Tài liệu Human Reproduction pdf

The male reproductive system has two main functions: It produces hap-loid male gametes, the sperm cells; and it must be able to deposit the sperm cells it produces inside the female... 1

After you have finished reading this chapter, you should be able to: Describe the structures and functions of the male and female

Where did you come from, Baby dear?

Out of the everywhere into the here.

George Macdonald, “Baby,”

At the Back of the North Wind

Introduction

Everyone loves the news stories about the taxi driver who delivers a baby forthe woman who could not get to the hospital on time At some point each

of us asks ourselves with awe for the first time, “Where did I come from?”

It has been said of the body that it is “fearfully and wonderfully made.”How true this is of the reproductive systems on which the survival of ourspecies depends In this chapter, you will study the systems of the humanbody responsible for our being here: the male and the female reproduc-tive systems

The male reproductive system has two main functions: It produces

hap-loid male gametes, the sperm cells; and it must be able to deposit the

sperm cells it produces inside the female (As described in Chapter 16,

360

Human Reproduction

17

fertilization in all mammals, including humans, occurs internally.) Inaddition, the male reproductive system provides a pathway for theremoval of urine (See Figure 17-1.)

The first function, sperm cell formation, occurs in the two testes The

formation of sperm requires a temperature that is a few degrees coolerthan the temperature of the rest of the body How is this temperaturereduction produced? The testes are suspended within a sac called the

scrotum Because it is not located within the body cavity, the temperature

of the scrotum is a few degrees lower than normal body temperature Thescrotum is an adaptation that has evolved to increase the chances of pro-ducing healthy sperm

Inside the testes are a great many tiny tubes, or tubules In fact, if allthe tubules in a single testis were laid end-to-end, they would reach fromone end of a football field to the other, seven times over! As normal diploidcells move through these tubules, they undergo meiotic cell division The

production of haploid gametes occurs in a process called gametogenesis.

The formation of sperm cells is called spermatogenesis Nowhere else in

the male’s body does meiotic cell division occur In the tubules, the

cells go through two divisions that produce four haploid spermatids from

each diploid cell The spermatids are actually immature sperm and are

pushed from the tubules into the epididymis, a tubule about 6 meters

Seminal vesicle Coccyx Large intestine

LIVING ENVIRONMENT BIOLOGY, 2e/fig 17-1 s/s (rev.10/22/03)

Figure 17-1 The male reproductive system.

long packed into each testis The spermatids remain there for severalweeks, completing their development into mature sperm cells.

Sperm cells are highly specialized cells that are able to move Eachsperm cell must be able to deliver a haploid set of chromosomes from themale to an egg cell in the female reproductive tract The structure of amature sperm cell is well adapted to its function Almost the entire head

of the sperm is the haploid nucleus, the all-important genetic informationthat is delivered to the egg Attached to the head of the sperm is a longtail that propels the sperm along Also present are large numbers of mito-chondria that produce ATP, which yields the energy the sperm use to pro-pel themselves to the egg (See Figure 17-2.)

From the epididymis, sperm move into the vas deferens, the

begin-ning of their pathway out of the body As sperm move through the vasdeferens, a number of glands add fluids Sperm and these added fluids

make up the semen In fact, most of the semen is not sperm Instead, it

consists mostly of fructose, a sugar that provides an additional source ofenergy for the sperm Prostaglandins are also found in the semen.Prostaglandins are chemicals that cause muscle contractions to occur inthe female reproductive system These contractions help move the spermtoward the egg In addition, semen has an alkaline pH This high pH neu-tralizes the acid in the male’s urethra and in the female reproductive sys-tem that might damage the sperm

The male reproductive system is adapted for internal fertilization The

penis is a structure that has evolved to deposit sperm safely within the

female’s body The tube that passes through the penis, the urethra, hastwo functions It is the pathway for sperm out of the body as well as thepathway for urine to leave the body after having been stored in the uri-nary bladder At the time of sexual excitement, a valve closes This valveprevents the bladder from releasing urine At the same time, nerveimpulses from the brain cause blood vessels in the penis to relax Bloodrushes in and fills empty spaces in the spongy tissue in the penis The

