Placentation in opossums

Embryo of Virginia Opossum (Didelphys virginiana) at 6 days
of gestation. Opened to show the vessels of the trilaminar yolk sac
and free-floating allantoic sac. From Selenka 1886.

Emil Selenka did not confine himself to rodents and primates (previous post and recent review). He also gave the first detailed description of embryonic development in a marsupial, the Virginia opossum. He bred them in the laboratory and thus had dated pregnancies.

All marsupials have a yolk sac placenta with both two-layered and three-layered areas (bilaminar and trilaminar omphalopleure); the latter has blood vessels that radiate from a sinus terminalis as can be seen in Selenka's illustration.



Neonate of Virginia Opossum. From Selenka 1886.

The Virginia opossum has a pretty short gestation even for a marsupial. The neonate above was born after 13 days. Development is supported largely by histotrophic nutrition, i.e. uptake of uterine gland secretions. The vascular part of the yolk sac is thought to be more important for gaseous exchange.

Note that the allantois makes no contact with the trophoblast. Unlike in the koala, wombat and bandicoots, there is nothing approaching chorioallantoic placentation. The allantois serves mainly as a receptacle for urine excreted by the mesonephros (here). 

Virginia Opossum (D. virginiana) by Cody Pope CC BY-SA 2.5
(Wikipedia Commons)
 

There are 87 species of didelphids in Central and South America, but the Virginia opossum is alone in extending its range to the USA and Canada.

Despite the species richness, placentation has been described for only five opossums; three from the same genus (D. virginiana, D. aurita and D. marsupialis) plus the gray short-tailed opossum (Monodelphis domestica) and gray four-eyed opossum (Philander opossum). I will save them for a later post.