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Several studies show effects of yolk androgens in avian eggs on the phenotype of the offspring. Yolk hormone concentrations decline strongly within the first few days of incubation. Although early embryonic uptake of yolk androgens is suggested by the presence of radioactivity in the embryo when eggs are injected with radiolabelled androgens, these studies do not verify the chemical identity of radioactive compound(s), although it is known that these androgens can be metabolized substantially. By using stable isotope-labelled testosterone and androstenedione in combination with mass spectrometry, enabling verification of the exact molecular identity of labelled compounds in the embryo, we found that after 5 days of incubation the androgens were not taken up by the embryo. However, their concentrations in the entire yolk albumen homogenates declined strongly, even when corrected for dilution by albumen and water. Our results indicate metabolism of maternal androgens, very likely to 5β-androstane-3α,17β-diol, etiocholanolone and their conjugated forms. The results imply that the effects of increased exposure of the embryo to maternal androgens take place either before this early conversion or are mediated by these metabolites with an as yet unknown function, opening new avenues for understanding hormone-mediated maternal effects in vertebrates.

Avian yolk androgens are metabolized instead of taken up by the embryo during the first days of incubation