Early embryology

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Early life goes through the following stages:

Formation of the bilaminar embryo

  • happens concurrently with development of syncytiotrophoblast
  • embryoblast forms two distinct layers:
    1. epiblast - large cells close to cytotrophoblast
    2. hypoblast - smaller cells, adjacent to blastocyst cavity
  • by end of second week, flat bilaminar disc is formed with rostral and caudal ends

Formation of the amniotic cavity

  • accumulation of fluid at about 9 day stage
  • fluid between epiblast and cytotrophoblast
  • see implantation diagram

Formation of the extraembryonic coelom (chorionic cavity)

  • fluid-filled spaces develop in the extra-embryonic mesoderm (12 days)
  • later coalesce into single cavity (chorionic cavity; 13 days)



  • cells invaginate to form a third layer between epiblast and hypoblast
  • initially called mesenchyme
  • later called mesoderm


Somitic mesoderm

  • segmental blocks of organization of mesoderm alongside notochord

Intermediate mesoderm

Lateral mesoderm

  • fluid-filled spaces appear (coelemic spaces)

Notochord formation

  • invagination of epiblast cells at rostral end of the primitive streak
  • happens in the middle of the third week
  • notochord induces differentiation around it
  • notochord disappears later
  • notochord extends as far as caudal plate

Neural tube

Folding in the embryo

  • during 4th week, neural tube grows disproportionately
  • This causes folding of the embryo both longitudinally and transversely (simultaneously)
  • folding causes the pericardial coelom to tuck in ventrally