Fertilization

During sexual intercourse, approximately 300 million sperm enter the vagina. Many are lost due to the acidic environment or flow out, but protective fluids allow a significant number to survive. The journey continues as sperm must pass through the cervix, which is temporarily open during ovulation, swimming through thinned cervical mucus. Millions more die, but some get trapped in cervical folds as a backup.

Inside the uterus, muscular contractions help sperm move towards the egg, but the woman's immune system destroys thousands more, mistaking them for invaders. Half of the remaining sperm head towards the empty fallopian tube, while the others swim towards the tube containing the unfertilized egg. Now, only a few thousand remain, battling against cilia that push the egg towards the uterus. Chemicals in the reproductive tract cause the sperm to become hyperactive, swimming faster and harder.

Finally, the few dozen surviving sperm reach the egg. They must push through the outer layer of cells, the corona radiata, and then attach to specialized receptors on the zona pellucida. This triggers the release of digestive enzymes from the acrosome, allowing a sperm to burrow through the layer and reach the egg cell membrane. The first sperm to make contact will fertilize the egg.

A single sperm attaches to the egg cell membrane, and within minutes, the outer membranes fuse, and the egg pulls the sperm inside. This event triggers changes in the egg membrane to prevent other sperm from attaching and releases chemicals that push them away, creating an impenetrable fertilization membrane. The zona pellucida hardens, trapping any unfortunate sperm inside. Inside the egg, the male genetic material spreads out and forms the male pronucleus with 23 chromosomes. The awakened female genetic material also forms a female pronucleus with 23 chromosomes.

Microtubules pull the male and female pronuclei together, and their chromosomes join, completing fertilization. At this moment, a unique genetic code is formed, determining gender, hair color, eye color, and many other characteristics. This new single cell, called a zygote, is the beginning of a new human being. Cilia then sweep the zygote towards the uterus, where it will implant in the rich uterine lining and continue to grow and mature for the next nine months until birth.