Mitochondria

Head

Tail

LIVING ENVIRONMENT BIOLOGY, 2e/fig 17-2 s/s

Figure 17-2 The mature sperm cell is well adapted to deliver the haploid set of chromosomes from the male to the egg cell of the female.

penis enlarges and becomes rigid The result is an erection Strong cular contractions occur in a sequence of events called an orgasm Semen

mus-is forced from the body during ejaculation The semen in a single

ejacu-lation contains up to 300 million sperm Any one of these sperm cellscould fertilize an egg and lead to the birth of a new human

Three important functions are performed by the female reproductive tem First, gametes (eggs) are produced in the ovaries Second, a pathway

sys-is provided for sperm cells to reach an egg Third, the female tive system provides a temporary home for the developing embryo (SeeFigure 17-3.)

reproduc-In females, gametogenesis produces haploid egg cells in the ovaries,

a pair of reproductive organs The production of egg cells, a process called

oogenesis, actually begins in a woman’s body before she is born

Approx-imately 2 million cells in the female fetus have already begun the firstphases of meiosis in the immature ovaries by the time of birth Nothingfurther happens to these cells until the female reaches puberty In an adultmale, sperm production occurs all the time; an average of 30 millionsperm are produced each day In a female, all potential eggs are presentwhen she is born Throughout her reproductive life, a female releases only

a few hundred of these eggs Usually only a single egg matures and isreleased each month During egg formation, meiosis produces four cellsafter two divisions However, only one mature egg is made Along withthis single mature egg, three other smaller structures are formed Because

LIVING ENVIRONMENT BIOLOGY, 2e/fig 17-3 s/s

b Ovary

(microscopic view)

Blood vessel

Ripe follicle Ovum

Germinal epithelium Young

follicles

Figure 17-3 The female reproductive system.

an egg cell needs a relatively large amount of raw materials, nutrientsstored in the cytoplasm are not divided equally between the egg and thethree other structures produced during meiosis Instead, just the large,single egg is packed with the nutrients needed to nourish the embryoright after fertilization.

This development of egg cells occurs within the ovary once a month.One mature egg cell is released from one of the ovaries The ovaries con-

tain follicles A follicle is a hollow bundle of cells with an egg inside The

follicle helps the egg mature As the egg matures, the follicle enlarges andfills with fluid The greatly enlarged follicle moves to the edge of the ovary

and bursts, releasing the egg and the fluid This event is ovulation, the

release of an egg from the ovary (See Figure 17-4.)

The egg cell gets swept into the oviduct (or fallopian tube), a long

tubular structure found next to each ovary If fertilization occurs, thesperm usually joins the egg in the oviduct The egg continues to move

along the oviduct to the uterus, a pear-shaped organ with thick

muscu-lar walls If the egg cell was fertilized, the embryo becomes attached to theinside wall of the uterus and continues to develop If fertilization did not

LIVING ENVIRONMENT BIOLOGY, 2e/fig 17-4 s/s

Primary

follicles

Developing follicles

Mature follicle

Ovary Oocyte

Corpus luteum

Disintegrating corpus luteum

Ovulation

Ovary Uterus

Oviduct

Figure 17-4 Ovulation is the

release of a mature egg from the

ovary into the oviduct.

occur, the egg cell breaks down within 24 hours of ovulation and is passedfrom the body, along with some blood and tissue from the uterine lining.(See Figure 17-5.)

