chapter 27 reproduction and embryonic development

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

Chapter 27

Reproduction and Embryonic Development


Baby Bonanza

• The increased use of fertility drugs

– Has caused an increase in the number of multiple births in the United States


• Fertility drugs

– Are sometimes too effective


ASEXUAL AND SEXUAL REPRODUCTION

27.1 Sexual and asexual reproduction are both common among animals

• In asexual reproduction

– One parent produces genetically identical offspring


• Asexual reproduction

– Enables an individual to produce many offspring rapidly


• Reproduction in some animals is accomplished through external fertilization

– In which the parents release their gametes into the environment where fertilization occurs

Eggs

Figure 27.1E


• Sexual reproduction

– May enhance reproductive success in changing environments


HUMAN REPRODUCTION

27.2 Reproductive anatomy of the human female

• Both sexes in humans

– Have a set of gonads where gametes are produced

– Have ducts for delivery of the gonads and structures for copulation


• A woman’s ovaries

– Contain follicles that nurture eggs and produce sex hormones

OviductOvaries

Follicles

Corpus luteumWall of uterus Uterus

Endometrium(lining of uterus)

Cervix(“neck” of uterus)

Vagina

Figure 27.2A


• Oviducts convey eggs to the uterus

– Where the eggs develop

Eggcell

Ovary

LM

20

0

Figure 27.2B


• The uterus opens into the vagina

– Which receives the penis during intercourse and forms the birth canal


• Other structures of the female reproductive system include

– Labia, the clitoris, Bartholin’s glands, and the cervix

Oviduct

Ovary

Uterus

Urinary bladder(excretory system)

Pubic bone

Urethra(excretory system)

ShaftGlans

Prepuce

Clitoris

Labia minora

Labia majora

Vaginal opening

Anus(digestive system)

Bartholin’s gland

Vagina

Cervix

Rectum(digestive system)

Figure 27.2C


27.3 Reproductive anatomy of the human male

• A man’s testes

– Produce sperm

Rectum(digestive system)

Seminal vesicle

Vas deferens

Ejaculatoryduct

Prostate gland

Bulbourethral gland

Vas deferens

Epididymis

TestisScrotum

Urinary bladder(excretorysystem)

Pubic bone

Erectiletissue ofpenis

Urethra(excretorysystem)

Glans of penis

Prepuce

Penis

Figure 27.3A


• Several glands

– Contribute to the formation of fluid that nourishes and protects sperm

Urinary bladder(excretorysystem)

Prostate gland

Bulbourethralgland

Erectile tissueof penis

Vas deferens

Epididymis

Testis

Seminalvesicle(behindbladder)

Urethra

Scrotum

Glans ofpenis

Figure 27.3B


• During ejaculation

– Sperm and the nourishing fluid, called semen, are expelled through the penis

Sphinctercontracts

Urinarybladder

Urethra region hereexpands and fillswith semen

Contractionsof vas deferens

Contractionsof prostategland

Contractionsof seminalvesicle

Sphinctercontracts

Contractionsof epididymis

First stage

Sphincter remainscontracted

Contractionsof musclesaround baseof penis

Sphincterrelaxes

Contractions of urethra

Second stage

Semen expelled

Figure 27.3C


• A negative feedback system of hormones

– Controls sperm productionStimuli from other areas in the brain

Hypothalamus

Releasing hormone Anterior

pituitary

Neg

ativ

e fe

edba

ck

FSH LH

Androgenproduction

Sperm production

Testis

Figure 27.3D


• Primary spermatocytes, which are diploid, are made continuously in the testes

– And undergo meiosis to produce haploid sperm

Primary spermatocyte

Differentiation andonset of Meiosis I

Meiosis I completed

Meiosis II

Differentiation

(in prophase of Meiosis I)

Secondary spermatocyte

(haploid; double chromatids)

Developing sperm cells(haploid; single chromatids)

Sperm cells

(haploid) Center ofseminiferous tubule

n n n n

n n n n

n n

2n

2n

TestisScrotum

Penis

Epididymis

TestisSeminiferous tubule

Cross section ofseminiferoustubule

Diploid cell

Figure 27.4A


• Each month one primary oocyte

– Matures to form a secondary oocyte, which can be fertilized

– Completes meiosis and becomes a haploid ovum

Diploid cell In embryo2n

Differentiation andonset of Meiosis I

Primary oocyte

(arrested in prophaseof Meiosis I)

