Third Week of Development: Trilaminar Germ Disc
Chapter Overview • Focus: The chapter discusses gastrulation, one of the most fundamental processes in embryonic development, leading to the formation of the trilaminar germ disc composed of ectoderm, mesoderm, and endoderm.• Significance: These three germ layers serve as the basis for all future tissues and organs, laying down the foundational architecture for the human body.
Gastrulation: Establishment of Germ Layers
• Primitive Streak Development:
o Initial Formation:
§ Appearance: The primitive streak first appears as a faint line on the dorsal
surface of the bilaminar disc. By day 15-16, it becomes a clearly defined structure with a narrow groove and slightly raised edges.
§ Primitive Node and Primitive Pit:
§ The primitive node is located at the cranial end of the streak and consists of a slightly elevated region.§ The primitive pit lies centrally within the node, indicating the site of significant cellular activity and initial axis formation.
§ Clinical Note: Errors in primitive streak formation can lead to serious
developmental defects, such as sacrococcygeal teratomas, which are masses formed from remnants of the primitive streak, containing tissues from all three germ layers.
• Cell Migration Dynamics:
- Epiblast Migration and Invagination
§ Chemotaxis and Morphogenesis: Epiblast cells migrate towards the
primitive streak, driven by chemotactic signals, particularly mediated by FGF8. Becomes flask-shaped before invagination beneath the epiblast layer
§ Flask-Shaped Transition: Upon arrival at the primitive streak, cells assume
a flask-shaped morphology, detach from the epiblast, and migrate inward - a process called invagination.
o Role of Fibroblast Growth Factor 8 (FGF8):
§ Cell Adhesion Downregulation: FGF8 downregulates E-cadherin, which
typically binds epithelial cells, enabling epiblast cells to lose cohesion and migrate. 1 / 2
§ Brachyury (T) Gene Activation: FGF8 also regulates the Brachyury (T)
gene, crucial for mesoderm cell specification and motility.
o Germ Layer Formation:
§ Endoderm Formation: First, invaginating cells displace hypoblast cells to
form the embryonic endoderm.
§ Mesoderm Formation: Subsequent cells that move between the epiblast and
the endoderm create the intraembryonic mesoderm.
§ Ectoderm Formation: Cells that remain in the epiblast become the ectoderm.
§ Epiblast as the Source: The entire trilaminar germ disc (ectoderm,
mesoderm, endoderm) is derived from the epiblast, highlighting its fundamental role as the source of all embryonic tissues.
Mechanisms of Notochord Formation
• Pathway of Notochord Development:
o Prenotochordal Cells Migration:
§ Prenotochordal cells migrate cranially from the primitive node through the midline to reach the prechordal plate.§ Initially, prenotochordal cells integrate into the hypoblast, resulting in a transient notochordal plate.
o Definitive Notochord Formation:
§ Notochordal Plate Formation: As the hypoblast is replaced by endoderm,
prenotochordal cells detach, forming a solid rod called the definitive notochord.
§ Cranial to Caudal Formation: Notochord development is a dynamic cranial-
to-caudal process, meaning cranial regions form first as the primitive streak regresses.
o Role in Axial Structure:
§ Basis for Axial Skeleton: The notochord serves as the primary inducer for
the developing neural tube and establishes the central axis for the vertebral column.
§ Notochord and Neural Induction: The notochord releases morphogens like
Sonic Hedgehog (SHH) that induce the overlying ectoderm to form the neural plate, which ultimately folds to become the neural tube.
Formation of Critical Body Axes • Anteroposterior, Dorsoventral, and Left-Right Axes:
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