Anatomy Unit 201: Fertilization & Cleavage

Fertilization

• Fertilization is the fusion of sperm and secondary oocyte, occurring in the ampulla of the uterine tube.
• The secondary oocyte is large, immotile, with a zona pellucida and corona radiata, and contains yolk granules in its abundant cytoplasm.
• The oocyte has 22 chromosomes plus one X chromosome, and cannot be fertilized after 24 hours due to degeneration.
• Sperm reach the oocyte through tail movement, uterine cilia, and chemoattraction from the oocyte; sperm lifespan is typically 48 hours.
• Capacitation is the process where the sperm's glycoprotein coat is removed, enabling fertilization; it takes about 7-8 hours in the female reproductive tract.
• The acrosome reaction involves sperm releasing enzymes like hyaluronidase, which help penetrate the zona pellucida.
• Phases of fertilization include sperm passage through the corona radiata, penetration of the zona pellucida, fusion of cell membranes, completion of meiotic division, and formation of male and female pronuclei, leading to zygote formation.
• Zona reaction solidifies the zona pellucida after sperm penetration, preventing additional sperm from entering.
• Outcomes of fertilization: completion of meiotic division, restoration of diploid chromosome count (46), determination of embryo's sex (46,XX or 46,XY), and initiation of cleavage.

Abnormalities of Fertilization

• Parthenogenesis occurs when an unfertilized oocyte begins division without sperm.
• Dispermy results in triploidy when an ovum is fertilized by two sperm, creating a zygote with 69 chromosomes.

Cleavage

• Cleavage is the process of rapid mitotic divisions of the zygote, leading to increased blastomere numbers.
• Initiates approximately 30 hours after fertilization, with the first division yielding two blastomeres.
• A morula (12–16 blastomeres) forms around three days post-fertilization.
• During cleavage, blastomeres divide within the zona pellucida, which prevents cell adhesion to the uterine tubes.
• As cell division continues, cavities form and merge to create a blastocele about four days post-fertilization.
• The blastocele separates cells into the trophoblast (future placenta) and inner cell mass (future embryo).
• The morula transforms into a blastocyst, which enters the uterus as the zona pellucida disintegrates, preparing for implantation.

Clinical Application

• Embryonic stem cells derived from the inner cell mass are pluripotent, capable of developing into any tissue type.

Test your understanding of fertilization and cleavage in human anatomy. This quiz covers definitions, phases, and key outcomes related to the fertilization process, as well as the cleavage of the embryo. Ready to assess your knowledge on these crucial concepts?