Kratz R.F., Siegfried D.R. Biology For Dummies

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Chapter 19: Reproduction 101: Making More Animals

✓ Secondary sex characteristics: These develop as an animal matures,

and they’re far less obvious than primary sex characteristics, which are

the male and female reproductive organs. Secondary sex characteristics

in humans include hair growth and distribution (beards in males), deep-

ening of the voice (in males), increase in muscle mass (males), increase

in amount and distribution of fat (in females), and development of

breasts (in females). Male deer grow antlers, male lions grow manes, and

male peacocks develop a fan of beautiful tail feathers.

Human reproductive cycles

Even though humans are capable of reproducing year-round, reproductive

cycles are still involved. Sperm can fertilize an egg any given day of the week,

but eggs can only be fertilized a few days out of every month. Human sexual

reproduction is therefore controlled by the monthly ovarian cycle (the devel-

opment of the egg in the ovary) and menstrual cycle (the periodic series of

changes associated with menstruation), both of which are controlled by hor-

mones (for more on hormones, see Chapter 18).

As described in the earlier “Oh, oh, oogenesis: Making eggs” section, an

oocyte needs to complete meiosis and mature into an egg before it can be

released by the ovary. Believe it or not, the brain runs this process.

The hypothalamus, which is found at the center of the brain, keeps a

check on how much of the hormones estrogen and progesterone are float-

ing through the bloodstream. When the levels decline, the hypothalamus

secretes a hormone called gonadotropin-releasing hormone (GnRH) that heads

straight for the pituitary gland (which is also found in the center of the brain)

and stimulates part of the pituitary to secrete follicle-stimulating hormone

(FSH) and luteinizing hormone (LH). FSH and LH begin the ovarian cycle

by kick-starting meiosis again and continuing development of the follicles

(another name for the oocytes that are suspended in meiosis) so they can

release an egg. As the follicles grow, they release the hormone estrogen.

When the hypothalamus detects rising levels of estrogen in the bloodstream,

it releases even more GnRH, causing the pituitary gland to release a large

amount of LH about midway through the ovarian cycle. LH stimulates the

release of the egg from the follicle in the ovary — in other words, LH triggers

ovulation. It also triggers the remaining follicle cells to develop into a mass

of cells called the corpus luteum, which secretes estrogen and progesterone

for the rest of the ovarian cycle (about two more weeks). These hormones

prepare the body for a possible pregnancy by spurring the tissues lining the

uterus to develop thicker blood vessels, which brings more nutrients into the

uterus.

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The hypothalamus can detect when the levels of estrogen and progesterone

have reached the point where the lining of the uterus is ready for implanta-

tion. If fertilization

✓ Has occurred: The fertilized egg implants in the lining of the uterus

and an embryo begins developing. That embryo immediately starts to

secrete the hormone human chorionic gonadotropin (hCG). The presence

of hCG ensures that estrogen and progesterone production continue so

the lining of the uterus remains nourished by larger blood vessels. After

the placenta (a blood-filled, nutrient-rich temporary organ) has formed,

the embryo gets its nutrients and blood supply through the umbilical

cord connecting the embryo to the placenta, which is connected to the

mother’s blood supply. Therefore, the production of hCG by the embryo

declines after the placenta is up and running.

Pregnancy tests detect the presence of hCG. Because hCG is produced

solely by fertilized eggs, only women with a fertilized egg in their bodies

should have detectable levels of hCG.

✓ Hasn’t occured: No hCG is produced, and the hypothalamus tells the

pituitary gland to stop producing FSH and LH. The lack of FSH and LH

stops the production of estrogen and progesterone, which causes the

lining of the uterus, sometimes referred to as the endometrium, to stop

receiving all that extra nourishment. The endometrium then starts to

disintegrate and eventually sloughs off and is carried out of the body by

the menstrual flow. The first day of menstrual flow is the first day of the

menstrual cycle (pictured in Figure 19-1). The drop in LH also causes the

corpeus luteum to disintegrate, ending the ovarian cycle.

