Fertilization & Sperm Function — Comprehensive Study Notes

Contributions of the epididymis to mature sperm

• The epididymis is where sperm gain motility and fertilizing ability.
• Proteins such as CRISP1 are added to the sperm head, which later allow binding to ZP3 in the zona pellucida.
• Membrane remodeling occurs, making sperm responsive to signals for capacitation and the acrosome reaction in the female tract.
• If the epididymis does not contribute, sperm would be immotile, unable to bind ZP3, and fertilization would fail.

Important secretions of reproductive structures

• Seminiferous tubules:
  • Site of sperm production.
  • Sertoli cells provide nutrients, ABP (androgen-binding protein), and signaling factors.
  • Sperm produced here are immature and require epididymal maturation.
• Prostate gland:
  • Secretes alkaline fluid to neutralize vaginal acidity.
  • Produces proteases to liquefy semen, improving sperm mobility.
  • Contributes zinc, citric acid, and clotting enzymes for stability and protection.
• Bulbo-urethral (Cowper’s) gland:
  • Produces mucus-rich fluid that lubricates the urethra and neutralizes urine acidity.
  • Prepares the urethral pathway for sperm to travel safely.

Steps of erection and ejaculation

• Erection:
  • Initiated by parasympathetic input → release of nitric oxide (NO) → vasodilation of penile arteries.
  • Blood fills the corpus cavernosum and spongiosum, creating erection.
• Ejaculation:
  • A sympathetic reflex with two phases: $1$) Emission; $2$) Expulsion.
  • Emission: sperm mix with fluids from seminal vesicles, prostate, and bulbourethral gland.
  • Expulsion: rhythmic contractions of urethral muscles force semen out.
• Together, these steps deliver sperm closer to the ovum, increasing fertilization chances.

Capacitation and its role in fertilization

• Capacitation = final maturation of sperm inside the female tract.
• Membrane changes: cholesterol is removed, increasing membrane fluidity and permeability to $Ca^{2+}$ and $HCO_3^{-}$.
• Intracellular changes: rise in $cAMP$, phosphorylation of proteins, and protein reorganization.
• Outcomes:
  • $Hyperactivation$ of motility (whip-like flagellar motion).
  • Prepares sperm for the acrosome reaction upon encountering ZP3.
  • Without capacitation, sperm cannot penetrate cumulus cells or the zona pellucida.

Progesterone signaling in sperm → hyperactivation, chemotaxis, acrosome reaction

• Source: progesterone is released by cumulus cells surrounding the egg.
• Mechanism: progesterone binds to receptors on sperm → opens CatSper channels → $Ca^{2+}$ influx.
• Consequences:
  • $Hyperactivation$: stronger flagellar beating helps sperm push through cumulus and zona pellucida.
  • Chemotaxis: sperm follow higher progesterone concentrations toward the oocyte.
  • Acrosome reaction: the Ca²⁺ rise primes sperm for enzyme release to digest the zona pellucida.

How sperm pass the barriers to fertilize the ovum

• Cumulus oophorus:
  • Sperm release hyaluronidase (PH20) to digest hyaluronic acid between cumulus cells.
  • Hyperactivated motility helps push through the cloud of granulosa cells.
• Zona pellucida (ZP):
  • Thick glycoprotein coat with ZP3 protein; sperm bind ZP3 via CRISP1.
  • Binding triggers a Ca²⁺ influx → acrosome reaction → release of hydrolytic enzymes to digest ZP.
• Oocyte membrane (oolemma):
  • When sperm reach the perivitelline space, Izumo (sperm) binds Juno (egg).
  • Plasma membranes fuse → sperm nucleus and centriole enter the oocyte cytoplasm.
  • Fertilization completes when pronuclei fuse → zygote forms.

✨ These notes summarize the key steps, proteins, and ions involved in sperm maturation, capacitation, and fertilization. If you’d like, I can add a compact mnemonic or one-line quick-reference cheat key to help memorize the sequence quickly for the exam.