CK-12 Biology I - Honors
Page 122
Changes in Males During Puberty Changes in Reproductive Organs
Testes grow larger Penis grows longer
Other reproductive structures grow Sperm production begins
Other Physical Changes
Pubic hair grows Facial and body hair grow
Bone density increases Long bones grow
Muscle mass and strength increase Bones in face grow
Adam’s apple grows Apocrine sweat glands develop
Shoulders widen Voice deepens
Cells that are targeted by testosterone are those that have testosterone receptors. Receptors are molecules in or on cells that bind to specific hormones. Testosterone receptors are on the nucleus of cells. After binding to testosterone, they enter the nucleus, where they bind to specific DNA sequences and regulate gene transcription.
Some of the changes in Table above involve maturation of the reproductive organs, including the penis. Traits such as adult penis size are called primary sex characteristics. Other changes, such as growth of pubic hair, are not directly related to reproduction. Characteristics of mature males such as pubic hair are called secondary sex characteristics.
Adolescent Growth Spurt
Another obvious change that occurs during puberty is rapid growth in height. This is called the adolescent growth spurt. In males, the rate of growth usually starts to increase relatively early in puberty. At its peak rate, growth in height is typically about 10 centimeters per year. 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.
Timing of Puberty
The ages at which particular changes of puberty occur differ from one person to another. However, the changes generally occur in the same sequence for most males. The sequence in which some of the more obvious changes occur is represented by the following stages:
Stage 3—Facial and underarm hair appear; the voice deepens.
Stage 2—The penis becomes longer; pubic hair appears.
Stage 1—The scrotum and testes grow larger.
In the U.S., the average boy begins Stage 1 of puberty at age 11.5 years. He begins the growth spurt in height by the second year of puberty, develops the ability to produce sperm a few years later, and continues to grow in height until age 17.5 years. Overall, he spends about six years going through puberty.
Depending on the genes he inherits, his diet, and many other factors, a boy may go through puberty a couple of years earlier or later than the average. This is usually normal, and early and late maturers generally have nothing to worry about. Nonetheless, a boy who is concerned that he is not developing normally should talk with his doctor.
Male Reproductive Organs
Figure 25.1
shows the male reproductive system. The main organs are the penis, testes, and epididymis. Several ducts and glands are also parts of the male reproductive system.
Penis, Testes, and Epididymis
The penis is an external genital organ with a long shaft and enlarged tip. It contains tissues that can fill with blood and cause an erection, which is stiffening and enlarging of the penis. A duct called the urethra passes through the penis. Sperm pass out of the body through the urethra. (During urination, the urethra carries urine from the bladder.)
The testes (singular, testis) are located in the scrotum, which is a sac of skin between the upper thighs. By hanging away from the body, the testes keep sperm at a temperature lower than normal body temperature. The lower temperature is needed for sperm production.
Each testis contains more than 90 meters of tiny, tightly-packed tubes called seminiferous tubules. They are the functional units of the testes, where sperm are produced and testosterone is secreted. A cross-section of a seminiferous tubule is shown in Figure below. The tubule is lined with spermatogonia and Sertoli cells. Spermatogonia are sperm-producing cells that you will read more about below. Sertoli cells help protect and nourish developing sperm.
Figure 25.2
This drawing shows a cross-section of a seminiferous tubule. Spermatogonia line the inside of the tubule, interrupted here and there by Sertoli cells. Spermatocytes, which are produced by spermatogonia, form the next layer of cells. Spermatids, which are produced by spermatocytes, form a third layer of cells.
In between the seminiferous tubules in the testes are interstitial cells, also called Cells of Leydig. These cells secrete testosterone. A high concentration of testosterone is necessary for sperm production. Testosterone is also needed throughout a man’s life to maintain his secondary sex characteristics.
The seminiferous tubules join together to form the epididymis. The epididymis is a coiled tube about 6 meters long lying atop the testes inside the scrotum (Figure above). Its functions are to help sperm mature and to 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. These are also shown in Figure above.
Ducts include the vas deferens and ejaculatory ducts. They transport sperm from the epididymis to the urethra in the penis.
Glands include the seminal vesicles, prostate gland, and bulbourethral glands. They secrete substances that become part of semen.
Semen is the fluid that is ejaculated from the urethra. Semen contains secretions from the glands as well as sperm. The secretions control pH and provide the sperm with nutrients for energy.
Production and Delivery of Sperm
A sexually mature male typically produces several hundred million sperm per day. Sperm production usually continues uninterrupted until death, although the number and quality of sperm decline during later adulthood.
