DEVELOPMENT OF
PLACENTA AND FETAL MEMBRANES
As the embryo grows, it must establish an efficient means of obtaining nutrients and eliminating waste products. It does this by establishing the placenta, an efficient interface between its vascular system and that of its mother.
Trophoblastic cells have marked
invasive capacities and grow into the walls of maternal blood vessels,
establishing contact with the maternal bloodstream. In early pregnancy,
trophoblastic cells frequently invade deep into the myometrium, but as
pregnancy progresses, invasion is limited by profuse proliferation of decidual
cells, which confine the trophoblastic invasion to the area just beneath the
attachment of the growing placenta. In the rare instances when decidual cells
fail to develop, implantation overlies an old scar or there are defects in the
development of the fibrinoid layer (Nita-buch layer), invasion of the uterine
wall by chorionic villi is extensive. This can result in a placenta accreta,
increta, or percreta.
In the recently implanted
blastocyst, the rim of trophoblastic cells, with the underlying mesodermic
stroma, constitutes the primitive chorion. At the same time, the amnion first
appears as a small cavitation in the mass of proliferating ectodermal cells in
the embryonic area. This cavity gradually enlarges and folds around the developing
embryo, so that eventually the latter is suspended by a body stalk (the
umbilical cord) in a closed bag of fluid (the amniotic sac).
During the early stage of
development of the amnion, another vesicle appears in the embryonic area and
for a time is much larger than the amnion. This is the yolk sac (not
illustrated), the function of which in mammalian development is not known. As
the embryo grows, the yolk sac decreases in size, until at term only a minute
remnant can be found near the site of the cord attachment to the chorionic
plate.
During the first 3 weeks after
implantation, a luxuriant growth of the rudimentary villi over the entire
blastodermic vesicle occurs, developing into a structure called the chorion
frondosum or “leafy chorion.” As the embryo, surrounded by the amnion, grows
and protrudes more and more into the uterine cavity, the decidua capsularis and
the underlying chorionic villi stretch and become flattened and atrophic. Most
of the villi disappear from this region, which is then called the bald chorion
or chorion laeve. Meanwhile the villi proliferate markedly in the highly
vascular decidua basalis. Here the chorion frondosum persists and becomes a
part of the fully developed placenta.
In rare cases the chorionic villi,
beneath the decidua capsularis, do not undergo atrophy but establish vascular connections with the decidua vera,
opposite the site of implantation, when the enlarging conceptus fills the
uterine cavity. In this condition, called placenta membranacea, the entire chorion
is covered with villi, and the thin placenta thus formed bleeds freely, does
not separate spontaneously, and is difficult to remove manually during the third
stage of labor.
The chorionic villi contain no blood
vessels during the first 2 weeks of gestation, and the embryo has not yet
developed a circulatory system. Nutrition is chiefly by osmosis. Toward the end of the third week,
certain cells in the mesodermic stroma differentiate into blood islands, around
which vascular walls soon appear. By branching and coalescence of these
vessels, the entire chorion becomes vascularized. Meanwhile, a fetal heart and
circulatory system have been developing. By the end of the fourth week,
connections are made between the vessels of the chorion and those of the fetus,
which have grown out through the body stalk, thus establishing a
fetal–placental circulation.
