. system The Female Reproductive

If you would like to learn more about puberty in females, go to

• www.science.nelson.com CA:.

LEARNING TIP------, As you study Figure 1, ask yourself, "What does this show?" Relate the information in Figure 1 to the information in the text.

Figure 1 The ovary contains follicles, which contain eggs. Once the eggs mature, they are released from the ovary.

Unit A Reproduction

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h Productive process. As . metes to t e re h . al Both males and females contribute ga . c ales have anot er v1t

mammalian iem il" d you learned in Chapter 3, ~owever, t nurture the fert 1ze egg . . t nal they mus . role. Because fertilization 1s m er ' . h next generat10n.

. . Th ·ve birth tot e from conception to birth. ey gi

Secondary Sexual CharacteriStics . h wth es 10 and 12 wit a gro

In females, puberty usually begins between ag y glands or breasts. If a 1 t of mammar ' spurt that includes the deve opmen d milk to nourish the baby.

. . h h ry g'lands pro uce woman gives birt , t e mamma h f underarm and pubic 0 h h d · b ty include• the growt 0

t er c anges urmg pu er. . . ) A ell the primary sexual hair and a widening of the hips (pelvic girdle · s w ' organs mature at this time.

Egg Production and Development . . Th h 1 1 11 d th·e ovum is well designed for its functions. It e uman egg, a so ca e , . . is much larger than th~ sperm because it contains many nutnents that will be used in cell division once the-egg is fertilized. The egg also has an outer ' barrier that prevents more than one sperm from entering.

Unlike the male, the female.has all her sex cells at birth. She may be born with las many as two million eggs, but most will degenerate, leaving approximately 400 000 at puberty. Once sexual maturity is reached during puberty, a single egg matures and is released each month.

The primary female reproductive organ is the ovary, where egg cells mature and are releaied (Figure· 1). Females have two ovaries located in the lower portion of the abdominal cavity. The ovaries have many follicles, each containing a single immature egg. The follicles also contain cells that nourish and protect the developing egg. The ovaries produce the female sex hormones, estrogen and progesterone.

H. Corpus luteum degenerates if the egg is not fertilized .

A. Nutrient-producing cells

surround the egg cell. B. Nutrient fluid collects.

G. Corpus luteum matures.

C. Mature follicle pushes against the wall of the ovary.

i f~~~oE-.~ D. Follicle ruptures.

E. Egg cell is released .

F. Corpus luteum develops from follicle . }

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The Menstrual Cycle The men,strual cycle is the female reproductive cycle. It begins at the onset of puberty. The menstrual cycle lasts approximately 28 days, although it can vary in length from 20 to 40 days in different women. The length of the cycle can change throughout a woman's reproductive life. In each cyde, usually a single egg matures and is released. The menstrual cycle keeps repeating until 111enopause, when it stops. Menopause most often 'occurs 1b' t omen between· 4o and 50 years of age.

Several follicles develop during each reproductive cycle, but usually only a single follicle and egg reach maturity in either ovary. Occasionally, both ovaries release an egg, which can result in twins if both eggs are fertilized. As the,follicle matures,,nutrient-producing cells develop around the egg cell (Figure 1·).

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The cells. will divide and form a nutrient 1rich cavity. Once the egg matures, it will burst through the. wall of the ovary. This release-from the ovary is called ovulation. The empty follicle is called the corpus luteum. The corpus , luteum matures and produces the hormones estrogen and progesterone.

The Path of the Egg After ovulation, the egg is propelled by finger-like projecti9ns that surround the opening to the oviduct. The oviduct ( also called the fallopian tube) is a tube that transports the egg to the uterus (Figure 20,, Peristalsis, which is wave-like muscular contractions, and tiny cilia move the egg through the oviduct. Figure 2 shows the two ovaries and the two oviducts. The uterus, also called the womb, is the organ that will receive the e,mbryo if the egg is fertilized. The lining of the uterus is called the endometri1im. It is _rich in nutrients, blood vessels, and mucus, and provides the nourishmen~ for the embryo. The cervix is the muscul;u opening between ~e uterus aJJ.d the vagina. The vagina receives the male penis and sperm du~ing sexua:l:ilintercourse, and it is the birth canal through which a baby comes out. The opening to the vagina is direcdy b€hind the opening to the urethra. Unlike m~l,es, whose u~ethra transpo:i;ts both urine and semen, the female urethra only transports urine.

oviduct · ' ~(fallopian tube)

egg

- ----ovary

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LEARNING TIP~. --'-f-"'-,

Make connections to your prior ~· knowledge; Ask yourself; "What oq I already know about the menstrual cycle?" Consider the informati(lln you have read, observed, and/or experienced.

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vagina ____ __. \

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Figure 2 The female reproductive system \

4.2

Ii ~ 1 'l The Female -Reproductive System

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Figure 3 This graph summarizes the eve~ts of the menstrual cycle in the ovary and the uterus.

LEARNING TIP---- -_Tables help you identify specific

- information quickly. As you study Tab/~ 1, look at the headings. The headings will help you focus on what is important in the table.

Unit A Reproduction

• fertilized, one of two events will h Depending on whether the egg is apPen:

menstruation or pregnancy.

