biology in focus - chapter 36

CAMPBELL BIOLOGY IN FOCUS

© 2014 Pearson Education, Inc.

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge

36Reproduction and Development


Overview: Pairing Up for Sexual Reproduction

Each sea slug produces sperm and eggs and thus acts as both mother and father to the next generation

Animal reproduction takes many forms A population outlives its members only by

reproduction, the generation of new individuals from existing ones


Figure 36.1


Concept 36.1: Both asexual and sexual reproduction occur in the animal kingdom

Sexual reproduction is the creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote

Asexual reproduction is the creation of offspring without the fusion of egg and sperm


Mechanisms of Asexual Reproduction

Many invertebrates reproduce asexually by budding, in which new individuals arise from outgrowths of existing ones

Also common among invertebrates is fission, the separation of a parent organism into two individuals of approximately equal size

Video: Hydra Budding


Figure 36.2


Fragmentation is breaking of the body into pieces, some or all of which develop into adults

Fragmentation must be accompanied by regeneration, regrowth of lost body parts

Parthenogenesis is the development of a new individual from an unfertilized egg


Sexual Reproduction: An Evolutionary Enigma

Sexual females have half as many daughters as asexual females; this is the “twofold cost” of sexual reproduction

Despite this, almost all eukaryotic species reproduce sexually


Figure 36.3-1

Asexual reproduction

FemaleFemale

Male

Sexual reproduction

Generation 1

Generation 2


Figure 36.3-2


FemaleFemale

Male

Sexual reproduction

Generation 1

Generation 2

Generation 3


Figure 36.3-3


FemaleFemale

Male

Sexual reproduction

Generation 1

Generation 2

Generation 3

Generation 4


Sexual reproduction results in genetic recombination The resulting increased variation among offspring

may enhance the reproductive success of parents in changing environments


Reproductive Cycles

Most animals exhibit reproductive cycles related to changing seasons

Reproductive cycles are controlled by hormones and environmental cues

Because seasonal temperature is often an important cue in reproduction, climate change can decrease reproductive success


Figure 36.4


In some species of whiptail lizards, the members of breeding pairs alternate roles

Reproduction is exclusively asexual, and there are no males

However, courtship and mating behavior still take place, with one female of each pair mimicking male mating behavior

The two alternate roles two or three times during the breeding season


Figure 36.5

(a) A. uniparens females

Male-like

Ovulation

Female Male-like

Female

Time

(b) The sexual behavior of A. uniparens iscorrelated with the cycle of ovulation.

Beh

avio

rH

orm

one

leve

lO

vary

size

OvulationProgesterone

Estradiol


Figure 36.5a

(a) A. uniparens females


Figure 36.5b

Male-like

Ovulation

Female Male-like

Female

Time

(b) The sexual behavior of A. uniparens iscorrelated with the cycle of ovulation.

Beh

avio

rH

orm

one

leve

lO

vary

size

OvulationProgesterone

Estradiol


For many animals, finding a partner for sexual reproduction may be challenging

One solution is hermaphroditism, in which each individual has male and female reproductive systems

Any two hermaphrodites can mate, and some hermaphrodites can self-fertilize

Variation in Patterns of Sexual Reproduction


Individuals of some species undergo sex reversals For example, in a coral reef fish, the bluehead

wrasse, a lone male defends a group of females If the male dies, the largest female in the group

transforms into a male Within a few weeks, this individual can begin to

produce sperm instead of eggs


External and Internal Fertilization

Sexual reproduction requires fertilization, the union of sperm and eggs

In external fertilization, eggs shed by the female are fertilized by sperm in the external environment

In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract


A moist habitat is almost always required for external fertilization to prevent gametes from drying out and to allow sperm to swim to eggs

Many aquatic invertebrates simply shed gametes into the surrounding water

In this case, timing of release of gametes is crucial to ensure that sperm and egg encounter one another

If external fertilization is not synchronous across a population, courtship behaviors might lead to fertilization


Figure 36.6


However fertilization occurs, the mating animals may use pheromones, chemicals released by one organism that can influence physiology and behavior of another individual of the same species


Ensuring the Survival of Offspring

Internal fertilization is typically associated with production of fewer gametes but the survival of a higher fraction of zygotes

Internal fertilization is also often associated with mechanisms to provide protection of embryos and parental care of young


The embryos of some terrestrial animals develop in eggs with calcium- and protein-containing shells and several internal membranes

