Human development week 2

Definitions & Key takeaways

At around day 8, the trophoblast gives rise to two layers; the cytotrophoblast and the syncytiotrophoblast. The cytotrophoblast which consists of mononucleated cells, makes the chorionic villi. On the other hand, the syncytiotrophoblast consists of multinucleated cells. These cells produce hCG (human chorionic gonadotropin) needed to keep the corpus luteum viable.

The corpus luteum needs to stay viable to keep producing progesterone needed to maintain the pregnancy until the placenta grows enough to take this task over. Also, the embryoblast differentiates into ventral hypoblast that makes the yolk sac, and the dorsal epiblast that later gives three embryonic germ layers.

Around days 9 to 12, the syncytiotrophoblast goes deeper into the decidua basalis for more nutrients needed to sustain growth, whereas the epiblast develops clefts that later coalesce to form the amniotic cavity. At around day 13, the hypoblast cells form the exocoelomic mesoderm cells outside the embryo. Finally, the epiblast gives rise to the three embryonic germ layers; endoderm, mesoderm, and ectoderm.

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During the second week of human development, the blastocyst attaches to the wall of the uterus. The blastocyst’s outer layer of cells are called trophoblast cells and they penetrate into the uterus, establishing a connection between the blastocyst and the mother.
The blastocyst’s inner layer of cells are called embryoblast cells and they turn into a new, flat, two-layered structure which eventually gives rise to all of the organs and tissues of the body.
By day 7 or 8, the blastocyst implants on the surface of the endometrial wall or decidua, and the area that it implants into is called the decidua basalis.
To snuggle deeper into the decidua basalis, trophoblast cells from the outer layer of the blastocyst assemble into two layers of cells.
One called the cytotrophoblast, which are mononucleated cells, and the other called syncytiotrophoblast, which are a multinucleated cluster of “cells”.
The syncytiotrophoblast expands into the decidua basalis. By day 9, the syncytiotrophoblast has pushed deeper into the decidua basalis, and by day 11, it’s almost completely buried within it - like a seed getting pushed into soil.
Around day 12, the decidua undergoes the decidual reaction - high levels of progesterone make the decidual cells enlarge and get coated in a sugar-rich, fatty fluid which can get absorbed by the syncytiotrophoblast and helps sustain the embryo early on.
Initially, the decidual reaction only occurs at the decidua basalis, but eventually it spreads throughout all of the decidua.
Around day 14 of development, cells of the syncytiotrophoblast start to protrude out to form little protrusions called primary villi - with each one looking a bit like a tree.
These primary villi “trees” form all the way around the fetus, and cells start to clear out from between the primary villi, leaving behind empty spaces called lacunae.
While this is all happening, arteries and veins from mom start to grow into the decidua basalis. Normally we think of red blood cells staying confined to blood vessels, but as the placenta develops, an interesting thing happens - tiny arteries merge with the lacunae.
So these empty spaces become filled with oxygenated blood. Veins also merge with lacunae and bring blood back to the mother’s heart.
Now over time, more and more of these little pools of blood develop and they start merging together to form a single large pool of blood with many arteries delivering blood into it and many veins taking blood away.
This large pool is called the junctional zone. So lots of fetal villi “trees” next to one another are basically submerged in the junctional zone.
While this happens on the “outside” of the blastocyst, the inner embryoblast cells assemble into two layers forming a flat structure called the bilaminar embryonic disc.
The hypoblast is the ventral layer of the bilaminar disc, and it consists of cells that start to line the fluid-filled cavity containing the embryoblast cells, the blastocoel, which then becomes known as the yolk sac.
Oddly enough, this yolk sac contains no yolk. Instead, the yolk sac is filled with fluid, called vitelline fluid, which washes across the embryo, nourishing it during this early stage.
It’s a bit like how small critters in the ocean get nutrients directly out of the water. Now the epiblast is the dorsal layer of the bilaminar disc, and it gives rise to all three of the germ layers of the embryo - the endoderm, mesoderm, and ectoderm.
The amniotic cavity develops just above the bilaminar disk, and it gets lined with epiblast cells. So we end up with two little balls, right next to each other.
Meanwhile, in the embryoblast, cells from the hypoblast layer start to differentiate into extraembryonic mesoderm cells, named because they are outside of the developing embryo.
These are some of the earliest mesoderm cells, and they start to form even while the embryoblast itself is a bilaminar disc.
These mesoderm cells line the inside of the cytotrophoblast and syncytiotrophoblast and begin creating space to form what will eventually become the chorionic cavity.
Alright, as a quick recap… In week two of human development, the syncytiotrophoblast continues to expand into the decidua and begins to reorganize into primary villi.
Blood vessels from mom grow nearby and create pools of blood near the villous trees. Inside the blastocyst, the embryoblast differentiates into the epiblast and hypoblast.
Some cells from the epiblast become the extraembryonic mesodermal cells, which will go on to line the chorionic cavity.