Menstruation f • h ·d t then the egg continues to the I no sperm are present mt e ov1 uc , . Uteru

and is discharged out of the vagina. Since no zy~ote has Implanted in the s uterus, the endometrium stops developing and IS shed. Menstruation is the shedding of the endometrium. This marks the first sta~e of _each menstrual cycle (Figure 3 ), the flow phase. Because the endometnum IS so rich in blood vessels, menstruation looks like blood is being released. Since no zygote has implanted, a new follicle starts to develop in the second phase the follicular phase. Once the follicle has developed, the third phase, ' ovulation, occurs. In ovulation, the egg leaves the ovary and travels toward the uterus. Once the egg is released, the final stage, the luteal phase, begins and the empty follicle develops into the corpus luteum. The corpus luteum' releases the hormone progesterone, which stimulates the endometrium to develop in preparation for the fertilized egg. During pregnancy, progester

ill l . w a so prevent any other eggs from maturing. Table I summarizes the events in the menstrual cycle.

follicle development ovulation corpus luteum

0 thickness of the endometrium ~-~,r---1c .,·._,.:_ !\ ' ,p

t.l: • ·ff ., 0 ,- p

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flow phase

0 5 days

follicular phase

14 ovulation

Table 1 A f h summary o t e female menstrual Of I ce

Phase

,,.

luteal phase

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Description of events Hormones produced flow phase Endometrium is shed

(menstruation). none

follicular phase Follicles in ovaries develop. Estrogen is produced Endometrium re-forms.

ovulation Egg is released from ovary. by follicles.

-luteal phase Corpus luteum forms.

Endometrium thickens. Estrogen and progeste~ne are produced by the corpus luteum.

28

Day (approximate)

1-5

6-13

14

15-28

pregnancy If the egg is in the oviduct and sp occur- One sperm penetrates the erm are present, then fertilization can · the oviduct. The two haplo · d outle~ membrane of the egg while it is in 1 nuc e1 fuse to pr d h. .

23 P,.,,rs of chromosomes Th O uce t e zygote with au · e zygote b · d" "d'

. urney through the oviduct Ab egrns lVl mg as ~t continues its JOf cells now called an embr . outhone week after, f f rtilization, the mass o , yo, reac es the uterus wh · __ .111 • 1 · the

endometrium. Pregnanc b . ' .ere it ww imp ant mto Y egrns when the · £ il' d . til the baby is born ap • . egg is ert 1ze and contmues un ' proximately nrne months later Emb · development happens during pregnancy E b . d · ryomc_ · f h · m ryoruc evelopment 1s an orderly senes o c anges that an emb d "\l ill l ryo un ergoes to become a fully formed baby. 1ou w earn more about p . regnancy and embryonic development in the next section.

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TRY THIS: Determining the Number of Eggs Released in a Lifetime

Skills Focus: predicting, recording (!i} ' --• Materials: calculator

1. A female has approximately 400 000 eggs available at puberty.

A. What event would reduce the number of eggs released from puberty to menopause? '

B. Approximately how many eggs have been released in a woman aged 40, who has given birth to four children?

2. Predict the number of eggs that you think a human female will release in her lifetime. Record your prediction.

c. Why do you think there is such a difference between the . number of available eggs and the number of eggs released? 1:

3. If one egg is released approximately every 28 days, I calculate how many eggs would be released in one year. 1.

Record your answer. '\ 4. Using the approximate age of 12 for when puberty1 begins, lf l'..J lJ ·I

and the approximate age of 50 for when menopause occurs, c,.ir\ calculate the number of eggs released. Record your answer. '

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Female Accessory Glands Another conspicuous feature that characterizes fe~ale hiarumals is the "-..

presence of mammary glands. Humans an_d other primates have two glands, while other mammals may have multiple glands. t h~ number of riiammary glands is related to the number of offspring that are1 produced in one pregnancy. During pregnancy, high levels of estrogen_and progesterone stimulate the mammary glands to develop more milk duct~~ Shortly after birth, progesterone decreases and another hormone,-called prolactin, causes milk to be produced to feed the baby. Each breast contains many milk-producing lobes, each with a duct th~t leads to the nipple. In most mammals milk is the sole source of nutriti0n for some time after birth. , \

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LEARNING TIP.,_ __ \\"----, Did you notice that Section 4.2 includes many new terms? Check the Glossary at the back of tnis textbook to find the meanings of terms that you are unsure of.

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\ l l 4.2 The lremale Reproductive System

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-<\ 'fy structures ' ' al , 9. Jidentt I. List the secondary sexual characteristics of fem e)e. L @1~) What is the function of mammary glan~J] / A 7} (b) Why are mammary glands considered y "£· " / 7 __ I' secondary sexual characteristics in humans? ---ti(

f Identify structures A, B, and C in Figure 4. . .. _ • @ 'tliiiiii~~ l7 ..--.,, - e> ~of -,,,,) A B

f;gure4

~ - Th d . . . . h fi al 4.; e en ometnum 1s important m t e em e _,. menstrual cycle. Describe its structure, location,

and function . .

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Figure 5

10 How does the corpus luteum develop? r::) What name is given to the process that releases \ V an egg from the ovary?

' 12. List the four phases of the menstrual cycle, and des

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cribe what is happening during each phase. 13. List, in order, the parts of the female reproductive

system that are involved in the sequence of events . from ovulation to fertilization to the birth of an ~9 Where does the endometrium fit into the

' menstrual cycle? •"'6y),t;~me the two female sex hormones, and :J.'J~scribe their roles.

} offspring. Indicate where fertilization takes place . _ ~ -( ~ On average, how many eggs does a woman

release in her reproductive life?

1 1,-- Nam~ the hormone ~at causes the production of milk shortly after birth. (a) What event happens during menstruation?

~J (b) ·What event or events will stop menstruation? / (c) What event happens approximately two

weeks before menstruation?

116 Unit A Reproduction

15. (a) When describing the menstrual cycle, the convention is to begin with the flow phase. Suggest why this is so.

(b) What is another name for the flow phase? 16. If the ovaries of a woman are removed, can she

still give birth to a baby? Support your answer.

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