Hypothalamic regulation of the adenohypophyseal-testicular axis in the male chick embryo

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.

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.

Examining a pathway for hormone mediated maternal effects--yolk testosterone affects androgen receptor expression and endogenous testosterone production in young chicks (Gallus gallus domesticus)

In vertebrates maternal androgens can substantially influence developing offspring, inducing both short and long term changes in physiology and behavior, including androgen sensitive traits. However, how the effects of maternal hormones are mediated remains unknown. Two possible pathways are that maternal androgens affect parts of the hypothalamus-pituitary-gonadal axis (HPG axis) or the sensitivity to androgens by affecting androgen receptor (AR) densities within the brain. To investigate both pathways, testosterone within the physiological range or vehicle only was injected into the egg yolk of unincubated chicken eggs and AR mRNA expression in different brain nuclei as well as plasma testosterone levels were measured in two week old male and female chicks that had hatched from these eggs. Our results showed a significant sex difference in plasma testosterone levels with males showing higher levels than females. Furthermore, AR mRNA expression as well as plasma testosterone levels were significantly lower in chicks hatched from testosterone treated eggs. These results suggest a compensatory mechanism for avoiding potential detrimental effects of high testosterone levels.

Neuropeptidomic analysis of the embryonic Japanese quail diencephalon

Background

Endogenous peptides such as neuropeptides are involved in numerous biological processes in the fully developed brain but very little is known about their role in brain development. Japanese quail is a commonly used bird model for studying sexual dimorphic brain development, especially adult male copulatory behavior in relation to manipulations of the embryonic endocrine system. This study uses a label-free liquid chromatography mass spectrometry approach to analyze the influence of age (embryonic days 12 vs 17), sex and embryonic day 3 ethinylestradiol exposure on the expression of multiple endogenous peptides in the developing diencephalon.

Results

We identified a total of 65 peptides whereof 38 were sufficiently present in all groups for statistical analysis. Age was the most defining variable in the data and sex had the least impact. Most identified peptides were more highly expressed in embryonic day 17. The top candidates for EE₂ exposure and sex effects were neuropeptide K (downregulated by EE₂ in males and females), gastrin-releasing peptide (more highly expressed in control and EE₂ exposed males) and gonadotropin-inhibiting hormone related protein 2 (more highly expressed in control males and displaying interaction effects between age and sex). We also report a new potential secretogranin-2 derived neuropeptide and previously unknown phosphorylations in the C-terminal flanking protachykinin 1 neuropeptide.

Conclusions

This study is the first larger study on endogenous peptides in the developing brain and implies a previously unknown role for a number of neuropeptides in middle to late avian embryogenesis. It demonstrates the power of label-free liquid chromatography mass spectrometry to analyze the expression of multiple endogenous peptides and the potential to detect new putative peptide candidates in a developmental model.

Secretion of inhibin and testicular expression of inhibin subunits in male duck embryos and newly hatched ducks

Concentrations of immunoreactive (ir-) inhibin in circulation, amniotic fluid, and testes of embryos and newly hatched ducks were determined from d 21 of incubation to d 1 of age. Plasma concentrations of FSH and LH were also determined by chicken radioimmunoassay (RIA) systems. In addition, gene expression and cellular source of inhibin were investigated by in situ hybridization and immunohistochemistry. The results showed that plasma ir-inhibin gradually declined from d 21 to d 24, followed by an increase on d 25 and remained high until d 1 after hatching. FSH in plasma was high on d 21 followed by a sharp decline toward d 25 after which FSH levels stabilized. A reverse relationship was observed between inhibin and FSH during the late stage of incubation. Embryonic testes contained high ir-inhibin levels. Testicular ir-inhibin levels were relatively high at early time points with a peak on d 23, and significantly decreased from d 23 to d 24 and stabilized thereafter. Amniotic fluid concentrations of ir-inhibin were relatively low and remained constant between d 21 and d 25. In situ hybridization demonstrated that the expression of inhibin alpha- and betaA-subunit mRNA was coexisted in the cells in the seminiferous tubules of testes on d 25. The immunoreactivity of inhibin betaA- and betaB-subunits was colocalized in the cells in the seminiferous tubules of testes on d 25. The results of dimeric inhibins determined by the ELISA method showed that inhibin B can be measured in embryonic testicular homogenate and pooled embryonic plasma. Although inhibin A was detected in testicular homogenate, it was under the detection limit in pooled embryonic plasma. In conclusion, these results indicate that cells in the seminiferous tubules of embryonic testes in ducks may secrete dimeric (bioactive) inhibins to circulation and that the FSH-inhibin feedback loop may become operational during the late stage of the incubation.