The ovarian cycle and the menstrual cycle occur simultaneously and are synchro-

nized to each other by hormones, but each cycle consists of different events.

✓ The ovarian cycle includes the development of the follicle, the secretion

of hormones by the follicle, ovulation, and the formation of the corpus

luteum. It occurs in the ovary, takes about 28 days to go from beginning

to end, and is controlled by GnRH, FSH, LH, and estrogen.

✓ The menstrual cycle includes the thickening of the endometrium to

prepare for possible implantation and the shedding of the endometrium

if there’s no implanted embryo. It occurs in the uterus, takes about 28

days, and is controlled by the levels of progesterone and estrogen.

Many more eggs are fertilized in a woman’s lifetime than she may realize.

Not every fertilized egg results in a bouncing baby boy or girl. If the hormone

levels aren’t right from the start, a fertilized egg may never implant or may

implant but not secrete enough hormones to maintain the pregnancy. If an

embryo doesn’t produce a sufficient amount of hCG, the pregnancy fails to

continue, and the embryo aborts itself (a spontaneous abortion is another term

for a miscarriage). An unusually heavy menstrual period that started a few

days late is often really the spontaneous abortion of a fertilized egg that didn’t

work out.

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Chapter 19: Reproduction 101: Making More Animals

Figure 19-1:

The men-

strual cycle.

Follicular

phase

Luteal

phase

Days

Proliferative phase Secretory phase

Menstruation

Immature follicle

Developing follicle

Mature follicle

Ovulation

Full corpus luteum

Shrinking corpus luteum

Progesterone

Estrogen

146 14 21 28

Talk about overstaying your welcome

After a follicle develops and releases an egg, the

empty follicle is called a corpus luteum, which

is Latin for “yellow body.” If the egg becomes

fertilized and implants in the uterus, the corpus

luteum hangs around to help out with the begin-

ning stages of pregnancy. It secretes proges-

terone for a few weeks until the placenta is fully

developed and can secrete progesterone on its

own. (The progesterone helps keep the lining

of the uterus rich with blood and nutrients for

the developing embryo.) Sometimes the corpus

luteum sticks around for a few months, but in

most cases, it eventually shrinks and withers

away during the pregnancy.

Notice how we said “in most cases”? About

10 percent of the time, the corpus luteum

hangs out in the ovary far longer than it should.

Sometimes it remains even if the woman isn’t

pregnant. When that happens, the corpus

luteum can turn into a cyst, which is aptly called

a corpus luteum cyst. Usually, this cyst on the

ovary isn’t a problem, unless it continues to

grow, twist, or rupture. Only then does it need

to be removed surgically.

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How humans mate

The most important thing you should know about mating is that members

of different species can’t successfully reproduce with each other. After all,

the whole point of sexual reproduction is to create a new generation that

contains the genetic information from the previous generations. Interspecies

sexual reproduction doesn’t work because different species contain different

numbers of chromosomes, and those chromosomes contain differing genes.

For instance, humans carry 46 chromosomes in each cell, whereas chimpan-

zees have 48 per cell. If a human and a chimp were to mate, the cell divisions

wouldn’t be equal, and a theoretical offspring (we won’t even go there) prob-

ably wouldn’t be able to survive.

Eggs are actually surrounded by a layer of proteins on top of the plasma mem-

brane that contains receptor molecules made solely for receiving sperm of the

same species. In human eggs, the zone that prevents fertilization by a different

species is called the zona pellucida. Only human sperm can crack the code to

get into the egg.

In the following sections, we stick to how humans make babies. (If your

parents have yet to have this conversation with you, you can let them know

they’re off the hook.)

