Development of the fetal membranes

Definitions & Key takeaways

The fetal membranes, also known as extraembryonic membranes, are associated with the developing embryo or fetus and protect and nourish it during gestation. These include the amnion, the yolk sac, the chorion, and the allantois. The chorion and amnion form the amniotic sac, which is filled with amniotic fluid in which the fetus is bathed in, safe from mechanical shock, and the allantois, which collects waste products from the embryo. The allantois degenerates early in pregnancy.

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The fetal membranes, sometimes called extraembryonic membranes, are tissues that form in the uterus during the first few weeks of development and develop along with the growing embryo.
These are the amnion, the yolk sac, the chorion, and the allantois, and each of these membranes starts out as a thin sheet of tissue that surrounds a fluid filled cavity.
The first membranes to form are the amnion and the yolk sac. At the beginning of week two, the embryo is not quite an embryo yet, it’s just a blastocyst.
And the blastocyst is partially embedded into the endometrium of the mother. The blastocyst has two parts: an outer cell mass called the trophoblast and an inner cell mass called the embryoblast.
The trophoblast is divided into the cytotrophoblast which forms the wall of the blastocyst, and the syncytiotrophoblast which is outside the wall and invades into the uterus.
The embryoblast is a bilaminar disc with a dorsal epiblast layer and a ventral hypoblast layer. During day 8, a tiny space, called the amniotic cavity, forms between the epiblast and cytotrophoblast.
The epiblast cells migrate to form a thin membrane called the amnion that surrounds the amniotic cavity, separating it from the cytotrophoblast.
During day 9, hypoblast cells begin to migrate to form a thin membrane lining the rest of the blastocoel, forming the walls of the yolk sac.
The yolk sac fills with fluid, called vitelline fluid, which washes across the embryo, nourishing it during this early stage.
So basically, by the middle of the second week, the two-layered embryonic disc is “sandwiched” between the amniotic cavity and the yolk sac.
By day 10, some epiblast cells differentiate into extraembryonic mesoderm, and they settle between where the amniotic cavity and yolk sac and where the cytotrophoblast is located, eventually creating a thick layer of extraembryonic mesoderm tissue between the two.
Then, over time, a space starts to form within this thick layer of extraembryonic mesoderm. The cells migrate either towards the amniotic cavity and yolk sac side or towards the cytotrophoblast side, leaving behind a space in the middle.
This space is called the extraembryonic coelom or chorionic cavity and it continues to expand, until there’s eventually just a thin layer of extraembryonic mesoderm cells lining the amniotic cavity and the yolk sac and lining the cytotrophoblast.
The one spot where the cavity doesn’t form, is at the body stalk which is where the embryoblast still remains attached to the wall.
At this point, the wall is composed of a layer of extraembryonic mesoderm, then cytotrophoblast, and finally syncytiotrophoblast, and together this is called the chorion.
The chorion develops chorionic villi, tree-like projections that invades into the endometrium of the uterus and breaks it down for nutrients.
And eventually the chorionic villi help form the fetal part of the placenta. During week 3, the embryo changes its shape - it folds along its two axes and goes from looking like a flat disc to a curved tube, so that the embryo looks like bit like a shrimp at this stage.
The folding also pinches the yolk sac so that it resembles a balloon that’s squeezed through a ring. The part of the yolk sac that goes within the embryo is called the gut tube and it becomes the digestive tract, the narrow portion is the vitelline duct, and the remainder keeps the name “the yolk sac”.
Finally, the last of the fetal membranes, the allantois, develops as a tiny outpouching from the rear part of the gut tube called the hindgut.
The allantois is a thin tube that grows up towards the umbilicus, and behind it develops the bladder. The allantois becomes a canal through which urine can exit the bladder before the urethra develops.
Between weeks 4-8, the amnion starts secreting a lot of amniotic fluid and this causes the amniotic cavity to swell and fold down and around the embryo - it becomes a fluid-filled sac protecting and insulating the embryo.
Eventually the amnion and the chorion fuse together and form the amniotic sac. Meanwhile during week 4, the allantois, the vitelline duct, and the body stalk combine to form the umbilical cord.
Over time, the nutritious yolk sac contents get used up and the yolk sac and vitelline duct shrink and and eventually disappear.
The allantois degenerates into a fibrous structure called the urachus, that remains attached to the urinary bladder. All right, as a quick recap: the four fetal membranes that develop in order to support and nourish the developing fetus include the amnion, the yolk sac, the chorion, and the allantois.
The chorion and amnion form the amniotic sac, which is filled with amniotic fluid in which the fetus is suspended by the umbilical cord.
The chorion also forms the fetal part of the placenta, helping in the exchange of nutrients, waste products and gases between the fetus and the mother.
The allantois, along with the vitelline duct of the yolk sac are incorporated into the umbilical cord but degenerate early in pregnancy.