Reproductive consequences of EDCs in birds: what do laboratory effects mean in field species?

The varied reproductive strategies of birds present a challenge in developing reliable indices for the assessment of effects of endocrine disrupting chemicals (EDCs). Precocial species, such as quail, appear to be most sensitive to EDC effects during embryonic development. Although the Japanese quail (Coturnix japonica) is a nonnative lab species, its reproductive strategy is similar to that of many free-ranging species. Because a great deal is known about the reproductive biology of this species and Japanese quail have a short generation time, this species is an ideal candidate for testing EDC effects. In this review, we present data collected in a two-generation design with embryonic exposure to estradiol benzoate (EB). This study was conducted to provide fundamental information for establishing reliable reproductive endpoints associated with estrogenic EDC exposure. Data were collected for a variety of endpoints, which were chosen as measures of reproductive capability and success. These reproductive fitness measures included fertility, hatching success, and offspring viability. Endocrine measures consisted of plasma hormone levels and gonad weight/condition. Neuroendocrine systems, such as the monoamine neurotransmitter systems, regulate hypothalamic gonadotropin releasing hormone (GnRH) and reproductive behavior. Therefore, these variables should potentially be very sensitive indicators. Behavioral measures included reproductive behavior. Results showed that embryonic estradiol exposure affected endocrine and behavioral responses in males and impacted productivity in females. Therefore, quails provide an excellent model to determine fundamental actions of EDCs. The laboratory trials then serve as a basis for the extrapolation of findings of controlled laboratory studies to effects that may be observable in free-ranging species.

Effect of follicle-stimulating hormone on different cell sub-populations in the ovary of newly hatched chicks treated during embryonic development

1. Cell sub-populations of the ovary of newly-hatched chicks were assessed following follicle stimulating hormone (FSH) treatment during embryonic development. Changes in cell number and the amount of oestradiol in serum were determined. 2. White Leghorn chick embryos received 1 microgram FSH applied to the chorioallantoic membrane at 13, 15, and 17 d of incubation. Within 24 h after hatching, animals were killed and blood was collected. The left ovary was immediately removed then weighed and processed by an enzymatic-mechanical dissociation method for total cell count. An air-drying method was also used for meiotic preparations to study the germinal cells. 3. The pre-follicular ovary is able to respond to FSH by inducing an increase both in the serum oestradiol concentration and in the number of steroidogenic cells and of poorly differentiated cells of the ovarian medulla. 4. FSH increases the number of oogonia, which are responsible for a sharp increase in the total population of germ cells in the FSH-treated ovary. 5. It is possible that FSH acts to increase the proliferation of oogonia and a delay in the meiotic prophase through a change in the microenvironment rather than by a direct effect on germ cells.

A stereological study of the different cell populations in chicken testes treated with follicle-stimulating hormone during embryonic development