Spermatogenesis
Spermatogenesis is the process of producing mature sperm. Sperm are haploid cells, meaning they have half the number of chromosomes as other cells of the body, which are diploid cells. Sperm must be haploid in order for normal sexual reproduction to occur. During reproduction, a sperm unites with another cell, called an egg. This is called fertilization. Unless both sperm and egg are haploid, the resulting offspring will not have the diploid number of chromosomes (see chapter titled Cell Division and Reproduction).
Sperm are produced in the seminiferous tubules of the testes and finish maturing in the epididymis. The entire process takes about 9 to 10 weeks. As shown in Table below, the production of sperm occurs in several steps, each involving a different type of cell and process.
Spermatogenesis begins when a spermatogonium with the diploid number of chromosomes undergoes mitosis to form primary spermatocytes, also with the diploid number. It proceeds as a primary spermatocyte undergoes the first cell division of meiosis to form secondary spermatocytes with the haploid number of chromosomes. A secondary spermatocyte undergoes the second meiotic cell division to form haploid spermatids. Spermatids mature into sperm, which are also haploid.
Spermatogenesis and Cell Division Type of Cell Number of Chromosomes Process
Spermatogonium Diploid Mitosis
Primary Spermatocyte Diploid Meiosis 1
Secondary Spermatocyte Haploid Meiosis 2
Spermatid Haploid Maturation
Spermatozoon (sperm) Haploid Fertilization
Spermatogonia, which line the seminiferous tubules in the testes, are diploid cells. They begin the process of spermatogenesis when they divide by mitosis to produce cells called primary spermatocytes, which are also diploid cells. Some spermatogonia divide just to produce copies of themselves. This ensures a constant supply of spermatogonia for future sperm production.
Primary spermatocytes go through the first cell division of meiosis to produce secondary spermatocytes. These are haploid cells. Secondary spermatocytes then quickly complete the meiotic division to become spermatids, which are also haploid cells.
Spermatids slowly mature into sperm, like the one shown in Figure below. Among other change
s, they lose excess cytoplasm from the head and grow a tail. The tail is a flagellum that lets them move by rotating like a propeller. The acrosome that covers part of the head produces digestive enzymes that help the head penetrate an egg. The mitochondria in the connecting piece produce energy that the sperm needs to “swim” through the female reproductive tract to reach an egg. However, sperm do not develop the ability to move until they complete their maturation in the epididymis. It takes sperm four to six weeks to travel through the epididymis and become fully mature. After they mature, they remain in the epididymis until they leave the body.
Figure 25.3
A mature sperm cell has several structures that help it reach and penetrate an egg. These structures include the acrosome, mitochondria, and tail. The nucleus, which makes up most of the head, carries copies of the fathers chromosomes.
Ejaculation
In order for reproduction to take place, sperm must leave the reproductive system. Sperm are released from the body during ejaculation. About 200 to 500 million sperm are released with each ejaculation. Ejaculation occurs when rhythmic muscular movements of the vas deferens propel sperm from the epididymis. The sperm are forced through the vas deferens and ejaculatory ducts and out of the body through the urethra. As sperm travel through the ducts, they mix with fluids from the seminal vesicles, prostate gland, and bulbourethral glands to form semen.
Lesson Summary
The male reproductive system forms before birth but does not become capable of reproduction until it matures during puberty.
The male reproductive system includes organs and other structures that produce sperm and deliver sperm and secrete testosterone.
Sperm are produced in the testes in the process of spermatogenesis and leave the body through the penis during ejaculation.
Review Questions
What are the two major functions of the male reproductive system?
List four physical changes that occur in males during puberty.
Name two male reproductive organs and identify their functions.
Describe how sperm leave the body.
Sexual dimorphism refers to differences between males and females of the same species. Based on what you read in this lesson, how does human sexual dimorphism change from birth to adulthood?
If a man did not have an epididymis, how would this affect his ability to produce mature sperm?
Make a flow chart showing the steps of spermatogenesis. Indicate the cells and process involved at each step.
What are the roles of testosterone in the male reproductive system, from the embryo to old age?
Further Reading / Supplemental Links
Stanley, Deborah, Sexual Health Information for Teens. Omnigraphics, 2003.
