CHAPTER 31 REPRODUCTION AND DEVELOPMENT. ASEXUAL AND SEXUAL REPRODUCTION Asexual reproduction...

CHAPTER 31

REPRODUCTION AND DEVELOPMENT

ASEXUAL AND SEXUAL REPRODUCTION

• Asexual reproduction produces offspring that are genetically identical to the parent.• Mitosis • Examples are

• fission in which one organism splits in two.

• budding occurs where part of the parent’s body becomes separated from the rest and differentiates into a new individual.


• Sexual reproduction occurs when a new individual is formed by the union of two cells.• These cells are called gametes, which are

formed by meiosis in the sex organs, or gonads.

• The union of a sperm and an egg produces a fertilized egg, or zygote.• In vertebrates, the zygote will develop by

mitosis into a multicellular organism.


• Parthenogenesis is a special type of reproduction in which offspring are produced from unfertilized eggs.• For example, in honeybees, a queen mates only

once and stores sperm.• If no sperm are released, the eggs develop

into drones, which are male.• If sperm are released, the eggs develop into

other queens or workers, which are female.


• Hermaphroditism is a reproductive strategy in which one individual has both testes and sperm and so can produce both sperm and eggs.• Most hermaphroditic organisms

require another individual to reproduce.

• Some hermaphroditic organisms change their sex through sequential hermaphroditism.• Protogyny is a change from

female to male.• Protandry is a change from

male to female.


• In mammals, sex is determined early in development.• If the embryo is XY, it is a male

and will carry a gene on the Y chromosome whose product converts gonads into testes.• The sex-determining gene is SRY

(sex-determining region of the Y chromosome).

• If the embryo is XX, it is a female and the gonads will become ovaries.

Y X

X X

(Follicles do notdevelop untilthird trimester)

Ovaries

Testes

Zygote

XY

Sperm

Ovum Ovum

Sperm

Zygote

Leydig cells

Seminiferoustubules

Develop in earlyembryo

No SRYSRYIndifferent

gonads

XX

EVOLUTION OF VERTEBRATE SEXUAL REPRODUCTION

• Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized the land.• In external fertilization, gametes are

released into the water.• Fish, frogs

• In internal fertilization, male gametes are introduced into the female reproductive tract.• Some fish, salamanders, reptiles, birds,

mammals


• Vertebrates with internal fertilization may be classified into different groupings.• Oviparity – the eggs are

fertilized internally but complete development outside the mother’s body.

• Ovoviviparity – the fertilized eggs complete development inside the mother and depend on yolk exclusively for nourishment before being born alive.

• Viviparity – the young develop within the mother and obtain nourishment from their mother’s blood before being born alive.


• Most fish and amphibians reproduce by means of external fertilization.• In most bony fish, the eggs contain only enough

yolk to sustain the developing embryo for a short time.• Fish mature rapidly, but there is high

mortality.


• Most cartilaginous fish use internal fertilization


• Amphibians reproduce in the water and have aquatic larval stages before moving to the land.• Development is longer than in fish, but the

eggs provide only slightly more yolk.

Adultsmating

Gilled larva

Red eft

Eggs


• Most reptiles are oviparous• The eggs are surrounded by a leathery shell

that is deposited as the egg passes through the oviduct.

• Most reptiles abandon their eggs after laying them.


• All birds are oviparous.• Most birds incubate their eggs

after they are laid.

Developing ovum

Ovum

Ovary

Mesentery

Rectum

Uterus(shell gland)

(a)

(b)

Opening to cloaca

Oviduct

b: © Wolfgang Kaehler/Corbis


• Some mammals are seasonal breeders, while others have multiple short reproductive cycles.• The females generally undergo the reproductive

cycle, whereas the males are more constant in their reproductive activity.

• Most females are “in heat,” or sexually receptive to males, only around the time of ovulation.• This period of sexual receptivity is called estrus.

• The reproductive cycle in females is called the estrous cycle.


• The most primitive mammals, the monotremes, are oviparous.• The duck-billed platypus and the echidna

incubate their eggs in a nest and, because they lack nipples, the young lick milk off their mother’s skin.

• All other mammals are viviparous.


• Viviparous mammals are divided into two subcategories based on how they nourish their young.• Marsupials give birth to

fetuses that are incompletely developed.• They complete their

development in a pouch of their mother’s skin, where they obtain nourishment from mammary glands.


