Week 3: Gastrulation, Notochord formation, and establishing fetal circulation

• At the end of week 3:
  • The primary uteroplacental circulation has developed and the embryo receives nutrients from the mother
  • The bilaminar disk is formed (hypoblast + epiblast)
  • The extraembryonic mesoderm (2 layers) and chorionic cavity are formed

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• There are 3 main features of the 3rd week:
  1. Gastrulation 
  2. Notochord formation
  3. Further developing the uteroplacental circulation – tertiary villi formation

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Gastrulation

Gastrulation = ‘The process by which the trilaminar disc is formed from the bilaminar disc‘

• Gastrulation begins with the formation of a groove – the primitive streak – on the epiblast surface
  • This originates at the caudal end (i.e. towards the anus) and extends cephalically (towards the head)
  • This establishes the long axis of the body which determines caudal and cephalic and left and right

• The primitive pit is then formed at the cephalic end of the primitive streak
  • This is surrounded by the primitive node

 

• Epiblast cells migrate towards the primitive streak and slip beneath it (invagination) to displace the hypoblast
  1. These first form the Endoderm which lines the hypoblast
  2. More cells come between the epiblast and endoderm to form the Mesoderm
  3. The remaining cells of the epiblast form the Ectoderm

• Collectively, these are called the trigeminal disc
  • This will become all the cells of the future baby (i.e. all of the baby comes from the epiblast and none from the hypoblast)

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Notochord formation

• The notochord is a key structure in developing the body map and organs
  • Develops during gastrulation together with  the trigeminal disc
  • Influences axis formation for many systems
    • I.e. determines where organs are placed
    • Affects both ectoderm (neurulation) and mesoderm (mesoderm layers) but is of endodermal origin (although ends up in mesoderm layer)

• Process of notochord formation
  1. The prechordal plate develops towards the cephalad region (head) of the embryo
     • Comes from epiblasts at the floor of the amnion cavity (amnioblasts)
     • Later develops into the buccopharyngeal membrane
  2. Endoderm prenotochordal cells extend from the primitive pit towards the prechordal plate
     • These prenotochordal cells (from endoderm) intercalate into the hypoblast to form the notochordal plate
       The Notochordal Plate
  3. The notochordal plate detaches from the endoderm to form the definitive notochord
     • This lies in the same plane as the mesoderm (but still formed from endodermal tissue)
     • This ultimately develops the body’s axes

       

        

• NB: The cloacal membrane and buccopharyngeal membrane are areas of anchored endoderm/ectoderm with no intervening mesoderm
  • The function of these is to establish the craniocaudal axis

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Further developing the uteroplacental circulation

As discussed in Week 2, at the start of Week 3 the primary uteroplacental circulation is developed

Primary villi are dipping into the lacunae (cytotrophoblastic core + syncytiotrophoblast outer) and gain nutrients by diffusion

• Secondary villi develop when extraembryonic mesodermal cells (Mesoblasts) penetrate into the primary villi
  • Thus the cytotrophoblastic core is replaced by mesoderm

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• Mesodermal cells differentiate into an endothelial cell type (which line blood vessels in adults)
  • This establishes the villous capillary system – thus forming the tertiary villi
  • This establishes haematotrophic circulation

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Summary of the 3rd week

• 3 main events:
  • Gastrulation – development of the 3 germ layers
  • Notochord formation – establishes body axes
  • Tertiary villi development – established uteroplacental circulation