Unit 7A STRUCTURE AND FUNCTION OF THE MALE REPRODUCTIVE SYSTEM LEARNING OBJECTIVES 1. Learn the structures of the male reproductive system. 2. Learn the functions of the male reproductive system. 3. Learn the makeup of semen and which structures contribute to this solution. 4. Learn the makeup of a sperm, how it moves, and how its enters an egg. 5. Learn the number of sperm produced/day and the number in an ejaculation of semen. INTRODUCTION Simple organisms, such as the amoeba, reproduce by simply splitting in half. Sexual reproduction requires two organisms, in most cases, that produce two different types of gametes (sperm and eggs). In lower organisms, these two cells meet in an aquatic environment outside the body of the female. In higher organisms, the two cells usually meet inside the body of the female who in some cases lays a fertilized egg. In most mammals, including humans, the fertilized egg develops into an immature organism inside the body of the female. This advanced type of reproduction involves rather complicated reproductive systems. In this unit we will examine the human male and female systems and their specific functions. Certain questions that we will attempt to answer include: 1) What is the purpose of reproduction? 2) What roles does the male system play? 3) What roles does the female system play? 4) How is the sperm able to reach the egg? 5) What is the reason there are so many more sperm than eggs? 6) What is the age of a mature sperm? Mature egg? 7) What is accomplished by the fact that the organs for producing eggs are inside the body cavity; whereas, the organs for producing sperm are outside the body cavity? 8) All living cells require a moist environment. How are sperm kept in a moist environment? 9) What other system uses parts of the same pathway as the male gametes? 10) Is there any difference in the length of time that the male and female are able to produce viable gametes? 85
11) Both types of reproductive organs, ovary and testis, start development within the abdominal cavity, how do the testes end up outside the abdominal cavity? 12) What happens to eggs and sperm that do not function in reproduction? STRUCTURE AND FUNCTION OF THE MALE REPRODUCTIVE SYSTEM The male reproductive system has three main functions: 1) produce sperm 2) produce hormones, and 3) deposit sperm in the female system. Figure 7-1 shows a side view of the male reproductive system. The sperm are produced by seminiferous tubules inside the testis (figure 7-2B). There are about 800 feet of seminiferous tubules within the two testes. The male produces sperm from the age of puberty until death with between 100 million and 200 million produced daily during his reproductive peak. There is some evidence that suggests that boys similar to girls in the United States are entering puberty at a slightly earlier age than previously thought. A study between the period from 1988-1994 found that a significant number of boys as young as 8 had signs of genital development which is considerably earlier than previous estimates. As physical signs such as appearance of pubic hair and enlargement of testes were used, one must be cautious of the subjective interpretations of these changes. The possible reasons for the earlier development are attributed to better nutrition, rising obesity, exposure to environmental chemicals, which can mimic sex hormones, and infant formula and other products containing soy, which can mimic sex hormones. An ejaculation of semen contains 200 400 million sperm in a 3-4 milliliter volume of semen. Between the tubules are located interstitial cells, which produce the male hormone, testosterone and some estrogen. The testes are enclosed by a layer of connective tissue, the scrotal sac (7-2A). The testes originate in the body cavity, but in the last 2 months of fetal life they descent through the inguinal canal into the scrotal sac. Testes remaining in the abdominal cavity produce no viable sperm. Temperature of the body cavity is about 37 C; whereas, the temperature in the scrotal sac is about 34 C. Figure 7-1. Male reproductive system. 86