• Placental mammals retain their young within the mother’s uterus.• The fetus is nourished by a placenta

which allows the fetus to obtain nutrients by diffusion from the mother’s blood.

• The young are born in a more developed state

• Drink milk after birth.

MALES

• Sperm is the male gamete and is highly specialized for its role as a carrier of genetic information.• Sperm do not successfully

complete their development at 37°C.• The sperm-producing

organs, called testes, are found in a sac called the scrotum, which maintains the testes at a temperature 3° cooler than the rest of the body.

TailMitochondrion

Nucleus

AcrosomeHeadCentriole

FlagellumBody

(top): © David M. Phillips/Photo Researchers

MALES

• The testis is composed of several hundred compartments packed with large numbers of tightly coiled tubes called seminiferous tubules.• These are the sites for spermatogenesis

• The process of spermatogenesis begins in germinal cells toward the outside of the tubule.

• As the cells undergo meiosis, they move toward the lumen of the tubule.

MALES

• After a sperm cell is manufactured within the testis, it is delivered to a long, coiled tube called the epididymis.• The sperm cell is not motile when it first arrives

at the epididymis and must remain there for at least 18 hours before motility develops.

• From the epididymis, the sperm is delivered to another long tube, the vas deferens.

• When sperm is ejaculated, it travels from the vas deferens to the urethra.

FEMALES

• In females, eggs develop from cells called oocytes.• These are located in

the outer layer of female gonads called ovaries.

• During each reproductive cycle, one or a few of these oocytes are initiated to continue their development.• This process is called ovulation.

FEMALES

• The fallopian tubes (also known as uterine tubes, or oviducts) transport eggs from the ovaries to the uterus.• The uterus is lined with a stratified epithelial

membrane called the endometrium.• The surface of the endometrium is shed

during menstruation.

FEMALES

• To fertilize an egg successfully, the sperm must make its way far up the fallopian tube.• The egg is moved down the fallopian tube by

contractions of smooth muscle lining the tube.• Sperm swim against the current created by

these contractions.• An egg loses its capacity to develop within 24

hours of ovulation.• Sperm can remain viable for up to 6 days.

FEMALES

• When the first sperm cell penetrates the oocyte’s protective layers the oocyte blocks the entry of other sperm.• The oocyte completes meiosis II,

forming a haploid ovum.

• When the female haploid nucleus joins with the male haploid nucleus, the egg is fertilized and becomes a zygote.• A fertilized egg attaches itself to

the endometrial lining to continue development.

2

1

3

Oviducts

Uterus

Cervix

Ovary

Vagina

(a)

(b)

Sperm

First polar body

Egg

Egg membrane

Granulosa cells

Zona pellucidaSperm

Nucleus

AcrosomeExtracellular space

Sperm nucleusfertilizing egg

HORMONES COORDINATE THE REPRODUCTIVE CYCLE

• The female reproductive cycle, called a menstrual cycle, is composed of two distinct phases.• The follicular phase in which an egg reaches

maturation and is ovulated.• The luteal phase where the body prepares for

pregnancy.• These phases are coordinated by a family of

hormones whose production is controlled by the hypothalamus.


• The follicular phase corresponds to days 0 to 14 of the reproductive cycle.• The anterior pituitary starts

the phase by secreting FSH and LH in small amounts.

• Initially, several follicles (a follicle is an oocyte and its surrounding tissue) are stimulated to grow.

• The follicle begins to secrete the female hormone estrogen.


• The low but rising levels of estrogen in the bloodstream act as negative feedback.• The output of FSH and LH are reduced.• This ensures that only one oocyte matures at a

time.

• A rise in estrogen signals the end of the follicular phase.


• The luteal phase occurs during days 14 through 28 of the reproductive cycle.• The higher levels of estrogen

begin to have a positive-feedback effect on FSH and LH secretion.

• The surge in LH causes ovulation and the wall of the follicle bursts.• The follicle is released into one

of the fallopian tubes.• LH directs the repair of the

ruptured follicle so that it fills in to become the corpus luteum.


• The corpus luteum secretes progesterone which also inhibits FSH and LH secretion.• Progesterone completes the body’s preparation of the

uterus for fertilization, including the thickening of the endometrium.

• If fertilization does not occur, production of progesterone slows and eventually stops.

• The decreasing levels of progesterone cause the thickened layer of the endometrium to be sloughed off.• This process, menstruation, usually occurs about

midway between successive ovulations.


