Avian extraembryonic membranes respond to yolk corticosterone early in development

During times of maternal stress, developing embryos can be exposed to elevated levels of glucocorticoids, which can affect development and permanently alter offspring phenotype. In placental species, the placenta mediates fetal exposure to maternal glucocorticoids via metabolism, yet the placenta itself responds to glucocorticoids to regulate offspring growth and development. In oviparous species, maternal glucocorticoids can be deposited into the egg yolk and are metabolized early in development. This metabolism is mediated by the extraembryonic membranes, but it is unknown if the extraembryonic membranes also respond to maternal glucocorticoids in a way comparable to the placenta. In this study, we quantified the expression of acyl-CoA thioesterase 13 (Acot13) as an initial marker of the membrane's response to corticosterone in chicken (Gallus gallus) eggs. Acot13 regulates fatty acid processing in the embryo, to potentially regulate resource availability during development. We addressed the following questions using Acot13 expression: 1) Do the extraembryonic membranes respond to yolk corticosterone early in development? 2) Is the response to corticosterone dependent on the dose of corticosterone? 3) What is the duration of the response to corticosterone? 4) Does a metabolite of corticosterone (5β-corticosterone) elicit the same response as corticosterone? We found that corticosterone significantly induces the expression of Acot13 on day four of development and that expression of Acot13 increases with the dose of corticosterone. Further, we found expression of Acot13 is significantly elevated by corticosterone on days four and six of development compared to oil treated eggs, but not on days eight and ten. Although this response is transient, it occurs during a critical period of development and could initiate a cascade of events that ultimately alter offspring phenotype. Finally, we found that 5β-corticosterone does not increase the expression of Acot13, indicating that metabolism inactivates corticosterone. Ultimately, this study provides insight into the mechanisms underlying how maternally deposited glucocorticoids can affect embryonic development.