Following spermatogenesis, sperm cells are stored in the lower portion of the epididymis. In order for ejaculation to occur, they are transported through

the epididymis (1), the vas deferens (2), the ejaculatory duct (3) and the urethra (4).

Obstruction of the epididymis, the vas deferens or the ejaculatory duct, if it is bilateral (on both sides), can result in aspermia (complete lack of semen) or azoospermia (complete lack of sperm in the semen). Obstructive azoospermia (sperm are produced but cannot be ejaculated) can also be caused by vasectomy (a medical procedure for sterilization, whereby the vas deferens is tied) or bilateral absence of the vas deferens, a genetic abnormality associated with the disease cystic fibrosis.

In obstructive azoospermia cases sperm cells may be retrieved directly from the epididymis or the testis, using a variety of surgical sperm retrieval techniques such as Percutaneous Epididymal Sperm Aspiration (PESA), Microsurgical Epididymal Sperm Aspiration (MESA) or TEsticular Sperm Aspiration (TESA), followed by IntraCytoplasmic Sperm Injection (ICSI).