UNSW Embryology

Week 2 Twinning

© Dr Mark Hill (2011)

Acknowledgements

Introduction

During the first week either monozygotic or dizygotic twins can be generated by a single or multiple fertilization event respectively. This topic is now covered in detail on the page Abnormalities - Twinning

Reading

• Human Embryology (2nd ed.) Larson
• The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud
• Before we Are Born (5th ed.) Moore and Persaud Ch
• Essentials of Human Embryology Larson
• Human Embryology Fitzgerald and Fitzgerald
• UNSW Embryology Selected Week 2 References

Objectives

Week 2 Overview- Implantation and Gastrulation

• Early Development 2
• Covering Week 2 of development
  • Finish formation of the Blastocyst
  • trophoblast, embryoblast
• Implantation
  • normal, ectopic
• Changes in the Uterus
  • Early utero-placental circulation
• Formation of
  • the Bilaminar Embryo
  • the 3 chambered conceptus
  • the Trilaminar Embryo
  • the Axial Process
• Early cell division
  • Cleavage of the Zygote
  • cleavage of zygote forms 2 blastomeres
  • cleavage with no cytoplasm synthesis
  • therefore individual cell volume decreases
  • initially synchronously, then asynchronously
• Formation of the Morula
  • there are now 16-20 cells
  • at the end of the oviduct
  • ball of cells
  • slow- centre cells, larger
  • fast- peripheral cells, flattened against zona pellucida
• Formation of the Blastocyst
  • Day 5
  • zona pellucida lost
  • uterine sercetions
  • breakdown ZP
  • support blastocyst (nutrition)
  • compaction
  • central cells now compacte
  • located on a region of the wall
  • blastoceol formation
  • cavity formed by compaction
  • fluid filled blastoceol
• Blastocyst
  • Trophoblast
    • forms the placenta
    • placental membranes
  • Embryoblast
    • Inner cell mass
    • forms entire embryo
• Early cell division
• Implantation- Trophoblasts
  • Occurs at day 6-7
  • adhesion
  • adplantation
  • implantation complete by about day 9

Quick Links

Week 2 Pages:

Introduction | Timeline Week 2 | Carnegie stages 5 to 7 | Abnormalities | Blastocyst Implantation | Sites of Implantation | Bilaminar Embryo | Embryonic Cavitites | Early Placentation | Ovary
Twinning
Molecular | Stem Cells | References | Text only | WWW Links | Movies - Week 2

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G