Human development week 3

Definitions & Key takeaways

The third week of human development is marked by the development of the primitive streak, which establishes the first body axis. At around day 14, the process of gastrulation takes place, which involves the epiblast differentiating into the trilaminar embryonic disk consisting of the ectoderm, the mesoderm, and the endoderm.

There is also the formation of the primitive groove or steak groove that runs down the center of the epiblast layer. At around day 17, a group of mesodermal cells forms the notochord, a structure that releases Sonic hedgehog protein that helps to orient tissue differentiation tissue differentiation. At around day 20 to 21, the notochord triggers the process known as neurulation, in which the ectoderm forms the neural plate, later on folding into the neural tube.

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During week 3 of human development, the blastocyst is fully embedded in the endometrial tissues, or decidua, and it undergoes a process called gastrulation that starts around day 14 of human development.
Gastrulation begins with the formation of the primitive groove, which is a narrow depression that goes down the center of the epiblast layer.
When viewed from above, the groove starts near the tail or caudal end of the embryo, and grows towards the head, or cranial end.
This results in the cranial-caudal axis, and the two sides of the groove represent the first instance of bilateral symmetry in the embryo - a left and right side to the body.
Closer to the midline where the groove is located is considered medial and closer to the edges is lateral. If you view the groove from the side, then you can see that the groove forms on the back, or dorsal side of the embryo - which makes the dorsal-ventral axis more obvious.
The round bilaminar disc also elongates, and starts to resemble a guitar pick, narrow at the caudal end and wide at the cranial end.
Let’s zoom out a bit, so we can see what this looks like as a tissue instead of individual cells, and we can compare them side by side.
At the cranial end of the primitive groove, a small mound of tissue develops called the primitive node, and a tiny dimple within it forms called the primitive pit.
The primitive groove, primitive node, and primitive pit together form the primitive streak. Try saying that 3 times quickly...
Okay - so as the primitive streak forms in the epiblast layer, some epiblast cells start to migrate towards the primitive groove, move down into the bottom of the groove, and then actually dive right into it.
It’s a bit like a child diving into a ball pit at a funhouse. The epiblast cells that slip through the primitive groove begin to differentiate to form new cell layers.
Some epiblast cells dive deep and form the embryonic endoderm layer - these freshly differentiated endoderm cells quickly replace the ventral hypoblast cell layer.
Other epiblast cells take a more shallow dive and form the embryonic mesoderm layer, which meets up with the extraembryonic mesoderm layer that was created earlier from the epiblast.
The extraembryonic mesoderm also divides into two layers the parietal mesoderm and the visceral mesoderm. Finally the epiblast cells that don’t dive into the groove at all, form the embryonic ectoderm layer.
So we end up with a three layered disc called the trilaminar disc with the ectoderm, mesoderm, and endoderm layers. These cell layers - called the germ layers - are multipotent, meaning they can differentiate into any tissue or organ.
Around day 15 of development, two areas of the ectoderm layer- one in the cranial region and one in the caudal region push ventrally and fuse with the endoderm layer, excluding the mesoderm layer entirely, and forming two bilaminar regions in an otherwise trilaminar disc.
The cranial bilaminar region develops into the oropharyngeal membrane which eventually disintegrates in week 4 to form the opening of the mouth.
The caudal bilaminar region develops into the cloacal membrane, which eventually disintegrates in week 7 to form the opening of the anus and genitourinary tracts.
Around day 17, a group of mesoderm cells form a solid rod of cells - kind of like the shaft of an arrow - and this structure is called the notochord.
The notochord is a transient embryonic structure, meaning that it doesn’t exist as a structure in the adult, in fact, the only remnant adults have of the notochord is that it contributes to the nucleus pulposus - which is the jelly-like center of the intervertebral discs.
Nevertheless, the notochord is extremely important during early development for a couple of reasons. First, the notochord is a solid structure and it helps influence how the embryo folds early on.
Second, the cells of the notochord secrete a protein called Sonic HedgeHog or Shh for short, which diffuses out through the trilaminar disc.
The closer a cell is to the notochord, the higher the concentration of Shh, so it’s a way for all of the cells to know where they are in three-dimensional space.
This helps the surrounding tissues differentiate and develop in the right way. On day 20, mesoderm cells around the notochord differentiate into three specialized types of mesoderm called the paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm, each of which goes on to make different tissues and organs.
The notochord also starts a process called neurulation where it stimulates the cells in the ectoderm layer to thicken and form the neural plate.
The neural plate starts to fold, and it dips down forming a neural groove with edges that are called neural folds. As the groove continues to grow ventral to the ectoderm layer, the neural folds comes together and they pinch off from the surface of the ectoderm layer, forming the neural tube.
The neural tube now sits between the mesoderm and the ectoderm. While the germ layers are becoming apparent, the syncytiotrophoblast cells - the outer cells of the blastocyst responsible for firmly implanting the embryo within the uterus - are further establishing the connective network between the embryo and the mother.
Primary villi along the outer trophoblast layer continue to get more and more submerged into the maternal blood of the junctional zone.
In addition, extraembryonic mesoderm cells migrate into the primary villi, forming secondary villi, which differentiate into small fetal blood vessels that collectively form the villous capillary system, which is the fetal contribution to the placenta.
At this point, the circulatory link between the embryo and the placenta is firmly established so that oxygen and nutrients can be pulled in from maternal blood as soon as the heart begins to beat.
All right, as a quick recap… In the third week of human development, the primitive streak establishes the first body axis, and triggers the epiblast layer to differentiate into the three germ layers forming the ectoderm, mesoderm, and endoderm.
Soon after, mesodermal cells form the notochord, which releases Sonic hedgehog protein which helps to define a three dimensional map for the nearby cells.
It also sparks the process of neurulation in the ectoderm which results in the formation of the neural tube. Finally, secondary villi develop within the primary villi and help to form the villous capillary system in the placenta.