Walker, Pam and Wood, Elaine, Understanding the Human Body: The Reproductive System. Lucent Books, 2002.
http://en.wikibooks.org/wiki/Human_Physiology/The_male_reproductive_system
http://www.kidshealth.org/parent/general/body_basics/male_reproductive.html
http://www.kidshealth.org/teen/sexual_health/changing_body/male_repro.html
http://www.medicalook.com/human_anatomy/systems/Male_Reproductive_System.html
http://en.wikipedia.org
Vocabulary
acrosome
Covers part of the sperm cell head; produces digestive enzymes that help the sperm head penetrate an egg.
adolescent growth spurt
Rapid growth in height seen during puberty.
Cells of Leydig
Cells that secrete testosterone, located in between the seminiferous tubules in the testes; are also known as interstitial cells.
epididymis
A coiled tube about 6 meters long lying atop the testes inside the scrotum; helps sperm mature and stores mature sperm until they leave the body.
fertilization
The uniting of a haploid sperm with a haploid egg.
luteinizing hormone
The main pituitary hormone responsible for puberty in males; stimulates the testes to produce testosterone.
male reproductive system
System with two major functions: producing sperm and secreting testosterone.
primary sex characteristics
Traits of reproductive organs seen in mature adults that are directly related to reproduction.
puberty
The period during which humans become sexually mature.
secondary sex characteristics
Physical traits of mature adults which are not directly related to reproduction.
semen
The fluid that is ejaculated from the urethra; contains sperm and secretions from the seminal vesicles, prostate gland, and bulbourethral glands.
seminiferous tubules
The functional units of the testes, where sperm are produced and testosterone is secreted.
sertoli cells
Help protect and nourish developing sperm, located in the seminiferous tubules.
sperm
Male gametes, or sex cells, which are necessary for reproduction; haploid.
spermatogonia
Sperm-producing cells, located in the seminiferous tubules; diploid.
spermatogenesis
The process of producing mature sperm.
testosterone
A masculinizing hormone and the chief sex hormone in males.
Points to Consider
By the time they finish puberty, males have developed the traits of mature adults of their own sex. They differ from mature females in many ways. How do these differences between sexually mature males and females come about?
What causes female to develop differently during puberty?
When do girls begin puberty, what changes do they go through, and what hormones control the changes?
Lesson 25.2: Female Reproductive System
Lesson Objectives
Explain how the female reproductive system develops before birth and matures during puberty.
Identify structures of the female reproductive system and their functions.
Describe how eggs are produced and how they are released from the ovaries.
Sequence the events of the menstrual cycle, and explain how hormones control the cycle.
Introduction
The female reproductive system is a collection of organs and other structures located primarily in the pelvic region. Most of the structures are inside the body. The female reproductive system has several functions:
producing eggs, which are female gametes
secreting female sex hormones
receiving sperm during sexual intercourse
supporting the development of a fetus
delivering a baby during birth
breastfeeding a baby after birth
During puberty, a girl develops into a sexually mature woman, capable of producing eggs and reproducing.
Sexual Development in Females
As you read in Lesson 25.1, the main differences between boys and girls at birth are their reproductive organs. Unlike males, females are not influenced by the male sex hormone testosterone during embryonic and fetal development. This is because they lack a Y-chromosome. As a result, females do not develop male reproductive organs.
Development Before Birth
Unless an embryo is stimulated by testosterone, the reproductive organs develop into female organs, such as the ovaries and uterus. By the third month of fetal development, most of the internal female organs have formed. Immature ova, or eggs, also form in the ovary before birth. Whereas a male produces sperm throughout his lifetime (after puberty), a female produces all the eggs she will ever make before birth.
Like baby boys, baby girls are born with all their reproductive organs present but immature and unable to function. Female reproductive organs grow very little during childhood. They begin to grow rapidly and to mature during puberty.
Changes of Puberty
From Lesson 25.1, you know that puberty is the period during which humans
become sexually mature. Puberty in girls differs from puberty in boys in several ways, including when it begins, how long it lasts, and the hormones involved. Girls begin puberty a year or two earlier than boys, and they complete puberty in about four years instead of six. In females, the major sex hormone is estrogen rather than testosterone.
Puberty in girls starts when the hypothalamus in the brain stimulates the pituitary gland to secrete hormones that target the ovaries. The pituitary hormones are luteinizing hormone, or LH, and follicle-stimulating hormone, or FSH. These hormones stimulate the ovary to produce estrogen.
Estrogen has many functions that you will read more about below. During puberty, estrogen promotes growth and other physical changes in females. For example, estrogen stimulates growth of the breasts and uterus. It also stimulates development of bones and contributes to the adolescent growth spurt in height. These and several other changes in females during puberty are listed in Table below: