“In vitro” fertilization, or IVF, is the fertilization of the oocyte outside of the human body in a culture. In this process, an oocyte is extracted from a female and exposed to ejaculate containing sperm cells. The fertilization, therefore, occurs outside of the female reproductive system. The zygote is then implanted in a female uterus for gestation and development. IVF is a process used in cases of infertility, surrogacy, and other cases of reproductive difficulties.

After fertilization and sperm cell penetration of the oocyte, the cortical reaction takes place. During the cortical reaction, a release of intracellular calcium ions triggers the exocytosis of cortical granules. Cortical granules are vesicles in the cortex of the oocyte that contain enzymes that prevent polyspermy. Exocytosis of the cortical granules releases their contents into the extracellular matrix—the zona pellucida in mammals—creating changes to prevent further sperm penetration.

Penetration of the oocyte by a sperm cell leads to an intracellular calcium release, trigging the exocytosis of cortical granules. The cortical granules release their contents—including proteases, peroxidases, and carbohydrates—into the extracellular matrix. These contents untether the perivitelline membrane and attract water into the perivitelline space, expanding it to form the hyaline layer. This is known as the slow block to polyspermy because it creates a permanent barrier to other sperm.

In mammals, the cortical reaction involves a very similar influx of intracellular calcium ions and exocytosis of cortical granules. The exocytosis releases the contents into the zona pellucida, where glycoproteins are cleaved to prevent further sperm cell binding. The hyaline layer then forms around the fertilized egg.

After sperm cell and oocyte plasma membrane fusion, the sperm pronucleus moves into the oocyte cytoplasm. The sperm and egg pronuclei migrate towards the center of the cell while performing DNA replication, where both nuclear membranes dissolve. The chromosomes are prevented from dispersing by a mitotic spindle that tethers the maternal and paternal genetic material together. The cell then begins mitosis, which fuses the genetic material into a single diploid genome.

During oogenesis, specifically ootidogenesis, the secondary oocyte halts development and arrests its growth at metaphase II of meiosis. This arrest is maintained through ovulation until fertilization in the fallopian tube. During fertilization, the cortical reaction in the oocyte acts as the slow block to polyspermy and starts with the release of intracellular calcium. This release of calcium ions allows the egg to resume meiosis and ends the metaphase II arrest. 

The fast block to polyspermy is the initial method that prevents polyspermy by the large number of sperm cells surrounding the egg. The fast block is triggered by sperm binding, which initiates a change in membrane potential. There is a large influx of sodium ions into the egg, creating a depolarization event that prevents further sperm penetration.

Both the fast and slow blocks to polyspermy prevent the penetration of a second sperm cell into the oocyte. The fast block to polyspermy is the initial and immediate block upon sperm binding. The fast block is an electrical block caused when oocyte membrane depolarization is started by an influx of calcium ions. The slow block is characterized by a change in zona pellucida makeup through the cortical reaction, which prevents sperm cell binding. The cortical reaction is triggered by the release of intracellular calcium.

Dizygotic twins are produced by the release and fertilization of two separate egg cells. Both zygotes implant in the uterus and develop. The resulting twins will not be genetically identical because they arise from different gametes.

Monozygotic twins are produced when a zygote divides into two embryos. This process is spontaneous and may result from blastocyst collapse. The resulting twins are genetically identical because they are produced from the same gametes.