Embryology 2nd and 3rd Week Development

Second and Third Weeks of Development

• By the end of the lecture, students will be able to analyze changes during weeks 2 and 3, recognize stem cell types, interpret gastrulation, and link gastrulation to teratogenesis and clinical issues.

Changes During the Second Week

• The blastocyst partly embeds in the endometrial stroma.
• The trophoblast differentiates into the cytotrophoblast and syncytiotrophoblast.
  • The cytotrophoblast is an inner, actively dividing layer.
  • The syncytiotrophoblast erodes maternal tissues.
• By day 9, lacunae form in the syncytiotrophoblast.
• Maternal blood enters the lacunae forming a primitive uteroplacental circulation.
• Cytotrophoblast forms cellular columns called primary villi, surrounded by the syncytium.
• The blastocyst is completely embedded and the surface defect heals by the end of the second week.
• The inner cell mass (embryoblast) differentiates into the epiblast and hypoblast, forming a bilaminar disc.
  • Epiblast cells create amnioblasts which line the amniotic cavity.
  • Endoderm cells are continuous with the exocoelomic membrane and together surround the primitive yolk sac.
• Extraembryonic mesoderm develops between the trophoblast, amnion, and exocoelomic membrane. The extraembryonic coelom, or chorionic cavity, forms.

Summary of the Second Week (Week of Twos)

• Trophoblast differentiates into two layers.
• Inner cell mass differentiates into two layers.
• Primary mesoderm splits into somatopleuric and splanchnopleuric mesoderm.
• Amniotic and yolk sac cavities form.

Changes During the Third Week (Trilaminar Germ Disc)

• Gastrulation: The bilaminar embryonic disc transforms into a trilaminar disc.
  • This process begins with a primitive streak that forms on the epiblast of the embryonic disc.
  • Epiblast cells migrate towards the primitive streak and underneath the epiblast, forming the endoderm layer
  • These cells form the mesoderm layer.
• Notochord Formation: Mesenchymal cells create a median cellular cord, the notochord, which develops cranially from the primitive node and pit.

Functions of the Notochord

• Forms the axial skeleton's axis.
• Stimulates the overlying ectoderm to form the central nervous system.
• Degenerates as vertebrae form. Its remnant is the nucleus pulposus of intervertebral discs.

Clinical Applications

• Gastrulation disruptions may occur due to genetic abnormalities, while toxic insults can lead to caudal dysgenesis (sirenomelia).
• Maternal diabetes is associated with extraembryonic abnormalities.
• Primitive streak remnants can develop into tumors in the sacrococcygeal region (sacrococcygeal teratomas).

This quiz focuses on the critical changes during the second and third weeks of embryonic development. Students will explore stem cell types, the process of gastrulation, and its connections to teratogenesis and various clinical implications.