Characteristic signs

• Fertilized oocyte
  
  
• Polar bodies
  
  
• Male and female pronucleus
  
  
• Formation of the zygote

© Dr. A. Senn et al, CHUV Lausanne

Characteristic signs

• Separation of the blastomeres
  
  
• Formation of the blastomeres (totipotent to ca. the 12-cell stage)
  
  
• Morula stage
  
  
• Compaction (8 to 16 cells)
  
  
• The polarization of the blastomeres becomes slowly visible

© Dr. A. Senn et al, CHUV Lausanne

Characteristic signs

• After the blastocyst cavity has formed, one speaks of a blastocyst (16 to 32 cells).
  
  
• Segmentation (inner and outer cell mass).
  
  
• Embryonic and abembryonic poles.

© Dr. A. Senn et al, CHUV Lausanne

Characteristic signs

• Free blastocyst.
  
  
• Hatching of the blastocyst through disintegration of the zona pellucida.
  
  
• Epiblast and hypoblast are formed and thereby the didermic embryonic disk.

© Dr. A. Senn et al, CHUV Lausanne

Characteristic signs

• Syncytiotrophoblast.
  
  
• Cytotrophoblast.
  
  
• Anchoring of the blastocyst on the endometrium and interaction between the embryo and maternal tissue.
  
  
• Implantation of the blastocyst in the mucosa of the uterus.

Characteristic signs

• Development of the bilaminar germ disk.
  
  
• Genesis of the amniotic cavity and the primary yolk sac (= primary umbilical vesicle).
  
  
• Invasion of the trophoblasts into the uterine mucosa.
  
  
• Solid trophoblast.

Characteristic signs

• Formation of the trophoblast lacunae and complete penetration of the embryo into the endometrium.
  
  
• Lacunar trophoblast.
  
  
• Definitive amniotic cavity.
  
  
• Primary umbilical vesicle (= primary yolk sac).

Characteristic signs

• Erosion of the maternal vessels: maternal blood in the lacunae of the throphoblast.
  
  
• The prechordal plate is formed (see stage 6).
  
  
• Extraembryonic mesoblast.
  
  
• Transformation of the primary into the secundary yolk sac (= secundary umbilical vesicle).