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.