A lot of things. The birth canal is a hostile environment that sperm can't survive alone. During ejaculation, several tissues come into play to create a mixture that ensures the sperm can reach their destination.
Sertoli cells, the cells that make sperm, secrete a fluid into the seminiferous tubules where the sperm are held. This fluid helps the sperm travel through the vas deferens and genital glands.
Next, cells in the ductus epididymis add glycerophosphocholine, which delays the sperm's final transformation into mobile cells. The ejaculatory duct adds fructose to fuel the cells, prostaglandins to prevent an immune system response in the vagina, Vitamin C to protect from oxidation, and zinc to protect DNA.
After that, the bulbourethral glands add more seminal fluid, galactose (a sugar like fructose,) and sialic acid to help the sperm bind with the egg cell. Finally, the prostate releases additional fluid and prostate-specific antigen (PSA) to improve sperm motility*. The fluid released by both glands is a strong base to counteract the acidity of the vagina.
By the time it leaves the body, semen contains the following:
Food for the sperm
Vitamins that protect sperm cells from damage
Fluid that shields the sperm from the acidic vagina and gives the sperm something to swim in
Enzymes to activate the sperm at the right moment, prevent attack by the woman's immune system, help the sperm swim, and finally combine with the egg cell
*High levels of PSA in semen can be an indicator of prostate cancer. The prostate also surrounds the urethra, which is why it's difficult to urinate with an enlarged prostate.
Posted 1896 day ago