Human reproductive systems

Before you dive into the details of human sexual reproduction, it helps to

know a little bit about the organ systems involved. As you can see in Figure

19-2, the male reproductive system is made up of the penis, the testes, and

the seminiferous tubules. Sperm is made in these tubules through the pro-

cess of spermatogenesis (which we explain earlier in this chapter).

The female reproductive system consists of the vagina, the uterus, the ova-

ries, and the fallopian tubes (see Figure 19-3). A woman’s egg cells are pro-

duced in her ovaries through the process of oogenesis (which we walk you

through earlier in this chapter).

Sexual intercourse

In order to prepare the body for sexual intercourse (the insertion of the man’s

penis into the woman’s vagina), men and women engage in activities that

increase arousal (responsiveness to sensory stimulation). When a man is

sexually aroused, his penis becomes erect because the erectile tissue within

the penis fills with blood. This erection allows the penis to stiffen so it can

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Chapter 19: Reproduction 101: Making More Animals

remain inside a woman’s vagina during intercourse. In females, the clitoris

is the sexually sensitive organ. It has erectile tissue and a glans tip, just like

a penis does. When a woman becomes aroused, the erectile tissue within

the clitoris (which is located toward the front end of the labia just under the

pubic bone) fills with blood, and the increased pressure causes drops of fluid

to be squeezed out of the tissue. This lubrication prepares the vagina for

sexual intercourse so the erect penis can be inserted into it easily.

When the penis is fully inside the vagina, the tip of the penis is as close as

possible to the woman’s cervix. The cervix is the bottom end of the uterus,

which extends down into the vagina. Sperm must travel through the cervix

to enter the uterus. During sexual intercourse, sperm travel from the man’s

epididymides (tubules in the scrotum that store produced sperm) to his vas

deferens, tubes that carry sperm from the scrotum to the urethra so they can

be ejaculated.

The actions that occur during sexual intercourse serve to bring the man and

woman to the climax of stimulation, which is followed shortly by orgasm.

Figure 19-2:

The male

reproduc-

tive system.

Vertebrae

Rectum

Seminal vesicle

Bulbourethral

(Cowper's) gland

Anus

Vas deferens

Epididymis

Testis

Urinary bladder

Pubic bone

Prostate gland

Urethra

Glans penis

Foreskin

Urethral

orifice

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Figure 19-3:

The female

reproduc-

tive system.

Ureter

Uterine tube

Uterus

Cervix

Urinary bladder

Pubic bone

Clitoris

Labium minora

Labium majora

Vertebrae

Rectum

Vagina

Urethra

Anus

Opening (orifice)

of vagina

Orgasm

Believe it or not, orgasm, the highly pleasurable climax of sexual intercourse,

serves a physiological purpose. As the sexual stimulation of a male intensi-

fies, the sperm move from the vas deferens into the urethra and secretions

from three glands — the seminal vesicles, the prostate gland, and the bul-

bourethral gland — all add their fluids to create semen (seminal fluid). The

semen contains the following “ingredients” that help promote fertilization:

✓ Fructose: This sugar gives the sperm energy to swim upstream.

✓ Prostaglandins: These hormones cause contractions of the uterus that

help propel the sperm upward.

✓ A pH of 7.5: This relatively high pH provides the basic solution in which

sperm can live and helps neutralize the acidic conditions of the vagina,

which would otherwise kill the sperm.

Orgasm occurs at the height of sexual stimulation and is signaled by muscu-

lar contractions and a pleasurable feeling of release. The muscular contrac-

tions cause semen to be expelled from the penis, which is called ejaculation.

The average amount of semen expelled during one ejaculation is less than 1

teaspoon, but it contains more than 400 million sperm.

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Chapter 19: Reproduction 101: Making More Animals

When orgasm occurs in a male, a sphincter muscle closes off the bladder to

prevent urine from entering the urethra. Shutting out urine allows the urethra

to be used solely for ejaculation at that time. (In males, the urethra is shared

by both the urinary tract and the reproductive tract; in females, the urethra is

solely part of the urinary tract.)

