First Week of Human Embryonic Development

Introduction

• Prenatal Developmental Periods:
  • Before Conception: Gametogenesis and fertilization.
  • After Conception:
    • Pre-embryonic Period: First and second week of development.
    • Embryonic Period: Third to eighth week after conception.
    • Fetal Period: Ninth week until birth.
• Focus of Lecture: Events and morphological changes in the first week of human embryonic development, leading to the formation of the blastocyst.

Events During the First Week

• Location of Fertilization: Ampulla of the fallopian tube.

• Movement of Conceptus:

  • Most events happen in the fallopian tube.
  • Conceptus moves towards the endometrial cavity (site of implantation) by the fifth to sixth day.
  • Facilitating Movement:
    • Ciliary activity of the fallopian tube.
    • Peristaltic activity of the fallopian tube, especially important in the isthmic region.

• Cleavage:

  • Defined as a series of mitotic cell divisions from zygote to blastomeres.
  • Leads to numerical increase and morphological changes.
  • Results in reduction in size of individual cells and compaction due to space constraints within the zona pellucida.

Morphological Changes

• Day 1:

  • Zygote Stage: Single cell with two pronuclei.
  • Mitotic Division: Results in two blastomeres (2-cell stage).

• Day 2:

  • 4-cell Stage: Further mitotic divisions occur.

• Day 3-4:

  • Morula Stage: 12-32 cells, characterized by compaction.

• Day 5:

  • Blastocyst Formation: Appearance of a cavity within the morula.
  • Blastocyst still covered by zona pellucida.
  • Increase in cell number and cavity size results in increased pressure (compaction).

• Day 6-7:

  • Hatching: Zona pellucida degenerates, blastocyst extrudes out.
  • Late Blastocyst: Larger cavity, absence of zona pellucida.

Parts of the Blastocyst

• Inner Cell Mass (Embryoblast): Becomes the embryo.
• Outer Cell Mass (Trophoblast): Becomes the placenta.
• Blastocyst Cavity: Fluid-filled space.
• Poles:
  • Embryonic Pole: Leads during implantation.
  • Abembryonic Pole: Opposite side of the inner cell mass.

Summary

• Key Events:
  • Movement from fertilization site to implantation site.
  • Cleavage and compaction.
  • Formation of blastocyst ready for implantation.
• Morphological Stages:
  • Zygote -> 2-cell -> 4-cell -> Morula -> Blastocyst.
• Decisions Made:
  • Distinction between cell populations destined to become the embryo and those forming the placenta.

The conceptus at the end of the first week is ready for implantation.