Second Week of Human Development

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Events in the 2nd week of development
- Formation of the epiblast and hypoblast
  - In the 2nd week of development (days 8 – 14) the embryoblast differentiates into 2 layers – the epiblast and hypoblast – termed the bilaminar disc
  - Epiblast: Columnar cells adjacent to the amnioblast. Form the amniotic cavity and the embryo during the 3rd week.
  - Hypoblast: Cuboidal cells adjacent to the blastocoel. Form the yolk sac and the lining of the chorionic cavity
  - The epiblast and hypoblast are fused at the proximally at the prechordal plate, which will later develop into the mouth
- Proliferation and differentiation of the trophoblast
  - Also in the 2nd week of development, the trophoblast divides into 2 layers – the cytotrophoblast and syncytiotrophoblast – These will form the embryonic component of the placenta (The maternal component is derived from the endometrium)
  - Cytotrophoblast: The inner layer of the chorionic villi. Is mitotically active. Cells of the cytotrophoblast migrate outwards and fuse to become the syncytiotrophoblast
  - Syncytiotrophoblast: Outer layer of the chorionic villi. It is mitotically inactive. These cells invade the endometrium, creating lacunae (spaces in between them) that fill with maternal blood
- Formation of the chorionic cavity (Extraembryonic coelom)
  - The trophoblast proliferates faster than its surrounding structures forming a gap between it known as the amniotic cavity. The amniotic cavity is lined by extraembryonic mesoderm arising from the hypoblast
  - The extraembryonic mesoderm splits into 2 layers – Somatopleuric (parietal) mesoderm and Splanchnopleuric (visceral) mesoderm –
  - Somatopleuric mesoderm: Lines the trophoblast
  - Splanchnopleuric mesoderm: Lines the amniotic cavity and the yolk sac, with an intermediate embryonic disk
  - A cavity forms between the somatopleuric and splanchnopleuric mesoderm. This is known as the Extraembryonic coelom. It is lined by the chorion (outermost cell layer surrounding the embryo)
  - A connecting stalk (a derivative of the extraembryonic mesoderm) connects the amniotic cavity and primitive yolk sac to the chorion. It is the precursor of the umbilical cord.

Second Week of Human Development

Jeffrey Kalei

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