Notes on Fertilization Lecture

Introduction

• Focus on fertilization after sperm ejaculation.
• Follow the sperm to the egg in the female reproductive tract.

Egg Location

• Egg is located in the ampulla of the fallopian tube.
• The egg (secondary oocyte) is arrested in metaphase II before ovulation.
• Surrounded by granulosa cells (Corona radiata).

Seminal Fluid Components

Seminal Vesicles

• Contributes 60-70% of seminal fluid.
• Important components:
  • Fructose: Energy source for sperm.
  • Prostaglandins: Stimulate uterine contractions (retropulsion).
  • Coagulase: Helps sperm latch onto vaginal walls.

Prostate Gland

• Accounts for 30-40% of seminal fluid.
• Important components:
  • Citrate: Energy source.
  • Prostate-specific antigen (PSA): Indicator for benign prostatic hyperplasia (BPH).

Sperm Structure

• Head: Contains nucleus with 23 chromosomes and acrosome (hydrolytic enzymes).
• Midpiece: Contains mitochondria for ATP production.
• Tail: Flagellum (9+2 arrangement) enables movement.

Sperm Function in Female Reproductive Tract

• Sperm movement facilitated by alkaline fluid; moves faster in alkaline.
• Prostaglandins cause uterine contractions, aiding sperm movement.
• Coagulase allows sperm to adhere within the vagina.
• Fibrinolysin helps release sperm after coagulation.

Capacitation

• Process of sperm activation as it travels through the female tract.
• Removes proteins and cholesterol, increasing motility (hypermotility).

Fertilization Process

Acrosomal Reaction

1. Binding to ZP3 receptors on zona pellucida.
2. Calcium influx triggers acrosome fusion, releasing enzymes (acrosin, proteases).
3. Enzymes break down zona pellucida for sperm penetration.

Fast Block to Polyspermy

• Sodium ions flow into the oocyte upon sperm binding, creating a positive charge that prevents additional sperm from binding.

Membrane Fusion

• Alpha subunit of sperm protein binds to oocyte membrane, leading to membrane fusion.
• Nuclear material from the sperm is released into the oocyte (ovum).

Slow Block to Polyspermy

• Calcium influx activates lysosomal enzymes to degrade zona pellucida proteins and harden the membrane, blocking further sperm entry.

Completion of Meiosis

• Calcium triggers the completion of meiosis II in the secondary oocyte.
• Results in a definitive ovum and a polar body.

Formation of the Zygote

• Fusion of male and female pronuclei results in a zygote (2N).
• Conception has occurred, marking the start of embryonic development.

Conclusion

• Summary of the fertilization process, emphasizing the intricate mechanisms involved.