Slides-Fertilization

D3.1 Fertilization

Fertilization Overview

  • Fertilization is defined as the union of sperm and egg, resulting in a diploid zygote.

  • The egg, once released from the follicle, travels down the oviduct where it may encounter sperm for potential fertilization.

Gametes Prior to Fertilization

  • The released egg is known as a secondary oocyte as it has not completed meiosis II.

  • Cytoplasm Components:

    • Contains cortical granules filled with enzymes.

    • Layers surrounding the egg:

      • Zona Pellucida: A jelly-like non-cellular layer composed of glycoproteins that prevents sperm entry.

      • Corona Radiata: A layer of follicular cells that nourish the egg.

  • Sperm Structure:

    • Composed of a head, midpiece, and tail.

      • Head: Contains haploid nucleus and acrosome (hydrolytic enzymes).

      • Midpiece: Houses mitochondria for energy.

      • Tail: A flagellum facilitating sperm movement.

Steps of Fertilization

  1. Sperm Movement and Capacitation:

    • Sperm navigate towards the oocyte, undergoing physiological changes for activation.

  2. Membrane Fusion:

    • Receptors on the sperm head bind to proteins on the oocyte membrane, causing the membranes to fuse.

  3. Egg Activation:

    • This fusion activates the oocyte, leading to intracellular calcium level rise, completion of meiosis II, and cortical reaction.

  4. Prevention of Polyspermy:

    • Changes occur in the egg’s plasma membrane and zona pellucida, preventing other sperm from fertilizing the egg.

  5. Sperm Entry:

    • The sperm head enters the cytoplasm; the midpiece and tail are discarded.

  6. Pronucleus Formation:

    • The sperm nucleus transforms into a male pronucleus; both male and female pronuclei have haploid genomes.

  7. Zygote Formation:

    • As nuclear membranes dissolve, chromatin condenses to form chromosomes, resulting in a diploid zygote that undergoes mitotic division.

In-Vitro Fertilization (IVF)

  • IVF is an artificial reproductive technology where egg and sperm are fused in a laboratory, followed by embryo transfer to the uterus.

Steps of IVF:

  1. Cycle Suppression:

    • The menstrual cycle is suppressed with medication to control ovulation timing (about 2 weeks).

  2. Fertility Drugs Administration:

    • Fertility drugs containing FSH stimulate the production of multiple eggs (superovulation).

  3. Maturation Injection and Egg Collection:

    • Human chorionic gonadotropin (hCG) matures follicles, allowing egg retrieval through follicular aspiration before rupture.

  4. Fertilization Assessment:

    • Eggs are placed in a petri dish with sperm; after 16–18 hours, fertilization is assessed.

  5. Embryo Culture:

    • If fertilization occurs, embryos are cultured in a specialized medium.

  6. Embryo Transfer:

    • Selected healthy embryos are transferred to the uterus on Day 3 or Day 5 to implant.

  7. Pregnancy Test:

    • Two weeks post-implantation, a pregnancy test is conducted to check success.