The secondary oocyte travels from the ovary down the fallopian tube after ovulation, while sperm cells are deposited into the female vagina by the penis. The sperm cells then make their way through the cervix and uterus into the fallopian tube, where one sperm cell fertilizes the oocyte.

Pre-ejaculate is a clear fluid that exits the penis from the urethra during times of arousal, but prior to orgasm. Pre-ejaculate, like ejaculate, is alkaline to combat the acidic environment of the vagina: however, pre-ejaculate is highly variable in sperm content and production among different individuals. Pre-ejaculate neutralizes the vaginal environment for sperm and acts as lubricant for sexual intercourse.

During sexual intercourse, arousal and stimulation lead to male orgasm. Orgasm is commonly accompanied by ejaculation. During ejaculation, pulses of semen exit the urethra due to the muscle contractions initiated by spinal nerve signals. 

During sexual intercourse, female lubrication is important in preventing pain and to ease penile penetration. Vaginal lubrication is triggered by sexual arousal of all kinds. Lubrication fluid is composed of secretions from Bartholin’s glands, the vaginal walls, and mucus glands in the cervix. This fluid is alkaline to promote a fertile environment for sperm cells. 

After ejaculation into the female vagina, capacitation of sperm cells takes place. During this process, sperm undergo molecular and biochemical changes that allow them to fertilize the oocyte. In this final stage of maturation, sperm cells lose many membrane proteins (including glycoproteins and steroids to ease binding to the oocyte), undergo an influx of intracellular calcium, and change their tail movement pattern. The changes that occur during sperm activation put the sperm cells into a state of hyperactivity, or heightened motility that aids fertilization.

The acrosome of sperm cells is a cap on the anterior end of the cell that contains digestive enzymes (i.e. acrosin and hyaluronidase) and surface antigens. During the acrosomal reaction, the acrosome fuses with the sperm’s plasma membrane and releases the acrosomal contents from the cell. The digestive enzymes break down the layers surrounding the oocyte and allow the sperm cell to approach the oocyte plasma membrane.

“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.