Placentation in watersnakes I: Placental histology and development in North American Nerodia (Colubridae: Natricinae)

• Published in: Journal of morphology (1931) 2017-05, Vol.278 (5), p.665-674
• Main Authors: Blackburn, Daniel G., Anderson, Kristie E., Aronson, Kera W., Burket, Mary K., Chin, Jessica F., San‐Francisco, Susan K., Callard, Ian P.
• Format: Article
• Language: English
• Subjects: Allantois - embryology; Allantois - ultrastructure; Animals; Biological Evolution; chorioallantois; Chorion - embryology; Chorion - ultrastructure; Colubridae; Colubridae - anatomy & histology; Extraembryonic Membranes - ultrastructure; Female; fetal membranes; Mammals; Nerodia; placenta; Placenta - embryology; Placenta - ultrastructure; Placentation - physiology; Pregnancy; United States; viviparity; yolk sac; Yolk Sac - embryology; Yolk Sac - ultrastructure
• ISSN: 0362-2525; 1097-4687
• DOI: 10.1002/jmor.20663

As part of a broad survey of placental structure, function, and evolution in reptilian sauropsids paraffin‐section histology was used to study microscopic anatomy of the uterus and fetal membranes of three species of North American watersnakes (Nerodia: Colubridae). The pre‐ovulatory uterus is poorly vascularized with inactive shell glands. These shell glands are activated during vitellogenesis but regress during pregnancy. Two placentas develop through apposition of the uterine lining to the chorioallantois and the yolk sac omphalopleure. Fetal and maternal components of the chorioallantoic placenta are progressively vascularized during development. Their epithelia are attenuated, but (contrary to a previous report), epithelia of neither the uterus nor the chorion are eroded. The fetal portion of the yolk sac placenta is an omphalallantois, formed of avascular omphalopleure, isolated yolk mass, and allantois. This placenta is progressively replaced by chorioallantoic placenta during mid‐ to late‐development through depletion of the isolated yolk mass. The chorioallantoic placenta is anatomically specialized for maternal–fetal gas exchange, and its expansion during development reflects the growing needs of the fetus for gas exchange. The yolk sac placenta is morphologically unsuited for gas exchange, but may serve other functions in maternal‐fetal exchange. In watersnakes (Nerodia), the placenta formed from the chorioallantois + uterus shows specializations for gas exchange. The yolk sac placenta lacks such specializations and becomes replaced during development by the chorioallantoic placenta as fetal needs for gas exchange increase.