The placenta, an endocrine organ that synthesizes and transports an array of steroid hormones during embryonic development, is imperative for embryonic survival. The placenta is a collective structure encompassing the maternal uterus and the embryonic membranes. The formation of three embryonic membranes; the amnion, chorion, and allantois, define the group known as amniotes which includes mammals, reptiles, and birds. Therefore, embryonic membranes are conserved across these vertebrate lineages.  Recent evidence that embryonic membranes synthesize and metabolize hormones in humans and rhesus monkeys suggests that these membranes could have similar functions in non-mammalian amniotes.

Is endocrine function of the embryonic membranes solely a characteristic of placental amniotes? I hypothesize that embryonic membranes also have an endocrine role in oviparous (egg-laying) species.

I have chosen to investigate this question by examining gene expression of several steroidogenic enzymes in the chorioallantoic membrane of the chicken (Gallus domesticus). An immense volume of literature exists on both chick embryonic development and development of? the chorioallantoic membrane (CAM). However, examination of steroidogenesis in the CAM has not previously been examined. As an established model organism, the chicken is an ideal species for an initial investigation into this question. Likewise, the chick CAM has been used extensively as an experimental model for angiogenesis, tumor growth and transplant studies.

I will investigate the localization of 3-beta-hydroxysteroid dehydrogenase (3-beta-HSD) in the CAM using in situ hybridization. I have chosen 3-beta-HSD for this study due to its role in synthesizing progesterone from pregnenolone. Progesterone is produced by the placenta and is important in the maintenance of pregnancy.

Because the chorion and allantois fuse to form the CAM, it is possible that the patterning of the 3-beta-HSD expression could be significant at the level of the tissue layer. In situ hybridization of sectioned tissues will be used to show specific localization of the mRNA within the different tissue layers of the CAM.