Human Embryo Development

Overview

• Development begins in the ovary.
• Unfertilized egg drifts towards the fallopian tube.
• Fertilization occurs when sperm encounters the egg.
  • Only one sperm manages to fertilize the egg.
  • Egg stiffens outer walls to prevent further entry.

Initial Cell Division and Implantation

• Over 5 days, the fertilized egg divides multiple times.
  • Reaches 32 cells within the stiffened outer wall.
• Formation of the blastocyst:
  • Inner cells develop into the human being.
  • Outer cells form the placenta and supporting tissues.
• Implantation in the uterine wall marks the initiation of pregnancy.

Early Development Stages

• 14 Days: Formation of primitive streak indicating bilateral symmetry.
• Cell Migration: Cells grow at different speeds, forming nested tubes (gut canal and other tissues).
• 28 Days: Embryo is ~1/4 inch long.
  • Limb buds and early inner ear development visible.

Detailed Development Timeline

• 32 Days: Eye pits form, arm buds taper.
• 33 Days: Nasal pits and hand plates begin.
• 37 Days: Embryo ~3/4 inch long.
  • Development of nose, ears, and basic limb components.
• 41 Days: Fingers, jaw, and lumbar curvature visible.
• 44 Days: Development of elbows and eyelids begins.
• 47 Days: Trunk straightens, limb extension, visible toes.
• 52 Days: 1 inch long.
  • Hands curve towards each other, external ears visible.
• 54 Days: Hands can touch, touch distinct toes.

Eight Weeks

• The embryo is about 1.25 inches long.
• Significant detail in development despite small size.