Some other animals retain the embryo, which develops inside the female

In many animals, parental care helps ensure survival of offspring


Figure 36.7


Concept 36.2: Reproductive organs produce and transport gametes

Sexual reproduction in animals relies on sets of cells that are precursors for eggs and sperm


Variation in Reproductive Systems

Many (but not all) animals have gonads, organs that produce gametes

Some simple systems do not have gonads, but gametes form from undifferentiated tissue

More elaborate systems include sets of accessory tubes and glands that carry, nourish, and protect gametes and sometimes developing embryos


Most insects have separate sexes with complex reproductive systems

In many insects, the female has a spermatheca in which sperm is stored during copulation


A cloaca is a common opening between the external environment and the digestive, excretory, and reproductive systems

A cloaca is common in nonmammalian vertebrates; mammals usually have a separate opening to the digestive tract


Human Male Reproductive Anatomy

The male’s external reproductive organs are the scrotum and penis

The internal organs are gonads that produce sperm and reproductive hormones, accessory glands that secrete products essential to sperm movement, and ducts that carry sperm and glandular secretions

Animation: Male Reproductive Anatomy


Figure 36.8

Seminalvesicle

(Rectum)

Ejaculatoryduct

Bulbourethralgland

Erectiletissue

Vas deferens

Prostate gland

Epididymis

Scrotum

Vas deferens

Testis Prepuce

PenisGlans

Urethra

(Pubic bone)

(Urinary duct)

(Urinary bladder)


Testes

The male gonads, or testes, produce sperm in highly coiled tubes called seminiferous tubules

Production of normal sperm cannot occur at the body temperatures of most mammals

The scrotum, a fold of the body wall, maintains testis temperature at about 2oC below the core body temperature


Ducts

From the seminiferous tubules of a testis, sperm pass into the coiled duct of the epididymis

During ejaculation, sperm are propelled through the muscular vas deferens and the ejaculatory duct and then exit the penis through the urethra


Accessory Glands

Semen is composed of sperm plus secretions from three sets of accessory glands

The two seminal vesicles contribute about 60% of the total volume of semen

The prostate gland secretes its products directly into the urethra through several small ducts

The bulbourethral glands secrete a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra


Penis

The human penis is composed of three cylinders of spongy erectile tissue

During sexual arousal, erectile tissue fills with blood from the arteries, causing an erection

The head of the penis, or glans, has a thinner skin covering than the shaft

The prepuce, or foreskin, is a fold of skin covering the glans; it is removed when a male is circumcised


Human Female Reproductive Anatomy

The female external reproductive structures include the clitoris and two sets of labia

The internal organs are a pair of gonads and a system of ducts and chambers that carry gametes and house the embryo and fetus

Animation: Female Reproductive Anatomy


Figure 36.9

Oviduct

(Rectum)Cervix

Major vestibulargland

Vagina

Vaginal opening

OvaryUterus(Urinarybladder)

(Pubic bone)

(Urethra)

BodyClitorisGlans

PrepuceLabia minoraLabia majora


Ovaries

The female gonads, a pair of ovaries, lie in the abdominal cavity

Each ovary contains many follicles, which consist of a partially developed egg, called an oocyte, surrounded by support cells


Oviducts and Uterus

Upon ovulation, a mature egg cell is released It travels from the ovary to the uterus via an oviduct Cilia in the oviduct convey the egg to the uterus,

also called the womb The uterus lining, the endometrium, has many

blood vessels The neck of the uterus is the cervix, which opens

into the vagina


Vagina and Vulva

The vagina is a muscular but elastic chamber that is the repository for sperm during copulation and serves as the birth canal

The vagina opens to the outside at the vulva, which consists of the labia majora, labia minora, hymen, and clitoris


Mammary Glands

The mammary glands are not part of the reproductive system but are important to mammalian reproduction

Within the glands, small sacs of epithelial tissue secrete milk


Gametogenesis

Gametogenesis is the production of gametes There is a close relationship between the structure

and function of the gonads

Video: Flagella in Sperm

Video: Sperm Flagellum


Figure 36.10aEpididymis

Seminiferous tubule

TestisCross section ofseminiferous tubule

Sertoli cellnucleus

Plasmamembrane

Lumen ofseminiferoustubule

MitochondriaNucleus

Acrosome

Midpiece

Tail

HeadNeck

Primordial germ cell in embryo

Spermatogonialstem cell

Mitotic divisions

Spermatogonium

Mitotic divisions

Mitotic divisions

Primary spermatocyte

Secondary spermatocyte

Spermatids(two stages)

