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Male Reproductive Anatomy

6 April, 2016 - 17:26

Sperm are immobile at body temperature; therefore, the testes are external to the body so that a correct temperature is maintained for motility. In land mammals, including humans, the pair of testes must be suspended outside the body so the environment of the sperm is about 2 °C lower than body temperature to produce viable sperm. If the testes do not descend through the abdominal cavity during fetal development, the individual has reduced fertility.

The scrotum houses the testicles or testes (singular: testis), and provides passage for blood vessels, nerves, and muscles related to testicular function. The testes are a pair of male gonads that produce sperm and reproductive hormones. Each testis is approximately 2.5 by 3.8 cm (1.5 by 1 inch) in size and divided into wedge-shaped lobes by septa. Coiled in each wedge are seminiferous tubules that produce sperm.

The penis drains urine from the urinary bladder and is a copulatory organ during intercourse (Figure 18.12; Table 18.1). The penis contains three tubes of erectile tissue that become engorged with blood, making the penis erect, in preparation for intercourse. The organ is inserted into the vagina culminating with an ejaculation. During orgasm, the accessory organs and glands connected to the testes contract and empty the semen (containing sperm) into the urethra and the fluid is expelled from the body by muscular contractions causing ejaculation. After intercourse, the blood drains from the erectile tissue and the penis becomes flaccid.

Semen is a mixture of sperm (about five percent of the total) and fluids from accessory glands that contribute most of the semen’s volume. Sperm are haploid cells, consisting of a flagellum for motility, a neck that contains the cell’s energy-producing mitochondria, and a head that contains the genetic material Figure 18.11). An acrosome (acrosomal vesicle) is found at the top of the head of the sperm. This structure contains enzymes that can digest the protective coverings that surround the egg and allow the sperm to fuse with the egg. An ejaculate will contain from two to five milliliters of fluid and from 50–120 million sperm per milliliter.

Figure 18.11 As seen in this scanning electron micrograph, human sperm has a flagellum, neck, and head. (credit: scale-bar data from Matt Russell) 

Sperm form in the walls of seminiferous tubules that are coiled inside the testes (Figure 18.12; Table 18.1). The walls of the seminiferous tubules are made up of the developing sperm cells, with the least developed sperm at the periphery of the tubule and the fully developed sperm next to the lumen. The sperm cells are associated with Sertolicells that nourish and promote the development of the sperm. Other cells present between the walls of the tubules are the interstitial cellsofLeydig, which produce testosterone once the male reaches adolescence.

When the sperm have developed flagella they leave the seminiferous tubules and enter the epididymis (Figure 18.12; Table 18.1). This structure lies along the top and posterior of the testes and is the site of sperm maturation. The sperm leave the epididymis and enter the vas deferens, which carries the sperm behind the bladder, and forms the ejaculatory duct with the duct from the seminal vesicles. During a vasectomy, a section of the vas deferens is removed, preventing sperm (but not the secretions of the accessory glands) from being passed out of the body during ejaculation and preventing fertilization.

The bulk of the semen comes from the accessory glands associated with the male reproductive system. These are the seminal vesicles, the prostategland, and the bulbourethral gland(Figure 18.12; Table 18.1). The secretions from the accessory glands provide important compounds for the sperm including nutrients, electrolytes, and pH buffering. There are also coagulation factors that affect sperm delivery and motility.



Figure 18.12 The reproductive structures of the human male are shown. 

Which of the following statements about the male reproductive system is false?

  1. The vas deferens carries sperm from the testes to the seminal vesicles.
  2. The ejaculatory duct joins the urethra.
  3. Both the prostate and the bulbourethral glands produce components of the semen.
  4. The prostate gland is located in the testes.
Table 18.1 Male Reproductive Anatomy
Male Reproductive Anatomy
Organ Location Function
Scrotum External Supports testes and regulates their temperature
Penis External Delivers urine, copulating organ
Testes Internal Produce sperm and male hormones
Seminal Vesicles Internal Contribute to semen production
Prostate Gland Internal Contributes to semen production
Bulbourethtral Glands Internal Neutralize urine in urethra