Fertilization Notes

Female Reproductive System

  • The female reproductive system consists of two ovaries connected to two oviducts, which are connected to the uterus.
  • The uterus becomes the cervix, which leads to the vagina.

Sperm Deposition and Movement

  • Sperm can be found in the cervix within a minute of deposition in the vagina.
  • The cervix dips down into the vagina due to muscular contractions, collecting sperm.
  • Sperm can be stored in the crypts of the cervix.
  • Cervical mucus nourishes sperm in the cervix and changes consistency during the reproductive cycle.
    • Just before ovulation, the mucus is thin and watery, allowing sperm to swim easily.
    • After ovulation, the mucus becomes thick and dense, hindering sperm movement.
  • Sperm lifespan is around 48 hours, while an oocyte's lifespan is about 14 hours.
  • Timing between sperm deposition and ovulation is crucial for fertilization.
  • Defense cells in the female reproductive tract attack sperm, as they are recognized as foreign cells.
  • Sperm motility and uterine contractions aid sperm movement.
  • Chemical attractants produced by the oocyte or follicular cells may attract sperm, but this is still debated.

Sperm Numbers and Obstacles

  • Males deposit 200-300 million sperm in an ejaculate.
  • The acidic environment in the vagina destroys many sperm.
  • Post-ovulation, thick cervical mucus prevents sperm from entering the uterus.
  • Female immune system cells attack and break down sperm.
  • Sperm may enter the wrong oviduct, as only one oocyte is usually released.

Sperm Storage and Capacitation in the Oviduct

  • Sperm reach the isthmus of the oviduct in 2-7 hours and are stored there.
  • Sperm undergo capacitation in the isthmus, a maturation process that prepares them for fertilization.

Capacitation

  • Capacitation involves two main events:
    • Sperm tails become hyperactive, enabling faster swimming and increased pressure on the oocyte layers.
    • Chemical changes prepare the sperm for the acrosome reaction.
  • Capacitation can take up to 10 hours.

Acrosome Reaction

  • The acrosome is a bag of digestive enzymes located on the sperm head that helps the sperm break through the coats surrounding the egg.
  • The oocyte is surrounded by granulosa cells and the zona pellucida.
  • The zona pellucida is a thick protein coat that protects the oocyte.
  • The acrosome releases digestive enzymes to break the connections between granulosa cells, allowing sperm to reach the zona pellucida.
  • The hyperactive tail helps sperm push through the granulosa cells.
  • When the sperm hits the zona pellucida, the acrosome ruptures, releasing a large amount of digestive enzymes.
  • These enzymes dissolve a tunnel through the zona pellucida, allowing the sperm to reach the oocyte and merge with it for fertilization.

Preventing Polyspermy

  • Polyspermy is when more than one sperm fertilizes an egg, leading to too much DNA and abortion of the fertilized egg.
  • Mechanisms to prevent polyspermy:
    • Depolarization: Upon sperm-egg contact, sodium rushes into the oocyte, changing its electrical state and repelling other sperm.
    • Cortical Granule Reaction: Cortical granules at the oocyte surface release material that binds to the zona pellucida, hardening it and preventing other sperm from burrowing through.

Fertilization Process

  • The video shows an oocyte with the zona pellucida, where a sperm is seen burrowing through it after the granulosa cells have been washed away.
  • After fertilization, the second polar body is formed, and the male and female pronuclei join.
  • The fertilized egg then undergoes cell division, with one cell becoming two, then four, and so on.