Fertilization and Early Development Quiz

Fertilization and First Week of Development

• Fertilization is the fusion of a spermatozoon and a secondary oocyte.
• It occurs at the ampulla of the uterine tube.
• The process takes approximately 24 hours.
• Fertilization occurs in 3 phases:
  • Penetration of the corona radiata
  • Penetration of the zona pellucida
  • Fusion of the oocyte and sperm cell membranes
• Sperm must undergo capacitation and acrosome reaction to penetrate the oocyte.
• About 300 million spermatozoa are released, but only a few hundred reach the site of fertilization, and only one fertilizes the oocyte.

Capacitation

• Sperm condition in the female reproductive tract.
• Occurs for about 7 hours.
• Mediated by oocyte fertilizin and sperm antifertilizin.
• Necessary for sperm to pass through the corona radiata and undergo acrosome reaction.

Acrosome Reaction

• Occurs after binding to the zona pellucida.
• Initiated by zona proteins.
• Results in the release of enzymes.
• Required enzymes include acrosin, neuraminidase, zona lysins (proteases)
• Only one sperm fertilizes an oocyte out of millions released.

Penetration of the Corona Radiata

• Capacitated spermatozoa pass through the corona radiata.
• Enhanced by hyaluronidase release from the sperm acrosome.
• Sperm movement (flagellum) also contributes to the penetration process.

Penetration of the Zona Pellucida

• Mediated by acrosome enzymes (acrosin, neuraminidase, esterases, and ZP3 ligand).
• Sperm contact with the zona pellucida triggers acrosome reaction.
• Enzymes facilitate penetration.
• Zona reaction occurs after penetration, altering the zona pellucida to prevent polyspermy.
• Lysosomal enzymes from cortical granules of the oocyte mediate the zona reaction.

Fusion of the Oocyte and Sperm Cell Membranes

• Initial sperm adhesion to the oocyte is mediated by integrins and their ligands (e.g., disintegrins)
• After adhesion, plasma membranes of sperm and egg fuse.
• Acrosomal head cap disappears during the acrosome reaction.
• In humans, the sperm head and tail enter the oocyte cytoplasm.
• The sperm's plasma membrane is left behind on the oocyte surface.

Response of Egg to Spermatozoon Entry

• Zona and cortical reactions: Cortical granules release enzymes altering the zona pellucida, preventing polyspermy.
• Second meiotic division of oocyte completes.
• Formation of male pronucleus from sperm nucleus.
• Metabolic activation of oocyte to undergo post-fusion development.

Results of Fertilization

• Restoration of diploid number of chromosomes.
• Determination of sex of embryo (XX female, XY male).
• Initiation of cleavage (cell division).
• Oocyte degeneration without fertilization.

Applied Anatomy

• Early pregnancy factor (EPF) secreted by trophoblast cells allows early detection of pregnancy.
• Dispermy (two sperm fertilize one egg) can lead to a triploid embryo and spontaneous abortion.
• Polyspermy is prevented by cortical and zona reactions.
• Assisted reproductive technologies are available for infertile couples (IVF, GIFT, ICSI).

Contraceptive Methods

• Oral contraceptives (OCPs) inhibit pituitary gonadotropins preventing ovulation and cause menstruation to happen naturally.
• Barrier methods (condoms, diaphragms) prevent sperm passage.
• Surgical procedures (vasectomy, tubal ligation) permanently prevent sperm or egg passage.
• Intrauterine devices (IUDs) potentially interfere with implantation.
• Depo-Provera and RU 486 are other contraceptive methods.

Cleavage

• Mitotic divisions follow fertilization, reducing the cytoplasm-nucleus ratio.
• First mitotic division occurs about 30 hours after fertilization, yielding two blastomeres.
• Compaction at the 8- or 9-cell stage occurs.
• Morula formation results in the embryo resembling a mulberry (12-16 cells).
• Cells differentiate into inner cell mass (embryoblast) and outer cell mass (trophoblast).
• Morula enters the uterine cavity on the third day.

Blastogenesis

• Morula transforms into a fluid-filled blastocyst.
• Blastocyst consists of blastocyst cavity, inner cell mass (embryoblast), and outer cell mass (trophoblast).
• Zona pellucida degenerates after blastocyst formation.
• Implantation of the blastocyst typically starts around the sixth day after ovulation.

Applied Anatomy (cont'd)

• Monozygotic twins originate from a single zygote, with the first mitotic division resulting in independent cell development.
• Mosaicism in cleavage may occur, leading to embryos with two or more cell lines having different chromosome numbers.