of the brain-pituitary-gonadal axis in the frog.

The effects of aromatase and 5 alpha-reductase inhibitors, antiandrogen, and sex steroids on Bidder's organs development and gonadal differentiation in Bufo bufo tadpoles

Embryos of toads (Bufo bufo) were treated with aromatase (4-OHA) and 5 alpha-reductase (17 beta C) inhibitors, antiandrogen (CPA), estradiol-17 beta, testosterone, and 5 alpha-dihydrotestosterone in order to study the role played by sex steroids in the development and sex differentiation of gonads. Test compounds were administered to tadpoles in water and morphometric and cytometric analyses were carried out on histological sections of the cephalic Bidder's organ (a rudimentary ovary) and of the gonadal region. In Bidder's organs, the number and size of oogonia and oocytes were modified by the treatments. However, the female commitment of the Bidder's organ occurs independently from steroid treatments that lead to an acceleration or slackening of the processes of proliferation and differentiation of oogonia. 4-OHA and androgens caused various degrees of inhibition of ovarian differentiation, with gonads maintaining an undifferentiated condition. Estrogen provoked a shift of the sex ratio towards the female sex, yet slackened gonadal growth. 17 beta C accelerated ovarian differentiation in females while CPA enhanced gonadal differentiation in both sexes by promoting the germ and somatic cell proliferation. We suggest that sex hormones may have a local regulatory role in gonadal differentiation during early developmental stages. Furthermore, the strong tendency of Bidderian germ cells to develop in the oogenetic way regardless of sex genotype and steroid treatments, and the quantitative sex differences found in the control Bidder's organs and gonads, suggest that other factors (such as intracellular mechanisms) may be involved in the initial steps of the process of germ cell differentiation.

Development of vasotocin pathways in the bullfrog brain

The brain of adult bullfrogs (Rana catesbeiana) contains six populations of cells which are immunoreactive for the neurohypophysial peptide arginine vasotocin (AVT). It is unknown when some of these cell populations first appear during development and when the sexual differences in AVT distribution first become apparent. We therefore used immunocytochemistry to examine development of AVT pathways in developing bullfrog tadpoles and in newly metamorphosed froglets of both sexes. AVT-immunoreactive (AVT-ir) cells were already present in the three diencephalic areas (magnocellular preoptic nucleus, suprachiasmatic nucleus and hypothalamus) at stage III (Taylor and Kollros stages), the earliest stage examined. Cell size in the magnocellular nucleus was not bimodally distributed in either tadpoles or froglets. AVT-ir cells in the telencephalic septal nucleus and amygdala did not appear until stage VI. There was no sexual difference in the density of AVT-ir cells or fibers in the amygdala of tadpoles or froglets. Finally, cells in the hindbrain pretrigeminal nucleus appeared much later--after stage XX. Thus, different populations of neurons begin to express AVT at unique times during development. The sexual dimorphism in AVT content observed in the amygdala of adult bullfrogs must appear during juvenile development or at adulthood.

Gonadal hormones inhibit the induction of metamorphosis by thyroid hormones in Xenopus laevis tadpoles in vivo, but not in vitro

Although the major hormones controlling amphibian metamorphosis are those of the thyroid, other hormones, notably prolactin and the adrenal steroids, modulate the effects of thyroid hormones (TH). Some authors report that the gonadal steroids stimulate the metamorphic actions of TH whereas others report inhibition. The aims of the present study were to determine the effects of gonadal steroids on TH-induced metamorphosis in Xenopus laevis and to determine the site of action of these steroids. In all cases, hormones were added to the water in which the tadpoles were swimming. The gonadal steroids, testosterone and 17 beta-estradiol, inhibited triiodothyronine (T3)-induced metamorphosis in living, premetamorphic tadpoles of X. laevis. Both steroids, at 3.4 microM, prevented the reduction in body weight and the shrinkage of head and alimentary canal brought about by 1 nM T3. In contrast, 3.4 microM corticosterone stimulated T3-induced metamorphosis. Addition of 100 nM T3 to the medium induced a large reduction in size of X. laevis tails cultured in vitro. The antagonistic effects of testosterone were not reproduced in such cultures, whereas the synergistic action of corticosterone was maintained. Testosterone had no effect upon the specific binding of T3 to X. laevis tail tissue, whereas corticosterone increased such binding. These findings indicate that, while corticosterone stimulates the metamorphic actions of T3 by acting directly in the peripheral tissues, the gonadal steroids, particularly testosterone, inhibit T3 by acting at a more central site. Prolactin is known to antagonize the metamorphic actions of T3 and one such central action could be the stimulation of prolactin synthesis. However, testosterone inhibited the prometamorphic actions of bromocriptine, which stimulates metamorphosis by inhibiting production of prolactin. Thus the central action of testosterone is unlikely to be a stimulation of prolactin production.

Estradiol secretion by the ovarian tissue, in response to hypophyseal stimulation, during ontogenesis of the bullfrog

Ovaries of tadpoles, froglets, young frogs, and mature frogs of Rana catesbeiana were cut into small pieces. They were incubated for 6 hr in Krebs-Ringer bicarbonate buffer as controls. Another series of ovaries of the same developmental stages were incubated with pituitary extracts in the buffer as experimentals. Media were then analyzed for estradiol secretion by radioimmunoassay. The results showed that estradiol secretion by tadpole ovaries during development was not affected by the addition of pituitary extracts of mature frogs in the media at any stage while those of young and mature frogs with pituitary extracts secreted more estradiol than those without. These findings indicate that tadpole ovaries are unresponsive to pituitary agents to produce estradiol while frog ovaries are dependent on some pituitary hormones to synthesize estradiol. Thus frog ovaries acquire dependence on the pituitary agent only after metamorphosis.

In vitro inhibition of estradiol secretion of tadpole ovaries by cyanoketone

We have demonstrated the presence of delta 5-3 beta-hydroxysteroid dehydrogenase (HSD) activity in tadpole ovaries of Rana catesbeiana. In the present study, we wish to determine whether estradiol (E2) secretion of tadpole ovaries could be influenced by cyanoketone (CK), a specific inhibitor of delta 5-3 beta-HSD. R catesbeina tadpoles at the premetamorphic climax were used, and pooled ovaries were incubated, 30 mg/tube, with CK at dosages of 0, 0.001, 0.01, 0.1, 1, and 10 micrograms/ml of Krebs-Ringer bicarbonate buffer for 6 hr. Media were collected for assay of E2 by radioimmunoassay (RIA). Results showed an inhibition of E2 secretion by CK that was positively correlated with CK dosage, but plateaued at doses of 0.1 microgram/ml and higher. This finding was comparable to that of G.F. Young, H. Kagawa, and Y. Nagahama (1982, Gen. Comp. Endocrinol. 47, 357-360) on adult fish ovaries. However, adult vertebrates depend on gonadotropins to regulate secretion of E2 while tadpoles, being immature, might secrete E2 independently of pituitaries. When the histochemical test for delta 5-3 beta-HSD activity was performed on in vitro CK-treated ovaries, there was a decrease of enzyme activity by CK. The RIA and histochemical findings may contribute to the concept of sex transformation in which a disturbance of steroidogenesis may induce sex reversal from females to males, at least in tadpoles.