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 On Behalf Of
Jeremy O'Wheel
Sent: Thursday, 25 September 2014 2:42 PM
To: brian fleming
Cc:
Subject: 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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