Embryo Development: Weeks 1 to 2

Introduction

• Focus on the development of the embryo up to week two.
• Topics: Gastrulation, implantation (brief), development of placenta (future video).

Recap of Week 1

• Fertilization and Cleavage: Process from zygote to blastocyst formation.
• Inner Cell Mass: Differentiates into embryoblast, forming the bilaminar disc.
• Trophoblast: Differentiates into cytotrophoblast and syncytiotrophoblast, part of chorion and placenta.

Trophoblast Development

• Cytotrophoblast: Well-defined cell margins.
• Syncytiotrophoblast: Cells proliferate, membranes disintegrate forming a syncytium, releases enzymes.
• Implantation: Blastocyst attaches via proteins (selectins/integrins) to uterine lining.
• Maternal Blood Vessel Connection: Syncytiotrophoblast reaches maternal blood vessels for nutrient exchange.

Hormonal Changes

• Syncytiotrophoblast Hormone Production: Releases beta-HCG to maintain corpus luteum, preventing endometrial shedding.
• Role of Progesterone: Maintains uterine environment to support embryo.

Bilaminar Disc to Trilaminar Disc

• Bilaminar Disc Components: Epiblast (top), Hypoblast (bottom).
• Primitive Yolk Sac: Below hypoblast, aids in nutrient delivery.
• Amniotic Cavity: Above epiblast.

Gastrulation Process

• Primitive Streak and Node Formation: Thickening of epiblast tissue.
• Epithelial Migration: Fibroblast growth factor 8 triggers epiblast cells migration.
• Layer Formation:
  • Endoderm: Epiblast cells replace hypoblast.
  • Mesoderm: Cells migrate through primitive groove and spread to form mesoderm.
  • Ectoderm: Remaining epiblast becomes ectoderm.

Development of Notochord

• Notochord Formation: Ectodermal cells migrate through primitive pit, move cranially.
• Role of Notochord:
  • Induces neurulation (neural tube formation).
  • Adult remnant becomes nucleus pulposus in intervertebral disc.

Key Structures

• Procordial Plate: Orientation for cranial-caudal axis.
• Buccopharyngeal and Cloacal Membranes: Indicate mouth and anus formation.

Summary

• Covered embryo development up to week two.
• Focus on gastrulation, trophoblast differentiation, and hormonal influences.
• Preview of future content: Week three development and placenta.