At the lower end of the uterus is the cervix, a narrow opening It is

through the cervix that the sperm traveled on their way to the egg cell

Connecting the cervix to the outside of the body is the vagina The

vagina is made up of muscular tissue It is into the vagina that sperm areejaculated from the penis Also, the vagina is the birth canal, throughwhich the infant passes as it leaves the mother’s body during childbirth.Unlike in males, the reproductive pathway in females is not combinedwith the pathway for excretion The urethra, through which urine passesfrom the urinary bladder, ends at an opening near the vagina

During one’s life, many changes and events occur in the body to makesexual reproduction possible Hormones coordinate these changes Themain endocrine gland in charge of producing these hormones is the ante-rior pituitary gland in the brain The pituitary gland is controlled by the

LIVING ENVIRONMENT BIOLOGY, 2e/fig 17-5 s/s

Progesterone Corpus

luteum stage

Menstruation stage

Day of menstrual cycle

Figure 17-5 Ovulation occurs in the middle of the menstrual cycle.

hypothalamus, a part of the brain This connection suggests that, in someway, thoughts stimulate the release of hormones in the brain that affectthe reproductive system.

Two main hormones, follicle-stimulating hormone (FSH) and

luteinizing hormone (LH), are secreted from the anterior pituitary These

hormones were first named for their functions in females Today, we knowthat these hormones have important functions in males, too

The effects of a hormone depend not on the hormone itself but onthe hormone’s target tissue In males, the target tissues for FSH and LH arethe testes FSH is required for sperm formation to occur LH causes the

testes to produce testosterone, the main male sex hormone The effects

of testosterone include the development of the male sex organs beforeand after birth Without the presence of testosterone, the fetus woulddevelop into a female Around the age of 11, the level of testosterone sud-denly increases in a boy’s body As a result, sperm production begins This

event is the beginning of puberty During puberty, the penis and the

testes begin to mature

Testosterone also affects various other tissues in the male Testosteronecauses pubic and underarm hair growth, causes changes in body propor-tions, increases muscle tissue, and deepens the voice Testosterone mayinfluence the development of acne and the beginning of body odors frombacteria attracted to sweat glands Because none of these characteristicsare directly related to sexual reproduction, they are called secondary sexcharacteristics In males, the level of testosterone in the body remainsmuch the same for about 40 years after puberty After that time, the level

of testosterone gradually begins to decrease

In females, FSH and LH from the anterior pituitary stimulate the

release of the major sex hormones, estrogen and progesterone, from the

ovaries Estrogen and progesterone are produced in the ovaries The onset

of puberty in females occurs somewhat earlier than in males At about age

10, the levels of estrogen and progesterone increase dramatically, causingthe uterus, vagina, and ovaries to mature Secondary sex characteristics,including the growth of pubic and underarm hair, breast development,acne, and body odors, also are influenced by estrogen and progesterone

In addition, a monthly cycle of events, known as the menstrual cycle,

begins Remember, in males, sperm production occurs all the time afterpuberty In females, the menstrual cycle occurs every month after puberty.Part of this cycle includes the release of an egg cell (or egg cells) from theovaries (See Figure 17-6.)

If you take a close-up look at the ovary, you can see that an egg ops in its own follicle The development of the egg is stimulated by

devel-FSH The follicle releases estrogen which, in turn, stimulates a suddenrelease of LH from the anterior pituitary The release of LH happens about

14 days into the menstrual cycle and causes ovulation—the release of the egg—to occur Eggs can be fertilized for up to 10 to 15 hours afterovulation

In addition to ovulation, another critical function occurs during themenstrual cycle The woman’s body must be prepared in case fertilizationoccurs Everything must be ready to nurture the developing embryo Dur-ing the first two weeks of the cycle, estrogen causes the lining of the uterus

to thicken There is also an increase in the amount of blood that flows tothis area

During the second half of the cycle, after ovulation has occurred, thefollicle (now without the egg cell) develops into a yellow-colored body,

the corpus luteum The corpus luteum begins to secrete progesterone,

the pregnancy hormone, which prepares the uterus for an embryo Spermcan survive in the female for up to 72 hours after sexual intercourse.Therefore, sexual intercourse a few days before or a few days after ovula-tion may result in pregnancy If pregnancy occurs, the embryo becomesattached to the inner lining of the uterus The growing tissue then begins

to release more hormones to keep everything in the right condition ever, if fertilization does not occur, the continued preparations in theuterus are unnecessary The body realizes this near the end of the four-week period of the menstrual cycle At this time, the level of LH decreases,the corpus luteum breaks down, and the level of progesterone decreases.Without this hormone, the uterine lining no longer remains intact.Instead, the lining breaks down The built-up tissue along with someblood and the unfertilized egg are released from the body This flow of

How-blood, called menstruation, lasts for about four days Then the levels of

FSH again begin to increase A new follicle releases an egg The cycle tinues (See table on page 368, which describes the menstrual cycle.)

by the release of hormones.

