Week 3 of Embryonic Development

Overview

• Formation of three embryonic layers is crucial for all body tissues and organs.
• Occurs during the third week of gestation.

Previous Weeks Recap

• Components from Week 2:
  • Epiblast and hypoblast form the bilaminar germinative disc.
  • Formation of the amniotic cavity and definitive yolk sac.
  • Key structures:
    • Syncytiotrophoblast
    • Cytotrophoblast
    • Chorionic plate with fixation pedicle.

Gastrulation

• Most significant event in Week 3: Establishment of three germ layers.
• Layers from outside to inside:
  • Ectoderm
  • Mesoderm
  • Endoderm
• Results in trilaminar embryonic disc.

Cuts to Study Gastrulation

1. Cut at Amniotic Cavity Level
2. Cut at Definitive Yolk Sac Level

Observations

• Buccal (or oropharyngeal) membrane forms the oral cavity.
• Formation of the primitive line on epiblast surface marks the beginning of gastrulation.
• Establishment of embryonic polarity:
  • Cranial and caudal ends identified
  • Dorsal (toward amniotic cavity) and ventral (toward hypoblast) surfaces established.

Formation of Trilaminar Germ Disc

• Migration of Epiblast Cells:
  • Cells migrate towards the primitive line, detach, and undergo invagination.
  • Endoderm Formation: Some cells displace hypoblast cells.
  • Mesoderm Formation: Cells between endoderm and epiblast.
  • Ectoderm Formation: Remaining epiblast cells.

Notochord Development

• Notochord Formation:
  • Cells invaginate at the primitive node and move cranially towards the prechordal plate.
  • Formation of notochordal plaque, leading to definitive notochord, crucial for neural tube induction (neurulation).
• Neuroenteric Duct: Temporary duct connecting amniotic cavities and yolk sac forms at primitive fossa.

Membranes and Diverticulum Formation

• Oropharyngeal & Cloacal Membranes:
  • Composed of endoderm and ectoderm.
• Allantois Formation:
  • Diverticulum extends into fixation pedicle, essential for urinary system development.

Growth Direction of Embryonic Disc

• Growth occurs in a cephalocaudal direction due to cell migration from the primitive line until the end of the fourth week.

Development of Chorionic Villi

• Primary Villi: Formed from cytotrophoblast invading syncytiotrophoblast.
• Secondary Villi Formation: Mesodermal cells invade primary villi.
• Tertiary Villi:
  • Mesodermal cells differentiate into blood cells and vessels, forming the capillary system that connects to the embryonic heart.

Types of Villi

• Stem/Anchor Villi: Extend from chorionic plate to decidua basalis.
• Free/Branched Villi: Grow from lateral areas of anchor villi, facilitate nutrient exchange.

Conclusion

• Overview of significant events during the third week of embryonic development.
• Encourage questions and comments for further discussion.
• Best wishes for exams!