There is an involuntary response that results in the scrotal sac shrinking and moving the testes closer to the body in a cold environment, and an adverse response in a warm environment. In the production of sperm, spermatogenesis, the cells undergo meiosis which reduces the chromosome number in the sperm to ½ what it is in the body cells. The sperm is a flagellated, tailed, single cell containing a nucleus and little cytoplasm (fig. 7-2C). The head contains 23 chromosomes. An acrosome fits like a cap over the head; it contains enzymes, which will assist the sperm to digest its way into the egg. The relatively long tail or flagellum provides motility to the sperm. A healthy sperm can move at a rate of 50 microns/second or 3-4mm/minute. The head of the sperm is about 7 microns in length, but the entire sperm is about 45 microns. The development of a spermatogonium (parent cell that will give rise to four sperm) to spermatozoa (sperm) requires 72 days. Another 2-4 days is required for maturation in the epididymis. Among the sperm are Sertoli cells, which provide nutrients for the sperm and phagocytose damaged sperm cells. The epididymis, contained within the scrotal sac, is a much coiled tube that acts as a storage place for sperm. Maturation occurs here and takes 2-4 days after which time the sperm are capable of fertilizing an egg. During ejaculation, sperm are propelled from the epididymis into the vas deferens. The vas deferens extends from the scrotum through the inguinal canal around and behind the bladder; the two vasa deferentia join to form a short ejaculatory duct. Sperm can be stored in the vas deferens for many days because their metabolic activity is low. The time depends upon the sexual activity of the male. Sperm that are not ejaculated during sexual activity are either released by nocturnal emission (wet dream) or are absorbed back into the body. The ejaculatory duct runs into the urethra. The urethra runs from the bladder through the head of the penis and drains both the reproductive and excretory systems. 87
Figure 7-2. Longitudinal section of the scrotal sac (A), cross section of seminiferous tubules (B), and sperm showing head, middle piece, and tail (C). The associated glands consist of the seminal vesicles, prostate gland, and Cowper s gland. The seminal vesicles contribute about 60% of the total volume of semen. The semen is the fluid, which contains the sperm and makes up the ejaculate. The vesicles are paired and empty into the ejaculatory duct. The fluid contains mucus, amino acids, and fructose. The prostate gland surrounds the urethra close to where it leaves the bladder. It secretes an alkaline fluid (ph 7.5-8.0) directly into the urethra. This helps to neutralize the acidity of the vagina and enhances the motility and fertility of sperm which have an optimum ph of 6.0 6.5. A large percent of older males have some problem with enlargement or cancerous growth of the prostate. Cowper s gland (bulbourethral gland) empties into the urethra. It secretes some fluid, but its function is not known. The penis serves to deposit the semen in the vagina of the female. A x-section of the penis shows there are three blood sinuses; one of these surrounds the urethra (fig. 7-3). The arteries fill with blood enlarging the sinuses and the veins are blocked preventing 88
the return of blood; this results in erection of the penis. At birth there is a layer of skin, foreskin that covers the head of the penis. Parents often choose to have this removed shortly after birth by a procedure referred to as circumcism. Figure 7-3. Cross-section of the penis showing the urethra and three sinuses. 89
OBJECTIVE QUESTIONS OVER MALE REPRODUCTIVE SYSTEM Unit 7A 1. The (A) interstitial cells (B) seminiferous tubules (C) Sertoli cells produce the sperm. 2. The (A) Cowper s gland (B) prostate gland (C) seminal vesicles produce(s) an alkaline fluid. 3. There is only one (A) testis (B) seminal vesicle (C) vas deferens (D) ejaculatory duct. 4. It takes about (A) one (B) three (C) five (D) 7 (E) 9 (F) 11 weeks for the production and maturation of sperm. 5. Sperm are usually stored in the (A) testis (B) epididymis (C) vas deferens (D) two of the preceding (E) all the preceding. 6. Sperm require a (A) lower (B) higher (C) about the same temperature to develop as/than eggs. 7. The length of a human sperm, including the tail is about (A) 45 microns (B) 100 microns (C) 1 millimeter (D) 1 inch. 8. There is/are (A) one (B) two (C) three (D) four sinuses in the penis. 9. The testes produce (A) testosterone (B) estrogen (C) both A and B (D) neither A or B. 10. Human sperm contain (A) 23 (B) 46 (C) 72 chromosomes. DISCUSSION QUESTIONS OVER MALE REPRODUCTIVE SYSTEM 1. What is an inguinal hernia? 2. What fluids contribute to the semen? 90
3. What is the reason that some experts advise wearing boxer shorts rather than briefs? 4. How long is the male able to produce sperm? 5. What is the advantage of reducing the number of chromosomes in the sperm from 46 to 23? 6. What physiological condition could prevent a male from achieving an erection? 7. What happens to the sperm that are being continuously produced if the male is not involved in sexual activity? 8. What is the reason that the male reproductive system is referred to as a closed system rather than the open system that occurs in the female? 91