10.2 Development of the placental villi

• Anatomy of the placenta
• The three villus types
  • The cytotrophoblast layer
• Connections between maternal and fetal tissues
  • Placental tissue structure
  • Development of the placenta
  • The decidua
  • Development of the villi
    

The decidua

At the implantation location, the maternal endometrium is changed by the decidual reaction (epithelial transformation of the fibroblasts of the uterine stroma, in that lipids and glycogen accumulate) and is called the decidua.

The decidua consists of various parts, depending on its relationship with the embryo:

• Decidua basalis, where the implantation takes place and the basal plate is formed. This can be subdivided into a zona compacta and a zona spongiosa (where the detachment of the placenta takes place following birth).
• Decidua capsularis, lies like a capsule around the chorion
• Decidua parietalis, on the opposite uterus wall

At around the 4th month, the fetus is so large that the decidua capsularis comes into contact with the decidua parietalis. The merging of these two deciduae causes the uterine cavity to obliterate.

Fig. 31, Fig. 32
The three deciduae in the second month of pregnancy. After the 4th month the growing fetus brings the decidua capsularis into contact with the decidua parietalis. The fusion of the two deciduae leads to the disappearance of the uterine cavity.