HUMAN EMBRYOGENESIS (STEP 5-7)

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Making Tubes

  • Step 5: During gastrulation the three germ layers form; the cell mass is now known as a gastrula
  • Step 5a: The primitive streak forms
  • Step 6: The notochord is formedImagen relacionada
Week 3 of development is the week of gastrulation. A germ layer is a layer of cells that will go on to form one of our organizational tubes. Our anatomy can really be boiled down to an inner tube (our digestive tract), and a series of tubes that wrap around it. The three germ layers that will translate into these tubes are the ectoderm, the mesoderm, and the endoderm.
Germ LayerWhat does the prefix mean?Goes on to form:
EctodermOuter, externalEpidermis (outer layer of skin), hair, nails, brain, spinal cord, peripheral nervous system
MesodermMiddleMuscle, bone, connective tissue, notochord, kidney, gonads, circulatory system
EndodermWithinEpithelial lining of the digestive tract; Stomach, colon, liver, pancreas, bladder, lung

The first step of gastrulation is the formation of the primitive streak (~ day 16). Let’s imagine the bilaminar disc as two tier cake. Imagine taking a knife and cutting into just the top layer (the epiblast) like you’re going to cut a slice.


Figure of two tier cake representing epiblast and hypoblast with cut through epiblast layer
This cut is the primitive streak, and it cuts from the caudal (anus) end in toward the end that will eventually become the head (the rostral end). This streak determines the midline of the body, and separates the left and right sides.  What we are actually seeing when we look at a primitive streak are moving cells. They are going from the epiblast and moving down so they end up between the original epiblast layer and the hypoblast. The first layer to invaginate dives the deepest and ends up closest to the hypoblast – this is the endoderm. The next layers will become the mesoderm, and the cells of the epiblast that continue to border the amniotic cavity are the ectoderm. We now have three germ layers, all of which will contribute to the developing embryo. In the picture below, the anus end is facing us.


Figure of bilaminar disc
Directly beneath the primitive streak the mesoderm (the middle germ layer) forms a thin rod of cells known as the notochord. The notochord helps define the major axis of our bodies, and is important in inducing the next step of embryogenesis, when we finally start to make our tubes! The notochord is a defining feature of the Chordate phylum, and will eventually become our intervertebral discs.

Neurulation

  • Step 6: Tubes form, making a neurula
  • Step 6a: The notochord induces the formation of the neural plate
  • Step 6b: The neural plate folds in on itself to make the neural tube and neural crest
  • Step 7: The mesoderm has five distinct categories
In the last post of this section there will be videos for you to watch and see this last part of the embryogenesis in case you don't want to read all of that information.

Now that we have successfully made the cell layers, we have to create the final 3D product. The first step in this rolling is the creation of the notochord. The notochord causes the ectoderm above it to form a thick flat plate of cells called the neural plate. The neural plate extends the length of the rostral-caudal axis. The neural plate then bends back on itself and seals itself into a tube known as the neural tube that fits underneath the ectoderm. The borders of where the neural plate had been get pulled under with it, and become the neural crest. The neural tube will become the brain and spinal cord.
The neural crest is sometimes called the fourth germ layer, because the cells that become the sympathetic and parasympathetic nervous systems, melanocytes, Schwann cells, even some of the bones and connective tissue of the face.

Illustration of neurulation process
Meanwhile, the mesoderm can be subdivided into the axial, paraxial, intermediate, and lateral plate mesoderms.  The paraxial mesoderm will give rise to somites, which will differentiate into muscle, cartilage, bone, and dermis. The intermediate mesoderm is the origin of our urogenital system – our kidneys, gonads, adrenal glands, and the ducts that connect them. The lateral plate mesoderm will give rise to the heart,blood vessels, the body wall, and the muscle in our organs.


Figure of stick figure labeled with urogenital system, heart, blood vessels, the body wall, and organ muscle


Figure of endoderm transforming into the digestive tract
Also at the same time, the endoderm is rolling into a tube as well – the digestive tract. The digestive tract is subdivided into the foregut, midgut, and hindgut. Each subdivision has its own nerve and blood supply. 


Diagram of gut formation
That’s what is going in with each of the three layers. While this is happening, the mesodermal layers are circling around the endoderm, and the part of the ectoderm that will become the skin is circling around both of the other layers. Some tubes, likes the neural tube, are closing, while the gut tube is connecting to the ectoderm to form the mouth and the anus.




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