Ovulation and Egg Transport

Ovulation and Egg Transport

• Ovulation involves the expulsion of antral fluid and the ovum from the ovary into the peritoneal cavity.
• The FSH and LH surge, triggered by estrogen, initiates the process.
• Corona radiata: A layer of granulosa cells surrounding the oocyte.
• Zona pellucida: A glycoprotein layer around the oocyte.
  • 13 microns thick in humans.
  • Composed of sulfated glycoproteins.
  • Mediates sperm binding and the acrosome reaction.
  • Prevents polyspermy (multiple sperm entry).
  • Acts as a filter during early cleavage divisions.
  • Prevents premature implantation.
• The ovary and uterine tube are not directly connected; the ovum is released into the peritoneal cavity.
• Egg transport:
  • Follicular rupture near the fimbriae (infundibulum) of the oviduct.
  • Fimbriae: Finger-like projections that sweep the egg into the oviduct.
  • Ciliated epithelial cells and smooth muscle contractions actively move the ovum to the ampulla.
  • Ampulla: The widest part of the fallopian tube where fertilization usually occurs.
  • Fertilization occurs at the ampulla or ampulla/isthmus junction.
  • The fertilized ovum travels to the isthmus and then the uterus.
  • Tubal transport takes 3-4 days.
  • Slow transport (72 hours) in ampulla; rapid phase (8 hours) through the isthmus and uterus.
  • Progesterone is necessary for egg passage through the uterotubal junction. It opens the uterine lumen.

Sperm Transport

• Sperm is deposited in the upper vagina (pH rapidly elevates to 7.2 from 4.3).
• Sperm transverses the cervix (fast and slow phases).
• Sperm travels through the uterus.
• Sperm enters uterine tubes.
• Sperm travels up the uterine tube via swimming and tube contractions.
• Only a few sperm are near the egg at a time.
• Fertilization occurs in the uterine tube.
• Sperm Transport: Upper Vagina
  • Seminal fluid, deposited during copulation, protects sperm from vaginal acidity.
  • Buffering effect lasts only minutes.
• Sperm Transport: Cervix
  • Cervix is a major barrier and acts as a reservoir.
  • Estrogen influences production of copious, transport-regulating mucus.
  • Cervical mucus composition varies through the menstrual cycle.
• Function of the Cervix:
  • Filters immotile sperm.
  • Is receptive to sperm during ovulation.
  • Protects sperm from the vagina.
• Rapid Transport in the cervix:
  • Rapid penetration of cervical mucus and transport to the uterine tubes (within an hour).
  • Muscular movements of the female tract contribute.
• Colonization of Sperm Reservoirs:
  • Many sperm are trapped in cervical crypts.
  • Facilitated by cervical mucus.
• Slow Release and Transport: Cervix
  • Sperm swims through mucus (2-3 mm/hr).
  • Storage in crypts, sequential release.
  • Prolonged passage is facilitated by mucus and reservoir.
• Sperm Transport in the Uterus:
  • Uterine smooth muscle contractions aid transport.
  • Sperm motility is not the primary means of uterine entry.
  • Sperm enters one oviduct (chance or chemoattractant from the egg).
• Sperm Transport in the Oviduct
  • Facilitates simultaneous transport of sperm and eggs.
  • Limited sperm presence in oviduct musculature.
  • Contraction frequency regulated by hormones, seminal plasma, and prostaglandins.
• Survival of Spermatozoa:
  • Sperm has a limited lifespan (80 hrs).
  • Cervical mucus helps in metabolic needs.
  • Sperm is separated from seminal plasma rapidly.
  • Capacitation occurs during passage.
• Loss of Spermatozoa:
  • Few reach the site of fertilization.
  • Elimination at barriers (cervix, uterotubal junction).
  • Damaged/immotile sperm are removed by phagocytosis.
• Capacitation:
  • Physiological changes in spermatozoa allowing penetration.
  • Glycoprotein surface alteration.
• Fertilization:
  • Sperm penetrating the corona radiata to intermingling of chromosomes.
  • Usually in the ampullary part of the uterine tube.

Fertilization Processes

• Penetration of corona radiata:
  • Around 12 spermatozoa surround the corona.
  • Hyaluronidase (from acrosome) may aid in penetration.
  • Sperm motility is crucial.
• Binding to Zona and Acrosomal Reaction:
  • Triggered in some species by zona pellucida components.
• Acrosomal Reaction:
  • Fusion of sperm plasma membrane with outer acrosomal membrane.
  • Release of enzymes (hyaluronidase, acrosin) to digest zona pellucida.
• Prevention of Polyspermy:
  • Fast block: Rapid depolarization of egg plasma membrane.
  • Slow block: Cortical reaction.
• Cortical Reaction:
  • Release of cortical granule contents.
  • Propagation of calcium wave.
  • Hydrolyze sperm receptor molecules in zona pellucida (zona block).
• Summary: Sperm's journey through the female tract is riddled with barriers before the successful fusion with the oocyte occurs.