Sperm is produced in the testicles and develops to maturity while traveling from the seminiferous tubules through the epididymis into the vas deferens. The process of producing sperm is called spermatogenesis, and it is hormonally driven. Brain hormones govern sperm production and are precisely controlled. The three main hormones needed for spermatogenesis are follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone. These hormones work together to help the body continuously produce sperm. Once the body has made enough hormones, sperm begin to develop within a system of tiny tubes called seminiferous tubules inside the testicles. Sperm start as simple round cells that must mature and transform to their tadpole-like shape. During puberty, these simple cells will develop into mature sperm. Adult men will continue to produce millions of sperm every day. The sperm then move to the vas deferens, which propels sperm forward with smooth muscle contractions. When a man is stimulated for sexual activity, the sperm are mixed with seminal fluid, a whitish liquid produced by the seminal vesicles and the prostate gland, to form semen. As a result of the stimulation, the semen, which contains up to 500 million sperm, is pushed out of the penis (ejaculated) through the urethra.