Quantitative morphological methods were used to analyse the histomorphometric changes and variations in the number and size of cells from diverse cellular populations in testes of newly hatched chicks treated with follicle stimulating hormone (FSH) during embryonic development. The tissue was fixed and embedded in Epon and sections were morphometrically measured under light microscopy, using point counting for volume densities and the Floderus equation to determine numerical density. The average volume of the individual cell was determined by dividing the volume density by the numerical density. Results indicate that FSH administration causes an increase in the number of Sertoli cells and spermatogonia, as well as enlargement of the individual Sertoli cells leading consequently to an increase in the diameter and volume density of the testicular seminiferous tubules. Results also reveal an increase in the volume density of the interstitial cords of the Leydig cells, this expansion is due to the enlargement of the individual Leydig cells and not to an increase in their number, which remains constant. We conclude that testes of chick embryos are able to respond to FSH treatment, as revealed by the changes in the number and size of the cells conforming the diverse cellular populations of the testis. FSH treatment during embryonic development induces histomorphometric changes in both the interstitial tissue and seminiferous tubules, accelerating their growth and differentiation.

Distribution and changes in mu- and kappa-opiate receptors during the midlife neurodevelopmental period of Coho salmon, Oncorhynchus kisutch

Parr-smolt transformation (PST) in coho salmon is associated with a plasma thyroid hormone (PT4) surge and a critical period of neural development that includes axonal sprouting, neurogenesis, and surges of select neurotransmitters. Here we provide a description of the selectivity, distribution, and the changes in the density of mu- and kappa-opiate receptors during PST, as revealed by quantitative in vitro autoradiography of [3H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol ([3H]DAMGO) and [3H]ethylketocyclazocine ([3H]EKC), respectively. The concentration of mu-receptors increased significantly in select cell groups in the early stages of parr-smolt transformation, until a peak was reached at the time coinciding with the peak of the PT4 surge. In other cell groups, the peaks occurred 1 or 2 weeks later. With one exception, this increase was followed by a decrease in concentration. The brain areas showing the highest concentrations are the dorsal nucleus of the ventral telencephalic area, the glomerular region, the granular layer of the valvula cerebelli, the nucleus diffuses of the inferior lobe, and the nucleus diffuses of the torus lateralis. Other regions with distinctly elevated mu-receptor concentrations are the stratum griseum centrale of the optic tectum and the preoptic area. The distribution of kappa-receptors is more diffuse, and the densities are considerably lower. The overlap in distribution of mu- and kappa-receptors is considerable, but significant exceptions are noted. For example, the dorsomedial nucleus of the dorsal telencephalic area, the habenular nucleus, and the dorsomedial nucleus of the thalamus exhibit a surge in density of kappa-receptors at the time of the PT4 surge, while the density of mu-receptors in these nuclei remain very low throughout parr-smolt transformation. The kappa-receptor containing cell groups are not identifiable until 3 weeks before the PT4 surge because of low densities. The most prominently labeled kappa-receptor regions are the ventral and dorsal nuclei of the ventral telencephalic area, the medial and dorsal zones of the dorsal telencephalic area, the optic tectum (all layers), the dorsomedial nucleus of the thalamus, the torus lateralis of the ventral hypothalamus, and the preoptic area. An increase of mu- and kappa-opiate receptor densities in specific brain regions may reflect roles in the alteration of brain organization, olfactory imprinting, neuroendocrine activity or other physiological activities. The overall distribution of these receptors are relatively more extensive in salmon than in other vertebrates so far studied.

Avian LHRH during embryonic development: measurement by competitive ELISA with a monoclonal antibody

A mouse monoclonal antibody directed against chicken Gln8-luteinizing hormone-releasing hormone (cLHRH-I) was developed and characterized. This antibody was used for the development of a competitive microtiter plate enzyme-linked immunosorbent assay for avian LHRH. The assay was validated for use with tissue and was used at a working range between 5 pg and 10 ng per sample. Using this procedure, cLHRH-I and II were assayed in whole brain extracts of Japanese Quail embryos. Samples were taken at regular intervals between Day 6 of incubation through Day 1 posthatch. There were 10 samples taken at each age with 2 replicates of the entire sampling regime. Data from males and females were pooled. LHRH concentrations were low, then rose to higher levels (15 pg/mg tissue) between Days 10 through 13 and decreased thereafter. These changes are likely to be correlated with the activation of the hypothalamic-pituitary-gonadal axis. This is particularly apparent in later embryonic development.