> > I wrote a little about the topic of the marsupial placenta in my 1993 book, “Fauna of Kakadu and the Top End”, a publication still used as a text by the University of NSW. > > Denise > > Denise Lawungkurr Goodfellow > PO Box 71 > Darwin River, NT, Australia 0841 > > PhD candidate, Southern Cross University, Lismore, NSW. > > Founding Member: Ecotourism Australia > Founding Member: Australian Federation of Graduate Women Northern Territory > 043 8650 835 > > > > > > > > > > On 25 Sep 2014, at 7:39 pm, Stephen Ambrose <stephen@ambecol.com.au> wrote: > >> Marsupial embryos are attached to the wall of the uterus by a placenta in >> the very early stages of development. The outer layer of cells in the >> fertilised, dividing egg is called the trophoblast. When the egg implants >> itself in the uterine wall, the trophoblast begins to differentiate into the >> different tissue layers that form the placenta and invade the maternal >> decidua (uterine lining). In eutherians (true placentals) the trophoblast >> secretes chemicals (e.g. phosphocholine) that help the embryo avoid >> detection by the maternal immune system, and develops suppressor cells which >> inhibit the actions of maternal antigens that enter the body of the foetus. >> Therefore, the foetus of eutherian mammals can remain implanted in the >> uterus during its major phase of growth. On the other hand, the trophoblast >> of metatherians (marsupials) has not evolved all the mechanisms to suppress >> the actions of maternal antigens that occur on the surface of the uterine >> lining. Therefore, although a marsupial placenta develops, there can only be >> a brief period of attachment, otherwise the mother’s immune system would >> eventually kill the foetus. Hence, the need for marsupial embryos to leave >> the uterus and develop as pouch young. >> >> The placenta in marsupials is derived from the part of the trophoblast that >> forms the embryo’s yolk sac. Hence, it is called the yolk sac placenta. In >> eutherians, the placenta is derived from the allantois, which is another >> embryonic sac that grows behind the yolk sac. Therefore, the eutherian >> placenta is known as the allantoic placenta. The difference in the origins >> of the eutherian and marsupial placentas probably explains why the latter >> lacks some of the mechanisms to effectively combat the mother’s immune >> system. >> >> So strictly speaking, marsupials should be regarded as placental mammals and >> I favour the use of the following clades (taxonomic groupings): Eutheria >> (allantoic placental mammals), Metatheria (marsupials) and Prototheria >> (monotremes). >> >> Stephen Ambrose >> Ryde NSW >> >> >> —–Original Message—– >> From: Birding-Aus [mailto:birding-aus-bounces@birding-aus.org] On Behalf Of >> Jeremy O’Wheel >> Sent: Thursday, 25 September 2014 2:42 PM >> To: brian fleming >> Cc: birding-aus@birding-aus.org >> Subject: Re: [Birding-Aus] FW: Bird tautology >> >> Marsupials aren’t “placental mammals” either, although I think they do have >> a placenta of sorts (but very small). Placental mammals are mammals in the >> infra class “Placentalia”. >> >> Jeremy >> >> >> >> >>
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