V. Bruggeman

Developmental endocrinology of the reproductive axis in the chicken embryo.

In mammals, the phenotype of the homogametic sex develops in the (relative) absence of steroids and the phenotype of the heterogametic sex is imposed by the early action of steroids. In contrast, the heterogametic sex in avian species is the female and the presence of estrogens and their receptors plays a crucial role in female sexual differentiation. The time- and sex-dependent expression of enzymes involved in steroidogenesis which determine the ratio of androgens/estrogens produced by the gonads has been extensively investigated during the last 5-6 years. These results all show that the lack of estrogen synthesis in the male appears to be due to the extremely low levels of 17beta-hydroxysteroid dehydrogenase and P450aromatase expression. In females, extensive expression of the aromatase gene (around day 5-6 of incubation), leading to estrogen synthesis, and specific expression of the estrogen receptor-mRNA in the left gonad results in the development of a functional left ovary. Other sex differences can be found in the expression of the inhibin subunit genes in gonads of chicken embryos and in circulating concentrations of inhibin, follicle stimulating hormone (FSH) and steroids. Sex reversal attempts have been made by varying incubation temperatures, by using anti-estrogens, androgens, aromatase inhibitors and synthetic steroids. In ovo administration of a sex steroid hormone or an inhibitor of endogenous sex steroid synthesis can cause phenotypical sex reversal. All these experiments show that the development of gonads in birds is very sensitive to changes in the embryonic hormonal environment, sometimes resulting in changes of postnatal reproduction and even growth.

Insulin-like growth factors in the regulation of avian ovarian functions

In the past three decades, overwhelming evidence has accumulated to show that insulin-like growth factor (IGF)-I and -II, their receptors and binding proteins (IGFBP) (the IGF system), have major roles to play in the regulation of ovarian function in mammals. Although studies in birds did not start until 5-6 years ago, the limited information thus far available suggests that the IGFs act as autocrine/paracrine regulators of follicular growth and differentiation, just as observed in mammals. The genes for IGF-I and -II, type-I IGF receptor, IGFBP-2, and IGFBP-5 are expressed in both granulosa and theca cells of the chicken ovary. The mechanisms by which the IGF system controls ovarian function in the avian species are complex and involve interactions with the gonadotrophins (LH and FSH), growth hormone, and even other growth factors. Effects are different between strains and nutritional status.

Body weight, fat content, liver weight and plasma leptin concentrations in broiler breeder females reared under ad libitum feeding, restricted feeding or combinations of both until age of first egg

supraoptic decussatio. Its complete trajectory could be followed until it reached the median eminence. Very intensely stained nerve fibers were easily detectable in both colchicin-treated and control animals, especially as they reached the median eminence (Figure). Additional immunopositive fibers were scarce throughout the brain. The observed distribution, with neuronal cell bodies in the paraventricular nucleus of the hypothalamus and projections towards the median eminence, strongly suggests that lLHRH-III might indeed be a hypothalamic hypophysiotropic releasing factor in the chicken. However, much more research will be needed in order to conclusively prove that lLHRH-III is the missing specific chicken FSHRF, as has been suggested in the rat (Yu et al., 1997). For this hypothesis to be true, it needs to be shown that the peptide is released into the hypophyseal portal blood system, that FSH cells express a specific receptor for the peptide and that FSH is released upon binding of lLHRH-III to its receptor. LRB is a Senior Research Associate of the Flemish Fund for Scientific Research (FWOVlaanderen). The present study was financially supported by a grant of the Flemish Fund for Scientific Research (FWO-Vlaanderen) and by a Faculty Mini-grant from the Texas A&M Program for Research and Graduate Studies to LRB.

Regulation of inhibin α- and βA-subunit messenger ribonucleic acid levels by gonadotropins and IGF-I in cultured chicken granulosa cells

Abstract A quantitative competitive reverse transcription polymerase chain reaction assay (QC RT-PCR) for quantifying the absolute levels of the expression of inhibin α- and β A -subunits in chicken granulosa cells showed that these subunits are expressed in different amounts depending on follicular maturation. The present study determined the regulation of the expression of these subunits. The individual effect of different doses of IGF-I, LH or FSH (1–100xa0ng/ml) or the combination of IGF-I with either LH or FSH at different concentrations, on the expression of inhibin α- and β A -subunit was determined on cultured granulosa cells of F 1 and the combined F 4 +F 5 follicle. Cells were cultured for 48xa0h in 6-well plates with or without added hormones. Culture medium was discarded, cells were washed and total RNA was extracted from the cells. Five hundred nanograms of total RNA was reverse transcribed using specific primers and coamplified with an internal standard, as described previously, to determine expression level in the cells. IGF-I, LH, and FSH enhanced the inhibin α-subunit mRNA levels in a dose dependent manner in both F 1 and the combined F 4 +F 5 whereas inhibin β A -subunit was not affected. The effects of FSH, LH were more expressed in F 1 follicles compared to F 4 +F 5 on the α-subunit. The addition of IGF-I and either LH or FSH during the culture period significantly increased the stimulatory effects of both LH and FSH on the expression of inhibin α-subunit in F 1 follicles but had no significant effect on the inhibin β A -subunit. The results suggest that the changing expression levels of inhibin α-subunit during follicular development are the result of the regulatory effect of the interaction between IGF-I and the gonadotropins and that the regulation of this subunit may be the main factor for the regulation of the protein inhibin levels. Other factors may be also implicated in the changing expression levels of the β A -subunit.

Autocrine/paracrine effects of inhibin and activin in chicken ovary

inhibin and/or activin and if this relative contribution to molecules with an opposing biological activity is mathematically dependent on the a / b A ratio. SAFI, M., BUYS, N., ONAGBESAN, O.M., BLEUGELS, B. & DECUYPERE, E. (1998) Quantification of inhibin/activin a and b A subunit messenger ribonucleic acid by competitive reverse transcription-polymerase chain reaction in chicken granulosa cells during follicular development. Biology of Reproduction, 59: 1047–1054.