Earlyspermatid Differentiation

(Sertoli cellsprovide nutrients)

Meiosis I

Meiosis II

n n

2n

2n

2n

n n n n

n n n nSperm cell


Figure 36.10aa

EpididymisSeminiferous tubule

TestisCross section ofseminiferous tubule

Sertoli cellnucleus

Spermatogonium



Spermatids(two stages)

Lumen ofseminiferous tubule

Sperm cell


Figure 36.10abPrimordial germ cell in embryo

Spermatogonialstem cell

Mitotic divisions

SpermatogoniumMitotic divisions

Mitotic divisions



Earlyspermatid Differentiation

(Sertoli cellsprovide nutrients)

Meiosis I

Meiosis II

n n

2n

2n

2n

n n n n

n n n nSperm cell


Figure 36.10ac

Plasmamembrane

MitochondriaNucleus

Acrosome

Midpiece

Tail

HeadNeck


Figure 36.10b

Primordial germ cell

Completion of meiosis Iand onset of meiosis II

Mitotic divisions

In embryo

Mitotic divisions

Mature follicle

Primary oocyte(present at birth), arrestedin prophase of meiosis I

Ovulation, sperm entry

Firstpolarbody

Degeneratingcorpus luteum

Rupturedfollicle

Ovulatedsecondary oocyte

Fertilized egg

Ovary

Secondary oocyte,arrested at metaphase of meiosis II

Oogonium2n

n

Secondpolarbody

Completion of meiosis II

2n

n

n

n

Primary oocytewithin follicle

Growingfollicle

Corpus luteum


Figure 36.10ba

Mature follicle


Rupturedfollicle

Ovulatedsecondary oocyte

Ovary

Primary oocytewithin follicle

Growingfollicle

Corpus luteum


Figure 36.10bb

Primordial germ cellMitotic divisions

In embryo

Mitotic divisions

Primary oocyte(present at birth), arrestedin prophase of meiosis I

Oogonium2n

2n


Figure 36.10bc

Completion of meiosis Iand onset of meiosis II

Ovulation, sperm entry

Firstpolarbody

Fertilized egg

Secondary oocyte,arrested at metaphase of meiosis II

n

Secondpolarbody

Completion of meiosis II

n

n

n


Spermatogenesis, the development of sperm, is continuous and prolific (millions of sperm are produced per day); each sperm takes about 7 weeks to develop

Oogenesis, the development of a mature egg, is a prolonged process

Immature eggs form in the female embryo but do not complete their development until years or decades later


Spermatogenesis differs from oogenesis in three ways All four products of meiosis develop into sperm, while

only one of the four becomes an egg Spermatogenesis occurs throughout adolescence and

adulthood Sperm are produced continuously without the

prolonged interruptions in oogenesis


Concept 36.3: The interplay of tropic and sex hormones regulates reproduction in mammals

Human reproduction is coordinated by hormones from the hypothalamus, anterior pituitary, and gonads

Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus and directs the release of FSH (follicle-stimulating hormone) and LH (luteinizing hormone) from the anterior pituitary


FSH and LH regulate processes in the gonads and the production of sex hormones

The major androgen is testosterone and major estrogens are estradiol and progesterone

Both males and females produce androgens and estrogens but differ in their blood concentrations of particular hormones

For example, males have testosterone levels about 10 times higher than females


Sex hormones regulate gamete production both directly and indirectly

They serve many additional functions including sexual behavior and the development of primary and secondary sex characteristics

Androgens are responsible for male vocalizations and courtship displays


Figure 36.11


FSH and LH act on different cells in the testes to direct spermatogenesis

Sertoli cells, found in the seminiferous tubules, respond to FSH by nourishing developing sperm

Leydig cells, connective tissue between the tubules, respond to LH by secreting testosterone and other androgens, which promote spermatogenesis

Hormonal Control of the Male Reproductive System

Animation: Male Hormones


Figure 36.12

Hypothalamus

Anterior pituitary

GnRH

Sertoli cells Leydig cells

Inhibin

Testis

Spermatogenesis Testosterone

FSH LH

Neg

ativ

e fe

edba

ck

Neg

ativ

e fe

edba

ck


Two negative feedback mechanisms control sex hormone production in males

Testosterone regulates the production of GnRH, FSH, and LH through negative feedback mechanisms

