2nd week of pregnancy

Also known as the week of two’s:

1. Two embryoblast layers form:

  • Endoderm (Hypoblast)
  • Ectoderm (Epiblast)

Therefore forming a bilaminar germ disc

2. Two trophoblast layers form:

  • Cytotrophoblast
  • Syncytiotrophoblast – has lacunae Image result for lacunae in syncytiotrophoblast

3. Two cavities form:

  • Amniotic cavity – the cytotrophoblast cells surrounding the amniotic cavity become amnioblast cells
  • Primary yolk sac – endodermal cells grow down, line cytotrophoblast forming Heuser’s membrane (exocoelomic membrane)

Heuser’s membrane/exocoelomic membrane – Short lived combination of hypoblast cells and extracellular matrix

Image result for heuser's membrane

4. Two extraembryonic mesodermal layers form:

In the cytotrophoblast, extraembryonic mesoderm forms, in which lacunae appear and fuse to form extraembryonic coelom. Therefore dividing the extraembryonic mesoderm into 2 layers.

  • Somatopleuric mesoderm – lines cytotrophoblast and amniotic cavity
  • Splanchnopleuric mesoderm – lines primary yolk sac
Image result for splanchnopleuric mesoderm


  • Secondary yolk sac also forms, when a large part of primary yolk sac separates and disappears leaving the smaller part. It is completely lined with endoderm.
Secondary Yolk Sac
  • Primary chorionic villi forms – cytotrophoblast projects into syncytiotrophoblast (more in the next chapter)

Syncytiotrophoblast – Placental barrier between maternal and fetal blood