Embryonic and trophoblastic areas of the marsupial
blastocyst as envisaged by Hartman
In the final chapter of her book (previous post), Zofia Kielan-Jaworowska writes on placental mammals, "Their enormous success depends in the first place on the acquisition of the trophoblast, on the basis of which the placenta is formed." In contrast, "Without the trophoblast, marsupials are unable to prolong their gestation period and are born at an extremely early level of anatomical development." This view or a version of it seems to be shared by other zoologists.
The souce of the misunderstanding can be traced to influential papers by Jason A. Lillegraven (e.g. here). The marsupial blastocyst lacks the inner cell mass of a placental mammal blastocyst. Early on, however, there are two distinct areas that often have been referred to as the embryonic and trophoblastic areas. Lillegraven argues, convincingly in my view, that the "embryonic area" is not equivalent to the inner cell mass and that a considerable portion of the "trophoblastic area" forms part of the developing embryo. In an anatomical sense the trophoblastic area is not the trophoblast as defined by Hubrecht (see previous post).
Nonetheless, Lillegraven also says, "But from a functional point of view (as applied to extra-embryonic parts of the conceptus having direct nutritive significance), the term "trophoblast" may be used to good advantage throughout the Amniota, whether development occurs within a shelled or a shell-free setting."
Thus marsupials do have trophoblast and of course they all have at least a yolk sac placenta and sometimes a chorioallantoic placenta to boot. As shown by Ulrich Zeller and Claudia Freyer (here) there are marsupials where the trophoblast forms a syncytium and exhibits invasive properties.
As noted in a previous post, there are sound reasons for distinguishing between Eutheria and Placentalia but it is little wonder that marsupial specialists bridle at the term "placental mammals."
