Week 3: Formation of Trilaminar Germ Disc

Week 3: Formation of the Trilaminar Germ Disc (Gastrulation)

• Primitive Streak and Body Axes Formation:

  • The primitive streak appears on the epiblast's dorsal surface, initiating gastrulation and defining the cranial-caudal axis.
  • The primitive node and oropharyngeal membrane appear. The oropharyngeal membrane will become the mouth, and the primitive streak guides cell migration.
  • The primitive streak and node direct cell migration and differentiation, creating the primary germ layers.

• Formation of Three Germ Layers via Gastrulation:

  • Epiblast cells migrate through the primitive streak to form:
    • Endoderm: Replaces hypoblast cells from the inner layer.
    • Mesoderm: Fills the space between endoderm and ectoderm.
    • Ectoderm: Formed from the remaining epiblast cells, forming the outer layer.

• Molecular Regulation of Embryonic Axes:

  • Cranial-Caudal Axis:
    • Caudalizing factors (Nodal, BMP4, WNT) are released at the primitive node (caudal end).
    • Cranializing factors (OTX2, LIM1, HESX1, CER-I) are released from the anterior visceral endoderm (AVE) to inhibit caudalizing factors in the cranial region.
  • Dorsal-Ventral Axis:
    • BMP4 promotes ventral development throughout the embryo.
    • Inhibitors (Goosecoid, Chordin, Noggin, Follistatin, Nodal) regulate dorsal development.
  • Left-Right Axis:
    • Defined by specific gene expression.
    • Cell migration and differentiation establish the left-right axis.

• Epithelial-Mesenchymal Transition (EMT):

  • Epiblast cells transition to mesenchymal cells, migrating to form the three germ layers.
  • EMT involves a loss of cell adhesion (e.g., E-cadherin), enabling cell migration and formation of mesoderm and endoderm.

• Endoderm and Mesoderm Formation:

  • Details about the formation of endoderm and mesoderm were not included in the provided text.

Week 3-4: Mesoderm Differentiation and Notochord Development

• Mesodermal Layer Specialization:

  • Paraxial Mesoderm: Forms somites (muscles, axial skeleton, dermis).
  • Intermediate Mesoderm: Forms urogenital structures.
  • Lateral Plate Mesoderm: Divides into somatic (limb skeleton, body cavity linings) and splanchnic (heart, blood cells, gut walls) mesoderm.

• Notochord Formation:

  • Primitive node and primitive streak guide inward migration of cells to form the notochord.
  • Cells migrating cranially from the primitive node form prechordal mesoderm which develops into prechordal cells, influencing forebrain development.
  • Cells from the primitive node form the prenotochordal process which eventually merges with the endoderm to form the notochordal plate.

• Prechordal cells and Prechordal Mesoderm:

  • Migrate cranially from the primitive node and the primitive streak, forming the prechordal mesoderm, which is located near the developing head region.
  • Contribute to head formation and influence forebrain development.

Days 17-21: Formation of Neural Plate and Neurulation

• Neural Plate and Neural Tube:

  • The neural plate, induced by signals from the notochord, forms at the cranial end (day 18).
  • The neural plate folds to form the neural tube, the precursor to the brain and spinal cord.

• Neural Crest Cells:

  • Form at the edges of the neural folds and migrate to form various structures (melanocytes, ganglia, Schwann cells, adrenal medulla).
  • Neural tube closure begins cranially and progresses caudally by day 21.

Day 21-28: Completion of Neural Tube Closure and Organ Primordia

• Completion of Neural Tube and Early Organ Formation:

  • Neural tube closure is complete by day 28, with sealing of anterior and posterior neuropores.
  • Initial organ structures (otic placodes, pharyngeal arches, pericardial bulges) become visible.

• Derivatives of Germ Layers:

  • Ectoderm: Epidermis, hair, nails, mammary glands, nervous system components.
  • Mesoderm: Muscles, bones, urogenital system, cardiovascular system.
  • Endoderm: Gastrointestinal tract, liver, pancreas, lungs.

Week 4: Pathologies and Embryonic Development

• Potential Developmental Defects:

  • Anencephaly: Failure of cranial neuropore closure, often fatal.
  • Spina Bifida: Failure of caudal neuropore closure, affecting spinal cord development.
  • Sirenomelia (Mermaid Syndrome): Fused lower limbs due to insufficient mesoderm development.
  • Sacrococcygeal Teratoma: Tumor from pluripotent cells in the notochord.

• Preventive Measures:

  • Folic acid supplementation reduces risk of neural tube defects.

• End of Embryonic Period:

  • Basic embryo structure with primary organ primordia is established by the end of week 4, marking the end of gastrulation.

This quiz focuses on the process of gastrulation and the formation of the trilaminar germ disc, including the roles of the primitive streak and the creation of the three primary germ layers: endoderm, mesoderm, and ectoderm. Additionally, it explores the molecular regulation of embryonic axes during development.