Introduction to Human General Embryology Developmental Genetics

This is formed within the trophoblast particularly in the syncytiotrophoblast. Its formation coincides with implantation.

During implantation, the syncytiotrophoblast invades into the maternal endometrium, and in this sense it is more invasive than any tumor tissue. As it comes into contact with blood vessels it creates lacunae, or spaces which later become filled with maternal blood. It is these lacunae that fuse to form lacunar networks.

As maternal blood flows in and out of the networks, exchange of nutrients and waste products occur with the fetus, forming the basis of a primitive uteroplacental circulation.


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The Amniotic Cavity

This is the cavity formed within the epiblast. The stages involved in its development include:

1. 
   The inner mass cell pulls away from the trophoblast forming a hollow that is surrounded by cells originating from the epiblast and called amnioblasts
   

2. 
   This cavity enlarges to form the amniotic cavity bounded by the amnioblasts and the rest of the epiblast.
   

3. 
   Further enlargement of this cavity occurs during embryonic folding.
   

4. 
   It is fluid filled (Amniotic fluid) and the volume of the fluid in a giving instance is of clinical importance. In one instance, an abnormally low volume might indicate abnormal development of the kidneys. The fluid constituents are of clinical significance as well e.g. amniocentesis.
   

This cavity is formed within the extraembryonic mesoderm.

First, an acellular epithelium proliferates between the exocoelomic membrane surrounding the exoceolmic cavity, and the cytotrophoblast, forming the extraembryonic mesoderm. This mesoderm surrounds the exocoelomic and amniotic cavities. With further development, the mesoderm increases in size and isolated spaces appear within it. It is these spaces will rapidly fuse to form a large cavity, the extraembryonic coelom or chorionic cavity.

1. 
   This fluid-filled cavity surrounds the conjoined amnion and primary yolk sac except just dorsal to the amnion where the extraembryonic mesoderm remains connected to both the amnion and the cytotrophoblast forming the connecting stalk
   

2. 
   The connecting stalk later becomes the major part of the umbilical cord.
   

3. 
   The extraembryonic coelom splits the extraembryonic mesoderm into two parts i.e. the extraembryonic somatic mesoderm, which lines the trophoblast and covers the amnion, and the extraembryonic splanchnic (or visceral) mesoderm, which covers the yolk sac.
   

4. 
   The extraembryonic somatic mesoderm and the two layers of trophoblast form the chorion, or fetal portion of the placenta.
   

5. 
   The chorion forms the wall of the chorionic sac within which the embryo and its amniotic and yolk sacs are suspended by the connecting stalk.
   

6. 
   As the extraembryonic coelom forms, the primary yolk sac decreases in size and a smaller, secondary, yolk sac forms.
   

7. 
   By the end of Day 13 and beginning of Day 14, the primitive yolk sac disintegrates into a collection of exocoelomic cysts. The much smaller secondary yolk sac is renamed the definitive yolk sac.