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25.1: Male Reproductive System

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Lesson Objectives

  • Identify male reproductive structures and their functions.
  • Explain how the male reproductive system develops.
  • Describe how sperm are produced.

Vocabulary

adolescent growth spurt
period of rapid growth that occurs during puberty
ejaculation
muscle contractions that propel sperm from the epididymes and out through the urethra in males
epididymis (plural, epididymes)
one of two male reproductive organs where sperm mature and are stored until they leave the body
luteinizing hormone (LH)
pituitary gland hormone that stimulates the testes to secrete testosterone and the ovaries to secrete estrogen
penis
male reproductive organ containing the urethra, through which sperm and urine pass out of the body
puberty
period during which humans become sexually mature
reproductive system
system of organs that produces gametes and secretes sex hormones
semen
fluid containing sperm and gland secretions that nourish sperm and carry them through the urethra and out of the body
sex hormone
chemical messenger that controls sexual development and reproduction
spermatogenesis
process of producing sperm in the testes
testis
one of two male reproductive organs that produces sperm and secretes testosterone
testosterone
male sex hormone secreted by the testes

Introduction

The reproductive system in both males and females consists of structures that produce reproductive cells, or gametes, and secrete sex hormones. A gamete is a haploid cell that combines with another haploid gamete during fertilization. Sex hormones are chemical messengers that control sexual development and reproduction. The male reproductive system consists of structures that produce male gametes called sperm and secrete the male sex hormone testosterone.

Male Reproductive Structures

The main structures of the male reproductive system are shown in Figure below. Locate them in the figure as you read about them below. You can also watch an animation about male reproductive structures at this link: http://www.medindia.net/animation/male_reproductive_system.asp.

Male Reproductive Structures. Organs of the male reproductive system include the penis, testes, and epididymis. Several ducts and glands are also part of the system. Do you know the reproductive functions of any of these structures?

Penis

The penis is an external genital organ with a long shaft and enlarged tip called the glans penis. The shaft of the penis contains erectile tissues that can fill with blood and cause an erection. When this occurs, the penis gets bigger and stiffer. The urethra passes through the penis. Sperm pass out of the body through the urethra. (During urination, the urethra carries urine from the bladder.)

Testes

The two testes (singular, testis) are located below the penis. They hang between the thighs in a sac of skin called the scrotum. Each testis contains more than 30 meters (90 feet) of tiny, tightly packed tubules called seminiferous tubules. These tubules are the functional units of the testes. They produce sperm and secrete testosterone.

Epididymis

The seminiferous tubules within each testis join to form the epididymis. The epididymis (plural, epididymes) is a coiled tube about 6 meters (20 feet) long lying atop the testis inside the scrotum. The functions of the epididymis are to mature and store mature sperm until they leave the body.

Ducts and Glands

In addition to these organs, the male reproductive system consists of a series of ducts and glands. Ducts include the vas deferens and ejaculatory ducts. They transport sperm from the epididymes to the urethra in the penis. Glands include the seminal vesicles and prostate gland. They secrete substances that become part of semen.

Semen

Semen is the fluid that carries sperm through the urethra and out of the body. In addition to sperm, it contains secretions from the glands. The secretions control pH and provide sperm with nutrients for energy.

Sexual Development in Males

The only obvious difference between boys and girls at birth is their reproductive organs. However, even the reproductive organs start out the same in both sexes.

Development Before Birth

In the first several weeks after fertilization, males and females are essentially the same except for their chromosomes. Females have two X chromosomes (XX), and males have an X and a Y chromosome (XY). Then, during the second month after fertilization, genes on the Y chromosome of males cause the secretion of testosterone. Testosterone stimulates the reproductive organs to develop into male organs. (Without testosterone, the reproductive organs always develop into female organs.) Although boys have male reproductive organs at birth, the organs are immature and not yet able to produce sperm or secrete testosterone.

Puberty and Its Changes

The reproductive organs grow very slowly during childhood and do not mature until puberty. Puberty is the period during which humans become sexually mature. In the U.S., boys generally begin puberty at about age 12 and complete it at about age 18. What causes puberty to begin? The hypothalamus in the brain “tells” the pituitary gland to secrete hormones that target the testes. The main pituitary hormone involved is luteinizing hormone (LH). It stimulates the testes to secrete testosterone. Testosterone, in turn, promotes protein synthesis and growth. It brings about most of the physical changes of puberty, some of which are shown in Figure below. You can watch an animation of these and other changes that occur in boys during puberty at this link: http://www.bbc.co.uk/science/humanbody/body/interactives/lifecycle/teenagers/.

