Fertilization Process: The Journey of Sperm to Egg

Introduction

• Fertilization is the complex process where a sperm unites with an egg to form a new human life.
• The journey involves a single sperm facing significant challenges to reach and fertilize the egg.

Initial Journey Post-Intercourse

• 300 million sperm enter the vagina during sexual intercourse.
• Many sperm die due to the acidic environment of the vagina, but some survive due to protective elements in the surrounding fluid.

Passing Through the Cervix

• The cervix, usually closed, opens during ovulation, allowing sperm to pass through.
• Cervical mucus thins to facilitate sperm passage.
• Many sperm die in the mucus, while some remain in the cervix for potential later travel.

Through the Uterus

• Uterine contractions aid the sperm in traveling toward the egg.
• The woman’s immune system attacks more sperm, mistaking them for invaders.

Journey to the Fallopian Tube

• Sperm are split: half head to the empty Fallopian tube, while the other half head to the tube containing the egg.
• Cilia in the Fallopian tube move the egg toward the uterus, forcing sperm to swim against the current.
• Fewer sperm (only a few thousand) remain as many are trapped or die.

Approaching the Egg

• Sperm become hyperactive, guided by chemical changes to their membranes.
• A few dozen reach the egg, which is surrounded by the Corona Radiata.
• Sperm must penetrate this layer and the Zona Pellucida.

Penetration and Fertilization

• Sperm attach to receptors on the Zona Pellucida, triggering enzyme release to penetrate it.
• The first sperm to contact the egg membrane will fertilize it.
• Subsequent changes prevent other sperm from attaching.
• The egg releases chemicals to repel other sperm and harden the Zona Pellucida.

Formation of Zygote

• Inside the egg, male genetic material expands, forming the male pronucleus with 23 chromosomes.
• Female pronucleus forms as the egg completes its division.
• Microtubules pull pronuclei together, combining chromosomes, forming a unique genetic code.

Conclusion

• A new single cell, the zygote, forms, marking the beginning of a new human life.
• The zygote is moved by cilia toward the uterus for implantation, beginning a 9-month maturation period leading to birth.