Embryonic Development: Weeks 2-3

Formation of the Bilaminar Embryonic Disc

• End of Week 2:
  • The bilaminar embryonic disc forms, consisting of:
    • Hypoblast
    • Epiblast

Gastrulation (Week 3)

• Overview:

  • Gastrulation is the process where the bilaminar disc differentiates into three primary germ layers.

• Primitive Streak Formation:

  • Occurs around 15 days after fertilization.
  • A thickened structure forms along the midline in the epiblast near the caudal end.
  • Purpose: Defines major body axes:
    • Cranial (head)
    • Caudal (tail)
    • Left and right sides
  • At the cranial end, the primitive streak expands to form:
    • Primitive Node: Contains a circular depression known as Primitive Pit.
    • Primitive Groove: Extends towards the caudal end.

• Cell Migration and Layer Formation:

  • Invagination: Epiblast cells migrate towards the primitive streak, detach, and move into the embryo's interior.
  • Definitive Endoderm Formation:
    • First invaginated cells replace hypoblast cells fully, forming the definitive endoderm by approximately Day 16.
  • Ectoderm Formation:
    • Remaining epiblast cells become the ectoderm, the most exterior layer.
  • Mesoderm Formation:
    • Some invaginated cells remain between ectoderm and definitive endoderm, forming the mesoderm.
  • Completion of Germ Layer Formation:
    • Once mesoderm and endoderm are formed, epiblast cell migration stops.

• Completion of Gastrulation:

  • Ectoderm formation continues from cranial to caudal end, establishing three distinct primary germ layers:
    • Ectoderm
    • Mesoderm
    • Endoderm
  • Marks the completion of gastrulation.