• If fertilization does occur high in the fallopian tube, the zygote undergoes a series of cell divisions, called cleavage, while traveling toward the uterus.• At the blastocyst stage, it implants in the lining of the uterus.• The embryo secretes human chorionic gonadotropin (hCG).

• This maintains the corpus luteum and prevents menstruation.

1

2

3

4

Egg

Fallopian tube

Fertilization

Developing follicles

OvulationOvary

Blastocyst

Implantation

Cleavage

Uterus

Morula

Corpusluteum

EMBRYONIC DEVELOPMENT

• During cleavage, the mammalian zygote divides rapidly into a larger and larger number of smaller and smaller cells.• The resulting tightly packed mass of about 36

cells is called the morula.• Each individual cell in the morula is called a

blastomere.• Further divisions of the blastomeres in the

morula lead to a hollow ball of 500–2,000 cells• This is called the blastocyst.


• The blastocyst contains a fluid-filled cavity called the blastocoel.• Within the blastocyst is an inner cell mass.

concentrated at one pole that goes on to form the developing embryo.

• The outer sphere of cells, called the trophoblast, releases hCG.


• The implantation of the blastocyst in the uterine lining initiates the formation of membranes.• Amnion encloses

the developing embryo.• Chorion forms from the trophoblast and interacts with uterine tissue

to form the placenta.• The placenta connects the developing embryo to the blood supply of the mother.


• Gastrulation occurs 10 to 11 days after fertilization and involves certain groups of cells moving inward from the surface of the inner cell mass.

• The lower cell layer of the inner cell mass becomes the endoderm and the upper layer becomes the ectoderm.

• The moving cells differentiate into mesoderm• They grow inward along a furrow called the

primitive streak.


• Neurulation is the stage of development that begins in the third week of embryonic development.• The three primary

germ layers begin their development into the tissues and organs of the body.


• The notochord forms first from mesoderm.• The neural tube forms from ectoderm.• Somites form along the side of the notochord

and will become muscles, vertebrae, and connective tissue.

• Between two layers of mesoderm, the coelom forms.

FETAL DEVELOPMENT

• Organogenesis, the process of forming body organs, begins in the fourth week of human pregnancy.

(a)

Tail

Lower limbbud

Forebrain

Future lens

Pharyngealarches

Somites

Neural tubeforming

Upper limbbud

(b)

Developingheart

Handplate

Futureexternalear

Head(growth accelerated)

Footplate

Umbilicalcord

Retinalpigment

FETAL DEVELOPMENT

• During the second month of pregnancy, great changes in morphology occur as the embryo takes shape.• It begins to look distinctly human

• Development is essentially complete at the end of the third month.• Only the lungs and brain need to develop more.• The developing human is now referred to as a

fetus instead of an embryo.

FETAL DEVELOPMENT

• The second trimester is a time of growth.• By the end of the

sixth month, the fetus still cannot survive outside the uterus without special medical intervention.

(c) (d)

Development is essentially complete.

The developing human is now referredto as a fetus.Facial expressions and primitivereflexes are carried out.

Arms and legs beginto move.

All of the major body organshave been established.

Bones actively enlarge.

Mother can feel baby kicking.

Following a period of rapid growth,the fetus is born.

Neurological growthcontinues after birth.

FETAL DEVELOPMENT

• The third trimester is a period of rapid growth.• All of the growth is fueled

by the mother’s bloodstream, passing into the fetal blood supply within the placenta.

• The placenta contains blood vessels that extend from the umbilical cord into tissues that line the uterus.

Amnion

Chorion

Placenta

Maternal vein

Maternal artery

Uterinewall

Umbilical cord

FETAL DEVELOPMENT

• At approximately 40 weeks from the last menstrual cycle, the process of birth begins, including labor and delivery.• Oxytocin and

prostaglandins work by positive feedback to stimulate uterine contractions

Uterus

Placenta(detaching)

Umbilicalcord

(b)

(c)

(d)

(a)

Placenta

UmbilicalcordUterus

Vagina

Cervix

FETAL DEVELOPMENT

• The mammary glands are prepared to nourish the baby after birth.• Prolactin stimulates milk production usually by

the third day after delivery.• Oxytocin is released following the infant

suckling at the breast and initiates milk release.

• Growth continues rapidly after birth.• Different organs grow at different rates.

• Neurological growth of the baby continues long after birth.

CHAPTER 31 REPRODUCTION AND DEVELOPMENT. ASEXUAL AND SEXUAL REPRODUCTION Asexual reproduction...

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