Sertoli cells secrete the hormone inhibin, which reduces FSH secretion from the anterior pituitary


Hormonal Control of Female Reproductive Cycles

Females produce eggs in cycles Prior to ovulation, the endometrium thickens with

blood vessels in preparation for embryo implantation If an embryo does not implant in the endometrium,

the endometrium is shed in a process called menstruation


Hormones closely link the two cycles of female reproduction Changes in the uterus define the menstrual cycle

(also called the uterine cycle) Changes in the ovaries define the ovarian cycle

Animation: Ovulation

Animation: Post-Ovulation


Figure 36.13

Anterior pituitary

Hypothalamus

GnRH

FSH LH

Control by hypothalamus(a)

FSH and LH stimulatefollicle to grow

Inhibited by low levels ofestradiol

FSH

LH

Pituitary gonadotropinsin blood

(b)

Growing follicle

Follicular phase OvulationEstradiol secretedby growing follicle inincreasing amounts

Ovarian cycle(c)

Stimulated by high levels of estradiol

Inhibited by combination ofestradiol and progesterone

LH surge triggersovulation

Maturingfollicle

Corpusluteum


Luteal phaseProgesterone andestradiol secretedby corpus luteum

Ovarian hormonesin blood

(d)

Uterine (menstrual) cycle(e)

Estradiol

Estradiol levelvery low

Peak causesLH surge(see )

Progesterone

Progesterone and estra-diol promote thickeningof endometrium

Endometrium

Menstrual flow phase Proliferative phase Secretory phase

0 Day

s

1

2

3

4

8

6

7

5 6

910

105 14 15 20 25 28


Figure 36.13a

Anterior pituitary

Hypothalamus

GnRH

FSH LH

Control by hypothalamus(a)


Inhibited by low levels ofestradiol

FSH

LH


(b)

Stimulated by high levels of estradiol

Inhibited by combination ofestradiol and progesterone


1

2

3

6

Day

s

0 105 14 15 20 25 28


4

Figure 36.13b


FSH

LH


(b)

Growing follicle


Ovarian cycle(c)


Maturingfollicle

Corpusluteum



3

8

6

7

Day

s

0 105 14 15 20 25 28


5

910

Figure 36.13c

Growing follicle


Ovarian cycle(c)

Maturingfollicle

Corpusluteum




(d)

Estradiol



Progesterone


4

87

6

Day

s

0 105 14 15 20 25 28


Figure 36.13d

Uterine (menstrual) cycle(e)

Endometrium

Menstrual flow phase Proliferative phase Secretory phase

Day

s

5

910


(d)

Estradiol



Progesterone


6

0 105 14 15 20 25 28


The Ovarian Cycle

The sequential release of GnRH then FSH and LH stimulates follicle growth

Follicle and oocyte growth and an increase in the hormone estradiol characterize the follicular phase of the ovarian cycle

Estradiol causes GnRH, FSH, and LH levels to rise through positive feedback

The final result is the maturation of the follicle


At the end of the follicular phase, the follicle releases the secondary oocyte

The follicular tissue left behind transforms into the corpus luteum in response to LH; this is the luteal phase

The corpus luteum disintegrates, and ovarian steroid hormones decrease by negative feedback mechanisms


The Uterine (Menstrual) Cycle

Hormones coordinate the uterine cycle with the ovarian cycle

Thickening of the endometrium during the proliferative phase coordinates with the follicular phase

Secretion of nutrients during the secretory phase coordinates with the luteal phase

Shedding of the endometrium during the menstrual flow phase coordinates with the growth of new ovarian follicles


Menopause

After about 500 cycles, human females undergo menopause, the cessation of ovulation and menstruation

Menopause is very unusual among animals Menopause might have evolved to allow a mother to

provide better care for her children and grandchildren


Menstrual Versus Estrous Cycles

Menstrual cycles are characteristic only of humans and some other primates The endometrium is shed from the uterus in a

bleeding called menstruation Sexual receptivity is not limited to a time frame


Estrous cycles are characteristic of most mammals The endometrium is reabsorbed by the uterus Sexual receptivity is limited to a “heat” period The length and frequency of estrus cycles vary from

species to species


Two reactions predominate in both sexes Vasocongestion, the filling of tissue with blood Myotonia, increased muscle tension

The sexual response cycle has four phases: excitement, plateau, orgasm, and resolution

Excitement prepares the penis and vagina for coitus (sexual intercourse)

Human Sexual Response


Direct stimulation of genitalia maintains the plateau phase and prepares the vagina for receipt of sperm

Orgasm is characterized by rhythmic contractions of reproductive structures In males, semen is first released into the urethra and

then ejaculated from the urethra In females, the uterus and outer vagina contract


During the resolution phase, organs return to their normal state and muscles relax


Concept 36.4: Fertilization, cleavage, and gastrulation initiate embryonic development

Across animal species, embryonic development involves common stages occurring in a set order

First is fertilization, which forms a zygote During the cleavage stage, a series of mitoses

divide the zygote into a many-celled embryo The resulting blastula then undergoes

rearrangements into a three-layered embryo called a gastrula


Figure 36.14

EMBRYONIC DEVELOPMENT

FERT

ILIZ

ATIO

N

GASTRULATION

CLEAVAGE

ORG

ANO-

GENESIS

Metamorphosis

Zygote

Blastula

Gastrula

Tail-budembryo

Larvalstages

Adultfrog

Sperm

Egg


Fertilization

Molecules and events at the egg surface play a crucial role in each step of fertilization Sperm penetrate the protective layer around the egg Receptors on the egg surface bind to molecules on

the sperm surface Changes at the egg surface prevent polyspermy, the

entry of multiple sperm nuclei into the egg


Video: Cortical Granule

Video: Calcium Release of Egg

Video: Calcium Wave of Egg


Figure 35.15-1

Basal body(centriole)

Spermhead

Sperm-bindingreceptors

AcrosomeJelly coat

Egg plasma membraneVitelline layer


Figure 35.15-2


Spermhead


AcrosomeJelly coat

Egg plasma membrane

Hydrolytic enzymes

Vitelline layer


Figure 35.15-3

Spermnucleus


Spermhead

Acrosomalprocess

Actinfilament


AcrosomeJelly coat

Egg plasma membrane

Hydrolytic enzymes

Vitelline layer


Figure 35.15-4

Sperm plasmamembrane

Spermnucleus


Spermhead

Acrosomalprocess

Actinfilament


AcrosomeJelly coat

Fusedplasmamembranes

Egg plasma membrane

Hydrolytic enzymes

Vitelline layer


Figure 35.15-5

Sperm plasmamembrane

Spermnucleus


Spermhead

Acrosomalprocess

Actinfilament


AcrosomeJelly coat Fertilization

envelope

Perivitellinespace

CorticalgranuleFused

plasmamembranes

Egg plasma membrane

Hydrolytic enzymes

Vitelline layer


The events of fertilization also initiate metabolic reactions that trigger the onset of embryonic development, thus “activating” the egg

Activation leads to events such as increased protein synthesis that precede the formation of a diploid nucleus

Sperm entry triggers a release of Ca2+, which activates the egg and triggers the cortical reaction, the slow block to polyspermy


Figure 36.16

First cell division

Onset of DNA synthesis

Fusion of egg and sperm nuclei complete

Increased protein synthesis

Formation of fertilization envelope complete

Binding of sperm to egg

Cortical reaction (slow block to polyspermy)Increased intracellular calcium level

Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)

Min

utes

Seco

nds

1

23468

1020304050 1

2345

10

2030406090


Cleavage and Gastrulation

Fertilization is followed by cleavage, a period of rapid cell division without growth

Cleavage partitions the cytoplasm of one large cell into many smaller cells

The blastula is a ball of cells with a fluid-filled cavity called a blastocoel

The blastula is produced after about five to seven cleavage divisions

Video: Cleavage of Egg


Figure 36.17

(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula

50 m


Figure 36.17a

(a) Fertilized egg

50 m


Figure 36.17b

50 m

(b) Four-cell stage


Figure 36.17c

50 m

(c) Early blastula


Figure 36.17d

50 m

(d) Later blastula


After cleavage, the rate of cell division slows The remaining stages of embryonic development

are responsible for morphogenesis, the cellular and tissue-based processes by which the animal body takes shape


During gastrulation, a set of cells at or near the surface of the blastula moves to an interior location, cell layers are established, and a primitive digestive tube forms

The hollow blastula is reorganized into a two- or three-layered embryo called a gastrula


The cell layers produced by gastrulation are called germ layers

The ectoderm forms the outer layer and the endoderm the inner layer

In vertebrates and other animals with bilateral symmetry, a third germ layer, the mesoderm, forms between the endoderm and ectoderm


Figure 36.18

Mesenchyme cells

Vegetal plate

Blastocoel

Blastocoel

Blastocoel

Blastopore

Blastopore

Mesenchyme cells

Vegetalpole

Animalpole

Filopodia

Archenteron

ArchenteronEctodermMouth

50 m

Digestive tube (endoderm)Anus (from blastopore)

Future ectoderm

Future endodermFuture mesoderm

Key

Mesenchyme(mesoderm formsfuture skeleton)


Figure 36.18a-1

MesenchymecellsVegetal plate

Blastocoel

Vegetalpole

Animalpole

Future ectoderm



Figure 36.18a-2


Blastocoel

Vegetalpole

Animalpole

Filopodia

Archenteron

Future ectoderm



Figure 36.18a-3


Blastocoel

Blastocoel

Blastopore

Vegetalpole

Animalpole

Filopodia

Archenteron

Archenteron

Future ectoderm



Figure 36.18a-4


Blastocoel

Blastocoel

Blastopore

Vegetalpole

Animalpole

Filopodia

Archenteron

ArchenteronEctodermMouth

Digestive tube (endoderm)Anus (from blastopore)

Future ectoderm


Mesenchyme(mesoderm formsfuture skeleton)


Figure 36.18b

Blastocoel

Blastopore

Mesenchymecells

Filopodia

Archenteron

50 m


Cell movements and interactions that form the germ layers vary among species

One distinction is whether the mouth develops at the first opening that forms in the embryo (protostomes) or the second (deuterostomes)

Sea urchins and other echinoderms are deuterostomes, as are chordates


Each germ layer contributes to a distinct set of structures in the adult animal

Some organs and many organ systems derive from more than one germ layer


Figure 36.19

ECTODERM (outer layer of embryo)

ENDODERM (inner layer of embryo)

MESODERM (middle layer of embryo)

• Epithelial lining of digestive tract and associated organs (liver, pancreas)

• Epithelial lining of respiratory, excretory, and reproductive tracts and ducts

• Thymus, thyroid, and parathyroid glands

• Skeletal and muscular systems• Circulatory and lymphatic systems• Excretory and reproductive systems (except germ cells)• Dermis of skin• Adrenal cortex

• Epidermis of skin and its derivatives (including sweat glands, hair follicles)

• Nervous and sensory systems• Pituitary gland, adrenal medulla• Jaws and teeth• Germ cells


Human Conception, Embryonic Development, and Birth

Conception, fertilization of an egg by a sperm, occurs in the oviduct

The resulting zygote begins cleavage about 24 hours after fertilization and produces a blastocyst after 4 more days

A few days later, the embryo implants into the endometrium of the uterus


The condition of carrying one or more embryos in the uterus is called gestation, or pregnancy

Human pregnancy averages 38 weeks from fertilization

Gestation varies among mammals, from 21 days in many rodents to more than 600 days in elephants

Human gestation is divided into three trimesters of about three months each


Figure 36.20

Cleavage

Fertilization

Ovulation

Ovary

Uterus

Endometrium

Implantation

Cleavagecontinues.

TrophoblastBlastocyst

Endometrium Innercellmass

(a) From ovulation to implantation

(b) Implantation of blastocyst

1

2

34

5

Cavity


During the first trimester, the embryo secretes hormones that signal its presence and regulate the mother’s reproductive system

Human chorionic gonadotropin (hCG) acts to maintain secretion of progesterone and estrogens by the corpus luteum through the first few months of pregnancy


Occasionally an embryo splits during the first month of development, resulting in identical or monozygotic twins

Fraternal, or dizygotic, twins arise when two eggs are fertilized and implant as two genetically distinct embryos

Some embryos cannot complete development due to chromosomal or other abnormalities

These spontaneously abort, often before a woman is aware of her pregnancy


During its first 2 to 4 weeks, the embryo obtains nutrients directly from the endometrium

Meanwhile, the outer layer of the blastocyst, called the trophoblast, mingles with the endometrium and eventually forms the placenta

Materials diffuse between maternal and embryonic circulation to supply the embryo with nutrients, immune protection, and metabolic waste disposal


The first trimester is the main period of organogenesis, development of the body organs

All the major structures are present by 8 weeks, and the embryo is called a fetus

Video: Ultrasound of Fetus


Figure 36.21

(a) 5 weeks (c) 20 weeks(b) 14 weeks


Figure 36.21a

(a) 5 weeks


Figure 36.21b

(b) 14 weeks


Figure 36.21c

(c) 20 weeks


During the second trimester the fetus grows and is very active

The mother may feel fetal movements as early as one month into the second trimester


During the third trimester, the fetus grows and fills the space within the embryonic membranes


Childbirth begins with labor, a series of strong, rhythmic contractions that push the fetus and placenta out of the body

Once labor begins, local regulators, prostaglandins and hormones, induce and regulate further contractions of the uterus


Lactation, the production of milk, is an aspect of maternal care unique to mammals

Suckling stimulates the release of prolactin, which in turn stimulates mammary glands to produce milk, and the secretion of oxytocin, which triggers milk release from the glands


Contraception

Contraception, the deliberate prevention of pregnancy, can be achieved in a number of ways

Contraceptive methods fall into three categories Preventing development or release of eggs and sperm Keeping sperm and egg apart Preventing implantation of an embryo


A health-care provider should be consulted for complete information on the choice and risks of contraception methods


Figure 36.22Male Female

Method Event MethodEvent

Vasectomy

AbstinenceCondomCoitusinterruptus(very highfailure rate)

Tubal ligation

AbstinenceFemale condom

Combinationbirth controlpill (or injection,patch, orvaginal ring)

Spermicides;diaphragm;progestin alone(as minipillor injection)

Morning-afterpill; intrauterinedevice (IUD)

Production ofsperm

Production ofprimary oocytes

Sperm transportdown male

duct system

Oocytedevelopmentand ovulation

Spermdepositedin vagina

Capture of the oocyte by the

oviduct

Spermmovement

through femalereproductive

tract

Transport of oocyte in

oviduct

Meeting of sperm and oocyte in oviduct

Implantation of blastocyst in endometrium

Union of sperm and egg


Figure 36.22a

Male FemaleMethod Event MethodEvent

Vasectomy

AbstinenceCondomCoitusinterruptus(very highfailure rate)

Tubal ligation

AbstinenceFemale condom

Combinationbirth controlpill (or injection,patch, orvaginal ring)

Spermicides;diaphragm;progestin alone(as minipillor injection)

Production ofsperm

Production ofprimary oocytes

Sperm transportdown male

duct system

Oocytedevelopmentand ovulation

Spermdepositedin vagina

Capture of the oocyte by the

oviduct


Figure 36.22b

Morning-afterpill; intrauterinedevice (IUD)

Spermmovement

through femalereproductive

tract

Transport of oocyte in

oviduct

Meeting of sperm and oocyte in oviduct

Implantation of blastocyst in endometrium

Union of sperm and egg

Male FemaleMethod Event MethodEvent


The rhythm method, or natural family planning, is temporary abstinence when conception is most likely; it has a pregnancy rate of 10–20%

Coitus interruptus, the withdrawal of the penis before ejaculation, is unreliable

Barrier methods block fertilization with a pregnancy rate of less than 10% A condom fits over the penis A diaphragm is inserted into the vagina before

intercourse


Intrauterine devices (IUDs) are inserted into the uterus and interfere with fertilization and implantation; the pregnancy rate is less than 1%

Female birth control pills are hormonal contraceptives with a pregnancy rate of less than 1%


Infertility and In Vitro Fertilization

Infertility, the inability to conceive offspring, affects about one in ten couples in the United States and worldwide

The causes are varied and equally likely to affect men and women

Sexually transmitted diseases (STDs) are the most significant preventable causes of infertility


Some forms of infertility are treatable Hormone therapy can sometimes increase sperm or

egg production In vitro fertilization (IVF) mixes oocytes with sperm

in culture dishes and returns the embryo to the uterus at the eight-cell stage

Sperm may be injected directly into an oocyte as well


Figure 36.UN01


Figure 36.UN02

Fertilizedegg

Polar body

Secondaryoocyte

Secondaryspermatocytes

Primaryoocyte

Primaryspermatocyte

Polarbody

Sperm

Spermatids

Spermatogenesis Oogenesis Human gametogenesis

2n 2n

n

n

n

n

n n

n n n n

n n n n

biology in focus - chapter 36

Science