Some of the changes that occur in boys during puberty are shown in this figure. Pubic hair grows, and the penis and testes both become larger.

Adolescent Growth Spurt

Another obvious change that occurs during puberty is rapid growth. This is called the adolescent growth spurt. In boys, it is controlled by testosterone. The rate of growth usually starts to increase relatively early in puberty. At its peak rate, growth in height is about 10 centimeters (almost 4 inches) per year in the average male. Growth generally remains rapid for several years. Growth and development of muscles occur toward the end of the growth spurt in height. Muscles may continue to develop and gain strength after growth in height is finished.

Production and Delivery of Sperm

A sexually mature male produces an astounding number of sperm—typically, hundreds of millions each day! Sperm production usually continues uninterrupted until death, although the number and quality of sperm decline during later adulthood.

Spermatogenesis

The process of producing mature sperm is called spermatogenesis. Sperm are produced in the seminiferous tubules of the testes and become mature in the epididymis. The entire process takes about 9 to 10 weeks. You can watch an animation of spermatogenesis at this link: http://wps.aw.com/bc_martini_eap_4/40/10469/2680298.cw/content/index.html. If you look inside the seminiferous tubule drawing shown in Figure below, you can see cells in various stages of spermatogenesis. The tubule is lined with spermatogonia, which are diploid, sperm-producing cells. Surrounding the spermatogonia are other cells. Some of these other cells secrete substances to nourish sperm, and some secrete testosterone, which is needed for sperm production.

Seminiferous Tubule. This drawing shows an enlarged cross section of a seminiferous tubule.

Spermatogonia lining the seminiferous tubule undergo mitosis to form primary spermatocytes, which are also diploid. The primary spermatocytes undergo the first meiotic division to form secondary spermatocytes, which are haploid. Spermatocytes make up the next layer of cells inside the seminiferous tubule. Finally, the secondary spermatocytes complete meiosis to form spermatids. Spermatids make up a third layer of cells in the tubule.

Sperm Maturation

After spermatids form, they move into the epididymis to mature into sperm, like the one shown in Figure below. The spermatids grow a tail and lose excess cytoplasm from the head. When a sperm is mature, the tail can rotate like a propeller, so the sperm can propel itself forward. Mitochondria in the connecting piece produce the energy needed for movement. The head of the mature sperm consists mainly of the nucleus, which carries copies of the father’s chromosomes. The part of the head called the acrosome produces enzymes that help the head penetrate an egg.

Mature Sperm Cell. A mature sperm cell has several structures that help it reach and penetrate an egg. These structures include the tail, mitochondria, and acrosome. How does each structure contribute to the sperm’s function?

Ejaculation

Sperm are released from the body during ejaculation. Ejaculation occurs when muscle contractions propel sperm from the epididymes. The sperm are forced through the ducts and out of the body through the urethra. As sperm travel through the ducts, they mix with fluids from the glands to form semen. Hundreds of millions of sperm are released with each ejaculation.

Lesson Summary

  • The male reproductive system consists of structures that produce sperm and secrete testosterone. They include the penis, testes, and epididymes.
  • The male reproductive system forms before birth but does not become capable of reproduction until it matures during puberty. Puberty lasts from about ages 12 to 18 years and is controlled by hormones.
  • Sperm are produced in the testes in the process of spermatogenesis. They mature in the epididymes before being ejaculated from the body through the penis.

Lesson Review Questions

Recall

1. What are the two major roles of the male reproductive system?

2. Name two male reproductive organs and identify their functions.

3. List three physical changes that occur in males during puberty.

4. Outline the process of spermatogenesis. What cells are involved in the process?

5. Where do sperm mature and how do they leave the body?

Apply Concepts

6. Sexual dimorphism refers to differences between males and females of the same species. Applying what you read in this lesson, describe how human sexual dimorphism changes from fertilization to adulthood.

Think Critically

7. Explain how and why boys change so much during puberty.

8. If a man did not have functioning epididymes, predict how his sperm would be affected. How would this influence his ability to reproduce?

Points to Consider

By the time they finish puberty, males have developed the traits of mature adults of their own sex. Adult males differ from adult females in many ways. Many of the differences come about because females and males develop differently during puberty.

  • How do you think females change during puberty?
  • Do you know when females begin puberty? Do you think it’s the same age as males?
  • What hormones do you think control puberty in females?

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