Dynamic Changes in Yolk Steroid Levels in Eggs During Development
The early endocrine environment is important to embryonic development because steroid exposure can induce permanent effects on offspring. In birds, maternal steroids are present in the yolk, which is thought to occur because steroids are lipophilic and the yolk has a high lipid content. Numerous steroids can be detected in bird yolks, with progestogens such as progesterone, pregnenolone, 17a-hydroxypregnenolone, pregnanedione, and pregnanolone being more abundant than the androgens. Once incubation begins, many of these steroids are subject to metabolism by the embryo in ovo, but very little is known about the specific routes of metabolism or what happens to steroids in the yolk during the later stages of development. To examine how steroid levels in the yolk change throughout development, chicken eggs were incubated and frozen at days 3, 6, 9, 12, and 15 of development and steroid levels in the yolk were quantified using LC/MS/MS. We found that some steroids, such as pregnenolone, etiocholanolone, progesterone, pregnanedione, and pregnanolone, showed a drop in concentration early in development but levels then transiently rose during the middle stages of development before dropping again after day 12. Our current interpretation of these findings is that maternally derived steroids that are present at the onset of development are metabolized before the embryo starts producing steroids that can be detected in the yolk. Towards the end of development, the embryonic steroids in the yolk are metabolized. Overall, these data suggest that steroid levels in the yolk of bird eggs are dynamic over the course of development, going through several periods of being elevated and then subsequently metabolized. The specific mechanisms regulating these changes in yolk steroid levels are currently being investigated.
Sudbrook, Teagan, "Dynamic Changes in Yolk Steroid Levels in Eggs During Development" (2020). Biological Sciences. 2.