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Male Reproductive Development

The male reproductive system begins developing at puberty in response to hormone signals from the brain.

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Male Reproductive Development

What changes happen during puberty?

A lot changes during this time. A boy has to start shaving, his voice deepens, he gets taller, as well as a few other changes.

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. In addition to the changes shown below, during puberty male facial hair begins to grow, the shoulders broaden, and the male voice deepens. 

Male changes during puberty

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.

   

 

Summary

  • 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.

Review

  1. What happens to a developing baby that lacks testosterone?
  2. List three physical changes that occur in males during puberty.
  3. Explain how and why boys change so much during puberty.

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    Vocabulary

    adolescent growth spurt

    Period of rapid growth that occurs during puberty.

    luteinizing hormone (LH)

    Pituitary gland hormone that stimulates the testes to secrete testosterone and the ovaries to secrete estrogen.

    puberty

    Period during which humans become sexually mature.

    testosterone

    Male sex hormone secreted by the testes.

    X chromosome

    One of two sex chromosomes; males have one X chromosome, females have two X chromosomes.

    Y chromosome

    One of two sex chromosomes; only males have a Y chromosome.

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