Fertilization and Early Development Quiz

Study Notes

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.