Placentation in the Amazonian manatee (Trichechus inunguis)

Abstract

Abstract. Evidence from several sources supports a close phylogenetic relationship between elephants and sirenians. To explore whether this was reflected in similar placentation, we examined eight delivered placentae from the Amazonian manatee using light microscopy and immunohistochemistry. In addition, the fetal placental circulation was described by scanning electron microscopy of vessel casts. The manatee placenta was zonary and endotheliochorial, like that of the elephant. The interhaemal barrier comprised maternal endothelium, cytotrophoblasts and fetal endothelium. We found columnar trophoblast beneath the chorionic plate and lining lacunae in this region, but there was no trace in the term placenta of haemophagous activity. The gross anatomy of the cord and fetal membranes was consistent with previous descriptions and included a four-chambered allantoic sac, as also found in the elephant and other afrotherians. Connective tissue septae descended from the chorionic plate and carried blood vessels to the labyrinth, where they gave rise to a dense capillary network. This appeared to drain into shorter vessels near the chorionic plate. The maternal vasculature could not be examined in the same detail, but maternal capillaries ran rather straight and roughly parallel to the fetal ones. Overall, there is a close resemblance in placentation between the manatee and the elephant.

Histological characterization of reproductive tract and fetal annexes of the West Indian Manatee (Trichechus manatus) from Brazil

“…The placenta was similar to that of Amazonian manatees with a well-developed villus region, syncytiotrophoblast, and the presence of maternal tissue cells and chorionic villi. The umbilical cord was similar to that of T. inunguis (Carter et al 2008).…”

Gestating for 22 months: luteal development and pregnancy maintenance in elephants

“…Dugongs (Dugong dugon) and manatees (Tricheus inuguis) are uniparous, but show the same pattern of multiple CLs clustering on the ovary of the gravid uterine horn [40,41]. Since sirenia also show a zonary, endotheliochorial placenta very similar to elephants [42], it is possible that the function and the formation of acCLs are the same. Reproductive processes usually appear relatively conserved across a genus and the special reproductive features found in elephants and sirenia may consolidate their close relationship.…”

Life-History Correlates of Placental Structure in Eutherian Evolution

“…In the simplest terms, there is still debate concerning gross placental morphology in certain species [e.g., placental shape in Ursus arctos (Benirschke 2010), interhemal barrier in Trichechus inunguis (Carter et al 2008)] and obvious differences in the qualities of certain placental conditions [e.g., invasiveness in the haemochorial Proboscidae (Allen et al 2002(Allen et al , 2003]. In such cases, categorisation of placental variation into discrete traits can be challenging, but also helps to absorb some of the uncertainty that is characteristic of any classification scheme.…”