THE FOUR PHASES OF THE MENSTRUAL CYCLE

Corpus Luteum Follicle Phase Ovulation Phase Menstruation

In most women, the menstrual cycle continues for at least 40 yearsfrom puberty Between the ages of 45 and 55, the levels of hormoneschange; the menstrual cycle becomes less regular and eventually stops

This stage, called menopause, marks the point at which a female is no

longer capable of reproducing Menopause is a normal occurrence in allwomen; however, the effects of menopause vary widely from one woman

to another In men, by contrast, sperm production continues throughoutlife, although the number of healthy sperm likely declines with age

Only human females and some other higher primates have a regularmonthly menstrual cycle Humans, however, are able to mate at any time.Females of almost all other mammal species will mate only at specific

times during a year These times are known as estrus or “heat.” It is only

during estrus that egg cells are released for fertilization Estrus occurs inwolves and deer once a year, in dogs usually twice a year, in cows andhorses once a month, and in rats and mice every few days

Ovulation, mating, and fertilization occur on a specific timetable Inmany animals, such as deer or sheep, estrus occurs during the autumn.This mating pattern evolved through natural selection and allows thedevelopment of the embryo to occur during the winter The young are

Check Your Understanding

How is it true that “the effects of a hormone depend not on the mone itself but on the hormone’s target tissue”—specifically in thecase of FSH and LH in both males and females?

hor-If unfertilized, ovum passes through uterus → corpus luteum disintegrates,

so it releases less estrogen and prog- esterone into blood

→ uterine lining

is shed and charged (3–7 days)

dis-Ruptured follicle (corpus luteum) continues to release estrogen and starts

to release terone → uterine lining fully ready to receive fertilized ovum (2–3 days)

proges-Pituitary gland secretes burst of FSH and LH → follicle ruptures → mature ovum is released from ovary into the oviduct (where it can be fertilized)

(3–4 days)

Pituitary gland

secretes FSH and LH

into blood → follicle

grows and matures

within ovary →

estrogen levels rise

and uterine lining

thickens (10–14

days)

born in early spring, when food is more plentiful The newborn then haveplenty of time to grow before the next harsh winter sets in.

Human females are among the few animals that allow mating at timeswhen no eggs have been released In other words, sexual intercourse inhumans can occur even when fertilization is not possible It has been sug-gested that this sexual pattern evolved in the human species in order todevelop strong bonds between males and females Such ties between thesexes are the basis of the human family unit Is there a connection amongbiology, behavior, and the strength of family ties?

a population of almost 1 billion people By the year 2000, the human ulation reached 6 billion If the current rate of growth continues, that num-ber will double within the next 40 years Today, the highest rates ofpopulation growth are in the poorest, least-developed countries Popula-tion growth is a matter of great concern throughout the world and is of crit-ical concern in certain countries As a result, family planning, includingthe use of various birth control methods, is being arranged not only byindividual couples but also by government agencies

pop-In countries such as pop-India, pop-Indonesia, Mexico, and China, the lation growth rate is very high The governments in these countries areconcerned about their ability to feed and care for an ever-increasing pop-ulation China has given the most attention to family planning In 1970,China began an official “one child–one family” policy In each commu-nity in China, government agencies work with couples to reach this goal.The rate of population growth has declined in China, but other socialproblems are developing For example, how will a country take care of itsolder population if there are fewer younger people working to supportthem? Other countries are watching China closely to see if it succeeds inmeeting its population growth goals Indeed, because China has almostone-quarter of Earth’s total human population, the success of family plan-ning in China is an important matter for the entire world (See table onpage 370, which lists the most populous countries.)