As with female reproduction, male reproduction is hormonally driven -- requiring a normally functioning hypothalamus and pituitary gland, again with the release of pulsatile GnRH stimulating FSH and LH. In men, LH primarily stimulates testosterone production while FSH stimulates the production of sperm (spermatogenesis). The testes must be capable of response to this endocrinolgic (hormonal) stimulus. In addition, there must be an intact ductal system to transport sperm to the urethra.

The male external genital consist of the scrotum, which houses the testes and associated ductal systems (epididymis and vas deferens), and the penis. The testes are covered by a tough fibrous layer called the tunica albuginea which divides the testes into lobules. Each of the 200 to 300 lobules contains one to three long and tightly coiled seminiferous tubules, within which sperm production occurs.

In contrast to female eggs, sperm are continually produced throughout a man's lifetime. During spermatogenesis and transport through the male reproductive tract, the male spermatozoa are transformed into a highly specialized cell with its own motility and enzymatic capacity to penetrate the egg. Epididymal function must be normal and the accessory glands must be functional to produce normal seminal plasma. An intact nervous system is also required to permit penile erection and normal ejaculation.

Male infertility can be divided into problems of ejaculation/erection, problems within the fine, small reproductive tract ducts and/or problems of sperm production. The latter is by far the most common of the three. 
Causes of abnormal sperm production are many but include: varicocele, genetic problems, infections, exposures to heat or toxic chemicals, drugs and medications and hormonal imbalance.

The primary test of sperm function is the semen analysis which will typically evaluate the volume of the semen, the sperm concentration and sperm motility. Additional more complex testing may be required during the course of an infertility evaluation.

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