In females, the height of sexual stimulation also causes intense muscular con-

tractions and a pleasurable feeling of release. The fluid released inside the vagina

helps create a watery environment that the sperm can swim in. The muscular

contractions of the uterus slightly open the cervix, which allows sperm to get

inside the uterus and also helps “pull” sperm upward toward the fallopian tubes.

Fertilization

After ejaculation, sperm have quite a bit of swimming to do before they find

the egg. They have to travel from wherever they’re deposited in the vagina,

through the muscular cervix, up through the entire uterus, and finally into

the fallopian tubes, which is where fertilization (the joining of sperm and egg)

actually occurs.

Because a human egg lives no longer than 24 hours after ovulation and human

sperm live no longer than 72 hours, intercourse that occurs in the three-day

period prior to ovulation or within the day after ovulation is the only chance

of fertilization during a given month.

If the sperm does find its way to the egg, it must penetrate the egg in order to

supply it with its 23 chromosomes. However, human eggs have several layers

of cells and a thick membrane surrounding them. To get through all that, the

sperm produces enzymes in a structure near its nucleus called the acrosome.

The acrosomal enzymes digest the protective layers of the egg (so the sperm

essentially “eats” its way into the egg). But the sperm isn’t alone in its efforts.

The egg helps the sperm get inside by going through physical and biochemi-

cal changes. After the sperm has successfully joined with the egg, the two

gametes create a cell that contains the full human chromosome count of 46.

How Other Animals Do It

Humans obviously aren’t the only animals that mate and undergo sexual

reproduction. All kinds of animals reproduce sexually. The question is: How

do they do it?

Following is a look at the mating styles of birds, bees, worms, and sea

urchins, as well as explanations of how the species’ zygotes form (a zygote

is the fertilized cell that results when two gametes join together). Note: We

highlight these animals’ mating styles and zygote formation because they

give an overview of the different strategies for sexual reproduction that occur

in the animal kingdom.

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✓ Bees: The process by which bees reproduce is called parthenogenesis,

meaning “virgin production.” (Partheno- is Greek for “virgin” [as in the

Parthenon], and genesis means “production.”) A queen bee receives all

the sperm she’ll ever be impregnated with during her nuptial flight when

she mates with the drones (male bees that never have a full chromo-

some count). The queen never mates with the drones again because she

doesn’t have to — she has stored their sperm cells inside her body, leav-

ing her in total control of when fertilization occurs.

When the queen lays eggs and releases the sperm, the eggs become

fertilized. Those fertilized eggs develop into females, many of which are

worker bees (these bees are born diploid but never produce gametes,

which means they can’t reproduce). The other fertilized eggs develop

into new queens. When the queen lays eggs but withholds sperm and

prevents fertilization, the unfertilized eggs develop into male drones.

The cells in drones’ testes develop into gametes that are received by the

new queen, which technically is one of their sisters. (If you’re confused

by what haploid and diploid mean, see Chapter 6.)

✓ Birds: Birds copulate, and the male bird deposits his sperm inside the

female bird. The egg becomes fertilized and is deposited outside the

female bird’s body to continue developing until it’s time to hatch. The

yolk of an egg (the yellow part) is where the developing embryo resides,

and the albumen (the white part) serves to nourish the embryo through-

out its development.

Just after fertilization, one spot on the yolk of an egg goes through a

series of divisions called cleavage. At the end of the cleavage divisions,

an embryonic disk called a blastoderm is created on one side of the

yolk. The blastoderm is the initial cell tissue that begins to develop into

a baby bird; it separates into an epiblast, which is the top layer, and a

hypoblast, which is the bottom layer. The epiblast cells migrate down

into the hypoblast along a line in the yolk, called a primitive streak, to

create the mesoderm, which goes on to develop the rest of the bird.

