Endocrine Activity of Extraembryonic Membranes Extends beyond Placental Amniotes

Abstract

BackgroundDuring development, all amniotes (mammals, reptiles, and birds) form extraembryonic membranes, which regulate gas and water exchange, remove metabolic wastes, provide shock absorption, and transfer maternally derived nutrients. In viviparous (live-bearing) amniotes, both extraembryonic membranes and maternal uterine tissues contribute to the placenta, an endocrine organ that synthesizes, transports, and metabolizes hormones essential for development. Historically, endocrine properties of the placenta have been viewed as an innovation of placental amniotes. However, an endocrine role of extraembryonic membranes has not been investigated in oviparous (egg-laying) amniotes despite similarities in their basic structure, function, and shared evolutionary ancestry. In this study, we ask whether the oviparous chorioallantoic membrane (CAM) of chicken (Gallus gallus) has the capability to synthesize and receive signaling of progesterone, a major placental steroid hormone.Methodology/Principal FindingsWe quantified mRNA expression of key steroidogenic enzymes involved in progesterone synthesis and found that 3β-hydroxysteroid dehydrogenase, which converts pregnenolone to progesterone exhibited a 464 fold increase in the CAM from day 8 to day 18 of embryonic development (F5, 68 = 89.282, p<0.0001). To further investigate progesterone synthesis, we performed explant culture and found that the CAM synthesizes progesterone in vitro in the presence of a steroid precursor. Finally, we quantified mRNA expression and performed protein immunolocalization of the progesterone receptor in the CAM.Conclusions/SignificanceCollectively, our data indicate that the chick CAM is steroidogenic and has the capability to both synthesize progesterone and receive progesterone signaling. These findings represent a paradigm shift in evolutionary reproductive biology by suggesting that endocrine activity of extraembryonic membranes is not a novel characteristic of placental amniotes. Rather, we hypothesize that these membranes may share an additional unifying characteristic, steroidogenesis, across amniotes at large.

Embryonic modulation of maternal steroids in European starlings ( Sturnus vulgaris )

“…In all of these studies, multiple metabolites (both conjugated and unconjugated) were detected, suggesting the presence of numerous metabolic enzymes during this period. The specific tissue of origin for these enzymes remains to be elucidated, but metabolic enzymes are expressed in the chorioallantoic membrane [18] and embryo [19] of the chicken. Additionally, very little is known about the potential biological activity of these metabolites.…”

Morphology of the sacral region and reproductive strategies of Metriorhynchidae: a counter-inductive approach

“…Recent research on viviparity and associated modes of fetal nutrition has produced some interesting results. Reproductive patterns in vertebrates, and the widespread occurrence of viviparity, are products of convergent evolutionary trends (Blackburn 2015), and some features that play a central role in embryonic development, such as the endocrine properties of extraembryonic membranes that have traditionally been viewed as a characteristic of placental amniotes, also characterise the chorioallantoic membrane of turtles, birds and crocodilians (Albergotti et al 2009;Cruze et al 2013).…”

Towards an Understanding of the Evolution of the Chorioallantoic Placenta: Steroid Biosynthesis and Steroid Hormone Signaling in the Chorioallantoic Membrane of an Oviparous Reptile1

“…We demonstrated that the chicken CAM exhibited mRNA expression of steroidogenic enzymes involved in P 4 biosynthesis, was capable of in vitro P 4 synthesis, and exhibited mRNA and protein expression of the progesterone receptor (PR). This indicated that steroidogenic extraembryonic membranes are not an exclusive characteristic of viviparous amniotes, and we hypothesized that steroidogenic extraembryonic membranes are an evolutionarily conserved characteristic of all amniotes [7].…”

“…While it is now understood that steroid hormone synthesis and signaling is a critical function of the placenta, the evolution of these functions in this organ is poorly understood. We have hypothesized previously that the ability of the amniote placenta to perform steroid biosynthesis and respond to steroid hormone signaling likely evolved in the extraembryonic membranes of an oviparous ancestor [6,7].…”

“…Therefore, one could hypothesize that the extraembryonic membranes of oviparous amniotes, sharing a conserved evolutionary history, could also perform steroidogenesis. Recently, our laboratory reported that the extraembryonic membranes of an oviparous amniote, the CAM of the chicken (Gallus gallus), has the capability to synthesize and respond to the signaling of progesterone (P 4 ) [7]. We demonstrated that the chicken CAM exhibited mRNA expression of steroidogenic enzymes involved in P 4 biosynthesis, was capable of in vitro P 4 synthesis, and exhibited mRNA and protein expression of the progesterone receptor (PR).…”