Notes on Fertilization and Sperm Transport in Female Reproductive Tract

Viable sperm must:

  1. Transverse the cervix: This process involves the sperm navigating through the cervical mucus, which can vary in consistency during the menstrual cycle, affecting the ease of passage.

  2. Transport through the uterus to the oviduct: This involves coordinated sperm motility and muscular contractions of the uterus that facilitate sperm movement towards the oviduct.

  3. Acquire fertilization capacity (capacitation): Capacitation is a crucial process that alters the sperm membrane, enhancing its ability to fertilize the oocyte.

  4. Experience the acrosome reaction: This reaction allows enzymes to be released from the acrosome, essential for penetrating the protective layers around the oocyte.

  5. Penetrate the zona pellucida: The zona pellucida is a glycoprotein layer surrounding the oocyte that sperm must breach to achieve fertilization.

  6. Bind to the oocyte plasma membrane: This is the final step where specific receptors on the sperm head interact with binding sites on the oocyte, leading to fertilization.

Transport Through the Uterus:

Phases:

  1. Rapid phase: Sperm reach the oviduct quickly post-breeding through uterine contractions, but remain non-viable initially.

  2. Sustained phase: During this phase, substances are secreted that help maintain the viability of sperm as they continue to be transported gradually to the oviduct, where they can survive for several days.

Capacitation:

Capacitation allows sperm to bind and fertilize the oocyte by undergoing biochemical changes.

  • Stripping of seminal plasma proteins occurs: This enhances the sperm's readiness for interaction with the oocyte.

  • Completed in the oviduct: This site is vital for increasing sperm motility and functionality, ensuring they are prepared for fertilization.

Acrosome Reaction:

Initiated upon binding to zona pellucida, this reaction includes the fusion of the acrosome membrane with the sperm membrane, releasing enzymes critical for zona penetration.

  • Involves vesiculation that aids in penetrating zona pellucida: This process enables sperm to digest the zona pellucida, allowing entry into the oocyte.

Binding to the Oocyte:

Microvilli of oocyte bind equatorial region of sperm, leading to the engulfment of the sperm cell.

  • Resulting in decondensing sperm DNA and stimulating cortical reaction: This reaction is crucial in preventing polyspermy, ensuring that only one sperm fertilizes the oocyte.

Sperm Viability in Female Tract:

  • Bitch: 9-11 days, Cow: 1.5-2 days, Mare: 4-5 days, Woman: 5-6 days.

Pregnancy Recognition:

Various species have specific signals and critical periods for pregnancy recognition after ovulation:

  • Cow: bFN-T at days 15-16, which helps maintain the corpus luteum.

  • Ewe: OIFN-T at days 13-14, important for preventing luteolysis.

  • Woman: hCG at days 7-12, crucial for supporting early pregnancy development.