Sperm cells bind to ZP3 glycoproteins in the zona pellucida—a layer surrounding the oocyte that is composed of glycoproteins. This binding triggers the acrosomal reaction in the sperm cell. Once inside the female reproductive tract, sperm cells undergo capacitation. The subsequent state of hyperactivity allows sperm cells to successfully move up the fallopian tubes towards the oocyte. Once a sperm cell encounters the cell layers surrounding the oocyte, the sperm binds and penetrates the layers of cells surrounding the oocyte until it reaches the zona pellucida.

The acrosomal reaction in sperm cells is another process that aids in fusing the sperm and oocyte. After binding to glycoproteins in the zona pellucida, the acrosome—cap of the sperm cell—fuses with the sperm plasma membrane. This fusion causes the release of acrosomal contents, which include enzymes that allow the sperm cell to move further towards the oocyte.

During the acrosomal reaction, the sperm cell acrosome fuses with the plasma membrane, releasing digestive enzymes. These digestive enzymes—acrosin and hyaluronidase—break down the layers surrounding the oocyte as well as parts of the oocyte plasma membrane itself. The plasma membranes of the sperm cell and oocyte then fuse, which completes the acrosomal reaction.

After the sperm cell and oocyte fuse, the sperm’s pronucleus enters the oocyte. The oocyte completes meiosis II while the sperm’s tail and mitochondria degrade. The pronuclei migrate towards one another within the oocyte and replicate genetic material in preparation for genome fusion and mitosis. The nuclear membranes of the sperm and egg pronuclei dissolve; however, a mitotic spindle tethers the chromosomes together and prevents the genetic material from diffusing. The genetic material then undergoes mitosis, which fuses the maternal and paternal chromosomes into one diploid genome.

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.