HUMAN FETAL DEVELOPMENT

1.   After eight weeks of development:

• all the organs + systems à been laid down.
•  embryo =  foetus.

 

2.   Extraembryonic membranes:

• amnion, chorion + allantois
• formed from embryonic cells
• in early stages of embryonic development.

3.   Amnion:
     Ø  develops from the epiblast.
     Ø  encloses embryo in a fluid-filled amnioc sac,
     Ø  protecting embryo (foetus) from:
          ·    abrupt temperature changes
          ·    mechanical impacts.

4.    Chorion:

      Ø  develops from trophoblast.
      Ø   surrounds the embryo + other extraembryonic membranes.

5.   Aallantois:

     ·    develops from part of embryo’s archenteron.

6.   Placenta:

     ·    consists of endometrial tissue + extraembryonic membranes.
     ·    a specialized organ
     ·    allows exchange of materials
     ·    between maternal +  foetal blood streams.

7.      Foetal part of the placenta

• arises from amniotic + chorionic membranes.
• umbilical blood vessels àformed from allantois.

8.      Materials are exchanged across placenta by:

• diffusion,
• active transport
• selective absorption.

9.      Nutrients, oxygen, water + antibodies:

• pass from the mother’s blood
• into the fetal blood circulatory system.

10.  Umbilical artery:

     ·    carries blood
     ·    from aorta of foetus
     ·    to placenta,

11.  Umbilical vein:

     ·    carries blood
     ·   from placenta
     ·   to posterior vena cava of foetus.

12.  Placenta is therefore:

     ·    site of gaseous exchange,
     ·    source of nutrients
     ·    excretory organ
     ·    source of antibodies.

13.  Although foetal + maternal blood streams:
      ·     flow very close to each other
      ·    but blood do not mix.