The bulbourethral gland is a part of the male reproductive system that contributes protein-rich fluid to semen and lubricates the urethra.

Prior to ejaculation, secretions from the bulbourethral gland, prostate gland, epididymis, and seminal vesicles enter the ejaculatory duct. The ejaculatory duct is the place where semen mixes and is stored until ejaculation.

Seminal vesicles secrete a large portion of the components that make up semen. A high concentration of fructose is included in these secretions. The fructose provides energy rich molecules for sperm cell movement.

In males, the urethra carries semen from the ejaculatory duct out of the penis. The urethra also carries urine from the bladder through the penis.

During copulation and sexual intercourse, the penis enters the female vagina. Ejaculation—the release of semen from the penis—also occurs in the vagina.

During capacitation, sperm cells undergo biochemical changes that allow for fertilization of the oocyte. One of these changes is an influx of intracellular calcium, which leads to an increase in intracellular cAMP levels. The high levels of calcium and cAMP promote hyperactivation—increased motility—through deeper tail bends. This change in sperm tail movement is accompanied by the adoption of a swinging movement by the head of the cell. These changes in motility patterns help sperm cell movement within the female reproductive tract.

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.