The potential role of the inhibin and activin family of proteins in the reproductive dysfunction of two domesticated avian species and two captive mammalian species
Hoffman, Jacquelyn Beth
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The transforming growth factor-B (TGF-B) superfamily of hormones includes inhibin, activin, and their receptors. Inhibin acts as a negative feedback regulator of pituitary FSH secretion while activin is a positive regulator of pituitary FSH secretion in addition to having a variety of ovarian autocrine and paracrine functions. By studying the expression of these hormones and their receptors, we may elucidate the mechanisms that govern follicular selection, maturation, and ovulation in mammalian and avian species. Additionally, reproductive pathophysiologies may be analyzed to understand the influences of captivity, domestication, and genetic selection, on the TGF superfamily of hormones. Inhibin RIA detection studies were performed on plasma samples from anovulatory and ovulatory female elephants (Elephas maximus, Loxodanta africana) during the luteal and non-luteal phases of their ovulatory cycles. Cyclical patterns of inhibin were only detected in ovulatory Asian and African elephants. Levels of inhibin detected in anovulatory elephants were not significantly higher than ovulatory elephants and thus were not indicative of granulosa cell tumors. Ovarian expression of mRNA for the inhibin/activin subunits was examined in two different genetic lines of turkey hens selected for egg production or rapid growth rate. Significant differences in subunit expression patterns and levels were detected between these two lines of hens. These differences in expression may potentially account for the differences in follicular hierarchy structure and egg production rates also observed in these birds. mRNA expression studies of activin type IA and type IB receptors were examined in theca and granulosa tissue from the ovaries of the broiler breeder hen. Both receptor types were detected in theca and granulosa tissues, with type IA being expressed at significantly greater levels in both tissues. Both type IA and type IB activin receptors were expressed at significantly greater amounts in the theca tissue of hierarchical and non-hierarchical follicles as compared to the granulosa tissue. Granulosa cell culture experiments indicate that gonadotropins depress the mRNA expression of both type I receptors in hierarchical follicles. When combined with previous activin type IIA receptor expression data, these results suggest that activin may elicit its main effect in the theca tissue of the broiler breeder hen ovary.