Fertilization Overview

Fertilization

• Defined as the fusion of sperm and secondary oocyte, resulting in a zygote.
• Primary site of fertilization: Ampulla of the uterine tube, which is the widest part.
• Characteristics of secondary oocyte: Large, immotile, covered by zona pellucida and corona radiata, contains abundant cytoplasm and yolk granules, has 22 chromosomes plus one X chromosome.
• Ovum can only be fertilized within 24 hours post-ovulation; otherwise, it degenerates.
• Sperm reach the site of fertilization through tail movement, uterine ciliary movement, and chemotactic signals from the oocyte. Sperm have a maximum lifespan of 48 hours.
• Capacitation: A crucial process where the glycoprotein coat is removed from sperm’s acrosome, occurring in the female reproductive tract over 7-8 hours.
• Acrosome reaction: Sperm release enzymes, including hyaluronidase and zona lysin, to penetrate the zona pellucida.
• Phases of fertilization include:
  • Passage through the corona radiata
  • Penetration of the zona pellucida
  • Fusion of oocyte and sperm membranes
  • Completion of the secondary meiotic division
  • Syngamy resulting in formation of zygote with 46 chromosomes
• Zona reaction: Following penetration, cortical granules cause the zona pellucida to become firm and impermeable to additional sperm.

Results of Fertilization

• Completion of the secondary meiotic division of the oocyte.
• Restoration of diploid chromosome number (46 total).
• Determination of the embryo’s sex (46,XX or 46,XY).
• Stimulation of cleavage processes.
• Abnormalities may occur:
  • Parthenogenesis: Development of an unfertilized oocyte.
  • Dispermy and triploidy: Fertilization by two sperm, resulting in a zygote with 69 chromosomes.

Cleavage

• Cleavage involves repeated mitotic divisions of the zygote, resulting in an increasing number of blastomeres.
• Begins approximately 30 hours post-fertilization, where the zygote divides into two blastomeres.
• Morula: When the zygote has divided into 12-16 blastomeres, usually formed around day three post-fertilization.
• Blastomeres continue mitotic division while contained within the zona pellucida, preventing cell adhesion.
• Formation of a blastocele, a fluid-filled cavity, occurs around day four.
• Blastocele separates blastomeres into:
  • Trophoblast: The outer cell layer forming the placental part.
  • Inner cell mass: The group of cells that will develop into the embryo.
• Transition from morula to blastocyst, which then enters the uterus, leading to gradual degeneration of the zona pellucida in preparation for implantation.

Clinical Applications

• Embryonic stem cells (derived from the inner cell mass of the blastocyst) are pluripotent and can generate any type of tissue, holding significant potential for medical therapies.

Explore the intricate process of fertilization, including the characteristics of the secondary oocyte and the crucial steps involved from sperm capacitation to the acrosome reaction. Understand the timing and physiological factors that influence successful fertilization in the human reproductive system.