✓ Earthworms: Earthworms are hermaphrodites, meaning they have both

female and male reproductive parts (specifically ovaries, testes, semi-

nal vesicles, vas deferens, and seminal receptacles). When earthworms

copulate, they face in opposite directions and put their citella together.

Citella are the external, smooth, nonsegmented parts of an earthworm;

their job is to secrete mucus and help the sperm get from the vas defer-

ens of one worm to the seminal receptacle of the other worm. Cocoons

form and are protected by a mucus sheath created by the citella. The

sperm and eggs are fertilized inside the cocoon, and the zygotes stay

enclosed in it until they hatch in the soil.

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✓ Planarian worms: These freshwater flatworms are fairly unique because

they can reproduce both asexually (by constricting their bodies and lit-

erally splitting in two) and sexually. All planarian worms are hermaphro-

dites. When two of these worms reproduce sexually, they use their male

organs to secrete and exchange sperm with each other. Then they use

their female organs to create zygotes. The zygotes develop into small

planarian worms that later hatch and mature into adults.

✓ Sea urchins: Male and female sea urchins look exactly the same on the

outside — they both have a ring of genital pores at the center of their

bodies. Males discharge sperm through their pores into the water, and

females discharge eggs through their pores into the water. Fertilization

is left to chance, but it’s helped by the fact that sea urchins live in close

contact, their eggs have a sticky coat to which sperm adhere, and ejacu-

lation by any one sea urchin signals the other males to ejaculate too.

Developing New Humans

Sexual reproduction involves the production of gametes and the act of

mating to join the gametes so fertilization can occur. After fertilization

takes place, the term development describes how the fertilized egg becomes

another new organism that possesses a mix of its parents’ DNA.

In the sections that follow, we walk you through the process of development,

from zygote to newborn. Note: We focus on the development of human off-

spring because that’s most relevant to your life. However, many similarities

exist between the development of humans and other animals.

From single cells to blastocyst

After the nucleus of a sperm and the nucleus of an egg fuse, fertilization is

complete, and the new, diploid cell is referred to as a zygote.

The zygote begins traveling down the fallopian tube, heading for the uterus

so it can implant in the uterine lining. As it travels, the zygote undergoes

cleavage, a rapid series of mitotic divisions that result in a multicellular

embryo (see Step 3 in Figure 19-4). The zygote divides into 2 cells, which

divide into 4 cells, which divide into 8 cells, which divide into 16 cells. At this

point, the zygote is a solid ball of cells called a morula.

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Figure 19-4:

Fertilization,

conception,

and early

embryonic

and fetal

develop-

ment of

humans.

Uterus

3. Zygote divides

Fallopian tube

Oocyte

Fimbria

Oocyte is released into fallopian tube

Embryo

(25 days)

Fetus (12 weeks)

Umbilical cord

Placenta

Head

Blastocyst

Morula

8-cell

Ovary

Embroyonic

disk

14 days

after fertilization

4-cell

2. Fertilization

1. Ovulation

2-cell

Mature follicle

Corpus luteum

Heart

Tail Body stalk

(future umbilical cord)

4. Conception

(Implantation)

Cell division continues, but the morula becomes filled with liquid, which

pushes the increasing number of cells out toward the periphery of the

embryo’s membrane, forming a hollow ball of cells called a blastula. In

humans, a group of cells inside the blastula become specialized to form

the embryo, and the blastula becomes a blastocyst. Different layers of cells

become specialized within the blastocyst, taking the first step toward form-

ing specialized tissues:

✓ The flattened cells along the edge of the blastocyst form the trophoblast.

✓ The fluid-filled cavity is called the blastocoel.

✓ The sphere of larger, rounded cells that’s destined to become the

embryo is called the embryoblast.

The trophoblast cells secrete an enzyme that helps degrade the lining of the

uterus. After the blastocyst “eats” its way into the wall of the uterus, it sinks

into the wall and implants itself. Conception occurs when the blastocyst

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