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A. Bilaminar disk
The blastula (also called blastocyst) has now successfully implanted in the uterus wall and further differentiation further develops rapidly.
2.
The embryoblast differentiates into two different cell layers, forming a bilaminar disk:
- Epiblast layer
- Hypoblast layer
At the same time, a new cavity emerges within the epiblast layer; the amniotic cavity.
The epiblast will further differentiate into the embryo while the hypoblast will differentiate into the supporting structures (amnio, yolk sac, placenta, etc).
After the implantation, the blastocyst will further grow and differentiate from a single layer (epiblast) into two cell layers:
- The ectoderm
- The entoderm
Then something crucial occurs; the development of the Primitive Streak.
The primitive streak is a (very) small and elongated depression or a ‘hole’ in the ectoderm layer. At one end of the streak, a group of cells cluster together forming the primitive node.
At the other end of the streak, something similar occurs, which is called the cloacal membrane (‘cloaca’ = end of the digestive track)
In this primitive streak, (some of) the ectoderm cells invaginate into the space between the ectoderm and the entoderm layer, thereby forming a new layer; the mesoderm layer.
This streak determines the very first orientation of the embryo; namely the orientation of the future body; in which direction the head will be located, and, in the opposite direction, the ‘tail’. And, with this orientation, this also determines what the right and the left side of the new organism will become.
As stated in the previous paragraph, the epiblast has now generated three germ layers.
The three germ layers are:
- Ectoderm
- Mesoderm
- Endoderm
The ectoderm layer will develop into the outer layer of the skin (epidermis) and the nervous system
The nervous system is quite a complicated organ which includes both the central and the peripheral nervous system, and, both motor and sensory system. And, also includes the major sensory organs (eyes, touch, temperature, pain etc).
The mesoderm provides for major tissues and organs in the body:
- Bones and cartilages
- Muscles (both striated and smooth muscles)
- Most of the cardiovascular system and the lymphatic system
- Other organs such as the kidneys, spleen, ureters, internal linings such as peritoneum
- And the dermis layer in the skin!
The endoderm, what does it provide?
Again, many things, such as:
- Liver, pancreas, and other large parts of the GI-system
- The epithelia in many organs such as in the GI tract, the trachea and airways in the lungs
- Glands that open in the GI tract
After the mesoderm is being formed between the ectoderm and the entoderm, and the primitive streak is developing in the ectoderm, a new development takes place (again!).
Gradually, as shown in the figure, an invagination occurs from the entoderm ‘pushing’ into a space in the mesoderm, parallel to the primitive streak.
After a few days, this invagination separates from the entoderm (which immediately closes), thereby forming an isolated tube-like structure, the notochord, located in the mesoderm layer.
Why does the embryo develop this notochord? (see previous section): to initiate a new structure, crucial for the embryo: the neural tube.
The notochord, which originated from the entoderm, now stimulates the neighbouring cells in the ectoderm to grow and differentiate while also stimulating the mesoderm to further grow too.
This leads to a local swelling and gradually develops into a crest of cells that pushes into the amniotic cavity.
These crests, left and right, grow towards each other and gradually fuse creating a tube, which then also separates from the ectoderm.
This neural tube starts to develop at the oral end of the embryo and gradually develops towards the distal part.
As we shall see, this neural tube will differentiate and generate multiple structures in the future foetus.
Ok, we have come a long way, but … our embryo is still a ‘flat’ sheet!
So, time for a new development which takes place in all three layers, more or less simultaneously. Some areas grow faster than others leading to a curving of the sheet or, as it is generally called, ‘folding’.
This gradual folding process takes place in both the longitudinal direction and in the transversal (= lateral) direction.
In the longitudinal direction, all three layers grow and curve around the yolk sac. In the cranial direction, the oropharyngeal membrane starts to appear, localizing the future mouth.
‘Above’ this membrane, the neural tube curves also around, which will form the brain while ‘underneath’ this curve, a region of cells group together to form the first indication of the heart tube.
In the caudal direction, similar curving occurs over the yolk sac and develop into the hindgut and the cloacal membrane, and also includes the connecting stalk, for the future umbilical region.
At the same time, folding also occurs in the lateral direction.
The right and left sides of the embryonic disc fold around the developing embryo, thereby squeezing the yolk sac.
