4. Early Embryonic Development

4.1 Cleavage Stages

  • Cleavage begins 30−36 hours after fertilization.
  • Characterized by rapid mitosis
  • Zygote → 2-cell → 4-cell → 8-cell (mitotic divisions)
  • Morula (Day 3)
  • Blastocyst (Day 5):
    • Inner cell mass (embryoblast) become the fetus.
    • Trophoblast (forms placenta)
  • Cells of early cleavage are totipotent and will divide to become every cell in the body.
    • A cloned blastocyst can be created by implanting a somatic cell into an ovum cytoplasm in a process called somatic cell nuclear transfer.
  • Embryoblast cells cultured in vitro are called embryonic stem cells and are pluripotent = able to become any type of cell in the body.
  • These cells might be used therapeutically to treat diabetes type I, Parkinson disease, or spinal cord injury.

4.2 Movement to Uterus

  • Embryo moves along fallopian tube via cilia.
  • Reaches uterus around Day 4–5 post-fertilization.

4.3 Blastocyst Hatching

  • Zona pellucida dissolves → embryo ready for implantation.