Evidence of steroid hormone activity in the chorioallantoic membrane of a Turtle (Pseudemys nelsoni).

Endocrine properties of extraembryonic membranes have traditionally been viewed as a characteristic of placental amniotes. However, our laboratory recently demonstrated that this ability extends to the extraembryonic membranes of two oviparous amniotes (chicken and alligator) indicating that endocrine extraembryonic membranes are not an innovation of placental amniotes and suggesting that this could be a shared amniote characteristic. In this study, we test our hypothesis that the chorioallantoic membrane (CAM) obtained from non-archosaurian obligate oviparous amniotes such as turtles, have the potential for steroid hormone activity. To investigate synthesis of a major placental hormone, we performed explant culture and found that the turtle CAM synthesizes progesterone in vitro in the presence of a steroid precursor. In addition, to examine whether the CAM has the ability to respond to steroid signaling, we quantified mRNA expression of the progesterone, androgen, and two estrogen receptors. Finally, to determine if steroid receptor mRNA is translated to protein, we performed immunolocalization of the progesterone receptor. Our data demonstrate that the turtle CAM exhibits steroid synthesis and has steroid hormone signaling capabilities. To that end, steroid hormone activity has now been demonstrated in the CAMs of three oviparous species that represent three independent lineages within oviparous Reptilia that have never exhibited viviparity; thus these data support our hypothesis that endocrine activity of extraembryonic membranes is a conserved trait of Amniota.