Toward a dynamic model of deposition and utilization of yolk steroids.

The discovery by Schwabl that maternal steroid hormones are transferred to the egg yolk and have effects on the phenotype of offspring revealed a new pathway for non-genetic maternal effects. The initial model relied on passive transfer. The thinking was that steroids passively entered the lipophillic yolk during yolk deposition and then were deposited in the yolk until they were passively delivered to the embryo as the yolk was used. Subsequent studies revealed that the system is much more dynamic than that. Here, we explore questions about how dynamic the system really is and look at questions like: Is transfer of maternal steroids to the yolk passive or is it actively regulated? At what stages of the maternal reproductive cycle are steroids transferred? During reproduction, how dynamic are the levels of yolk steroids? Especially in the case of potentially deleterious steroids (e.g., androgens in female offspring; glucocorticoids), once deposited can they come out of the yolk over time? Can they be metabolized by the yolk or by the embryo? During incubation, how much do steroid levels in the yolk change? Can steroids diffuse from the yolk to the embryo prior to yolk utilization? Does the embryo contribute to yolk steroid levels as it develops? We believe that comprehensive answers to questions like these will eventually allow us to generate a much more accurate and complete model of the transfer and utilization of yolk steroids and that this model will be much more dynamic and active than the one initially proposed.