Overview
This lecture explains the detailed process of human fertilization, including sperm structure, seminal fluid roles, and the stepwise actions enabling a single sperm to fertilize an egg and form a zygote.
Location & Preparation of the Egg
- The egg (secondary oocyte) is released from the Graafian follicle due to luteinizing hormone and is arrested in metaphase II.
- The oocyte is located in the ampulla of the fallopian tube, surrounded by granulosa cells (corona radiata).
Seminal Fluid & Accessory Glands
- Seminal vesicles contribute 60–70% of seminal fluid, providing fructose (sperm fuel), prostaglandins (stimulate uterine contractions), and coagulase (aids latching in vagina).
- The prostate gland contributes ~30% of fluid, including citrate (energy), fibrinolysin (breaks coagulation), and PSA (prostate health marker).
Sperm Structure & Function
- Sperm consists of a head (nucleus, acrosome with digestive enzymes), midpiece (mitochondria for ATP), and tail (flagellum for movement).
- Flagellum has a 9+2 microtubule arrangement, driving motility.
Chemical Roles in Seminal Fluid
- Fructose powers sperm movement via mitochondrial ATP production.
- Prostaglandins cause uterine contractions to help move sperm upward (retropulsion).
- Coagulase causes sperm to latch in the vagina; fibrinolysin later releases them.
- Relaxin increases sperm motility.
- Seminalplasmin acts as an antibiotic in the female tract.
Sperm Journey & Capacitation
- Sperm swim through alkaline seminal fluid, enabling faster movement.
- Capacitation is removal of proteins and cholesterol from the sperm head, increasing motility and allowing binding to the egg.
Fertilization Events
- The sperm first binds to ZP3 receptors on the egg’s zona pellucida.
- The acrosomal reaction releases enzymes (acrosin, proteases) to digest a path through the zona pellucida.
- Calcium ion influx triggers enzyme release during this reaction.
Blocks to Polyspermy
- Fast block: When the sperm fuses to the egg membrane, sodium ions rush in, creating a positive charge that prevents other sperm from fusing.
- Slow block: Fusion triggers the egg’s smooth ER to release calcium, activating cortical granules (lysosomes) to harden the zona pellucida and degrade ZP3 receptors, permanently preventing further sperm entry.
Completion of Meiosis & Zygote Formation
- Calcium signals the egg to complete meiosis II, yielding the definitive ovum and a polar body.
- The male and female pronuclei (each with 23 chromosomes) fuse to create a diploid zygote (2N), beginning embryonic development.
Key Terms & Definitions
- Capacitation — The process of removing proteins and cholesterol from the sperm head to enable fertilization.
- Acrosomal reaction — Release of enzymes from the acrosome allowing sperm to penetrate the zona pellucida.
- Zona pellucida — Protective glycoprotein layer around the egg, containing ZP3 receptors for sperm binding.
- ZP3 receptors — Specific proteins on the zona pellucida that bind to sperm during fertilization.
- Fast block to polyspermy — Rapid electrical change in egg membrane preventing other sperm entry.
- Slow block to polyspermy — Release of enzymes that harden the zona pellucida, permanently blocking additional sperm.
- Zygote — The first diploid cell formed from the fusion of male and female gametes.
Action Items / Next Steps
- Review the steps of fertilization, focusing on key chemical events and blocks to polyspermy.
- Prepare for questions on sperm structure and the roles of seminal fluid components.