Male Reproductive Anatomy

Recommended Reading

Read section 26.3, pp. 810-815.  Figure 26.7 on pp. 812-813 is a good reference; particularly 26.7a, b, and c.  Refer to p. 824-5 for information on erection and erectile dysfunction drugs.

In the male reproductive system, the gonads, where spermatozoa are produced, are called the testes (singular: testis). Spermatozoa (henceforward referred to as "sperm" for short) need to develop at a temperature that is slightly lower than core body temperature, so during development the testes move out of the abdomen through the inguinal canal and come to reside in the scrotum, a sac-like structure posterior to the penis. Cryptorchidism is the term for the condition where one or both of the testes fail to descend into the scrotum by birth. If the testes remain undescended, sperm development is disrupted, causing sterility. Cryptorchidism is also associated with an increased risk for the development of testicular cancer.

The testis is packed with coiled seminiferous tubules.  The seminiferous tubules are where spermatogenesis occurs.  Sertoli cells are important support cells located within the seminiferous tubule.  Sertoli cells secrete hormones and paracrine factors that support spermatogenesis. Another important support cell is found in the spaces between the seminiferous tubules is called the interstitial cell or Leydig cell. Leydig cells produce testosterone, the principal male androgen.  Androgens are steroid hormones that are necessary for spermatogenesis, but that also affect other tissues in the body to cause the male secondary sexual characteristics.  The sperm that are produced in the testis move next into a structure called the epididymis, that sits atop each testis. Within the epididymis, which consists of one extremely coiled tubule, fluid is reabsorbed so that the spermatozoa become concentrated.  Male reproductive success depends upon a high concentration of sperm in the ejaculate; less than 15,000,000 sperm per ml is considered infertile.  The male ejaculate, called semen, contains sperm and secretions from the accessory glands:  the seminal vesicles, the prostate gland, and the bulbourethral glands.  (The bulbourethral glands are not visible on our model.)

A series of ducts are necessary to convey the sperm to the female reproductive tract. The ductus deferens (also called the vas deferens) is the duct that starts at the tail of the epididymis at the bottom of the testis, and travels into the abdominal cavity. The structure that contains the ductus deferens and the testicular blood vessels is called the spermatic cord. After it enters the abdominal cavity, the ductus deferens travels posteriorly across the top of the bladder, heading inferiorly at the posterior side of the bladder. The ductus deferens widens into an ampulla where sperm are stored.  It unites with the duct from the seminal vesicle to form the ejaculatory duct, a short passageway that penetrates through the prostate gland and leads to the prostatic urethra.  The urethra is a passageway that conveys both semen and urine out to the end of the penis, and consists of three parts:  the prostatic urethra, the membranous urethra, and the penile urethra.  The membranous urethra is a short segment that passes through skeletal muscles (the external urethral sphincter) and is not visible on our model.  As per their names, the prostatic urethra is contained within the prostate gland and the penile urethra is contained within the penis.   

The penis is a copulatory organ designed to become erect and deposit sperm in the female reproductive tract. Penile erection depends upon three erectile tissues: the two corpora cavernosa (singular: corpus cavernosum) and the corpus spongiosum, which surrounds the penile urethra. The corpus spongiosum is narrow along most of its length, but then expands to fill the glans penis (the distal part of the penis).  The erectile tissues contain large vascular spaces that become engorged with blood when the penis is erect. This process depends on the activation of neurons that release nitric oxide to cause relaxation of vascular smooth muscle in penile arterioles.  Nitric oxide stimulates an intracellular enzyme that generates cyclic GMP (cGMP), which promotes decreased intracellular Ca⁺⁺ and relaxation of vascular smooth muscle.  Drugs for the treatment of erectile dysfunction inhibit the enzyme phosphodiesterase, which breaks down cGMP.  By blocking phosphodiesterase, the effects of cGMP are prolonged, allowing erection to occur.  The control of penile erection is discussed on p. 824-5 of your textbook. 

Part of the model is removed in the next image. This allows for a sectional view of the bladder and prostate gland.