2n Present at birth

Completion of Meiosis Iand onset of Meiosis II

Secondary oocyte

(arrested at meta-phase of Meiosis II;

released from ovary)

n n

Entry of sperm triggerscompletion of Meiosis II

Ovum

(haploid) n nSecondpolar body

Firstpolar body

Figure 27.4B


• The development of an ovarian follicle

– Involves many different processes

Corpus luteum

Degeneratingcorpus luteum

Start: Primary oocyte

within follicle

Growingfollicles

Mature follicle

Ovary

Ruptured follicleOvulation

Secondaryoocyte

Figure 27.4C


27.5 Hormones synchronize cyclic changes in the ovary and uterus

• The ovarian cycle includes

– Changes in the ovary that occur about every 28 days

• The menstrual cycle

– Involves changes that occur in the uterus


An Overview of the Ovarian and Menstrual Cycles

• Events in the menstrual cycle

– Are synchronized with the ovarian cycle, which occurs about every 28 days


• Uterine bleeding, called menstruation

– Includes the breakdown of the endometrial lining

– Usually persists for 3–5 days

• After menstruation

– The endometrium, the lining of the uterus, regrows


• Five hormones

– Synchronize the events in the ovarian cycle

Table 27.5


Hormonal Events Before Ovulation

• Approximately every 28 days

– The hypothalamus signals the anterior pituitary to secrete FSH and LH

• FSH and LH

– Trigger the growth of a follicle


• As the follicle grows, it secretes estrogen

– Which causes a burst in FSH and LH levels, leading to ovulation


Hormonal Events at Ovulation and After

• After ovulation

– The follicle becomes the corpus luteum

• The corpus luteum secretes both estrogen and progesterone

– Which exert negative feedback on the hypothalamus and pituitary, causing a decline in FSH and LH levels


• As FSH and LH levels drop

– The hypothalamus can once again stimulate the pituitary to secrete more FSH and LH, and a new cycle begins


Control of the Menstrual Cycle

• The menstrual cycle

– Is directly controlled by estrogen and progesterone


• If fertilization of an egg occurs

– A hormone from the embryo maintains the uterine lining and prevents menstruation


• The ovarian and menstrual cycles

A

B

Control by hypothalamus

Hypothalamus

Releasing hormone

Anterior pituitary

Inhibited by combination ofestrogen and progesteroneStimulated by high levelsof estrogen

FSH LH

Pituitary hormonesin blood

LH

FSH

FSH LH

LH peak triggersovulation andcorpus luteumformation

Ovarian cycle

Growing follicle

Pre-ovulatory phase

Maturefollicle

Ovulation

Corpusluteum

Post-ovulatory phase

Degeneratingcorpus luteum

Estrogen Progesterone and estrogen

Ovarian hormonesin blood

Estrogen

Progesterone

EstrogenProgesterone and estrogen

C

D

E Menstrual cycle

Endometrium

0 5 10 14 15 20 25 28Days

MenstruationFigure 27.5

1

4

6

2

5

3

78


27.6 The human sexual response occurs in four phases

• The excitement phase

– Prepares the sexual organs for coitus

• The plateau phase

– Is marked by increases in breathing and heart rate


• Orgasm follows

– Characterized by rhythmic contractions of the reproductive structures

• The resolution phase

– Completes the cycle and reverses the previous responses


CONNECTION

27.7 Sexual activity can transmit disease

• Sexual intercourse

– Carries the risk of exposure to sexually transmitted diseases (STDs)


• STDs common in the United States

Table 27.7


CONNECTION

27.8 Contraception can prevent unwanted pregnancy

• Contraception

– Is the deliberate prevention of pregnancy

Table 27.8


• Contraception can be accomplished

– Through various methods

Skin patch

Diaphragm

Spermicide

Condom

Birth control pills

Figure 27.8


PRINCIPLES OF EMBRYONIC DEVELOPMENT

27.9 Fertilization results in a zygote and triggers embryonic development

• Embryonic development begins with fertilization

– The union of sperm and egg to form a diploid zygote


The Properties of Sperm Cells

• Only one sperm

– Fertilizes an egg

Co

loriz

ed

SE

M 5

00

Figure 27.9A


• During fertilization

– A sperm releases enzymes from the acrosome that pierce the egg’s coat

Plasma membrane Middlepiece HeadNeck

Mitochondrion(spiral shape)

Nucleus

Acrosome

Tail

Figure 27.9B


The Process of Fertilization

• Sperm surface proteins bind to egg receptor proteins

– Sperm and egg plasma membranes fuse, and the two nuclei unite


• Changes in the egg membrane

– Prevent entry of additional sperm

• The fertilized egg (zygote)

– Develops into an embryo


Figure 27.9C

• The process of fertilization

Nucleus

Acrosome

Sperm

Plasmamembrane

Spermhead

Acrosomalenzymes

Jellycoat

Vitellinelayer

Plasmamembrane

Receptor proteinmolecules

Spermnucleus

Cytoplasm

Eggnucleus

Egg cell

Zygote nucleus

The spermapproachesthe egg

1

Proteins on thesperm head bind toegg receptors

3

The plasmamembranes of spermand egg fuse

4

The sperm nucleusenters the eggcytoplasm

5

A fertilizationenvelope forms

6

The sperm’sacrosomal enzymesdigest the egg’s jellycoat

2

The nuclei of sperm and egg fuse

7


27.10 Cleavage produces a ball of cells from the zygote

• Cleavage is a rapid series of cell divisions

– That results in a blastula, a ball of cells

Zygote 2 cells

4 cells

8 cells

Many cells(solid ball)

Blastula(hollow ball)

Cross sectionof blastula

Blastocoel

Figure 27.10


27.13 Changes in cell shape, cell migration, and programmed cell death give form to the developing animal

• Cells of the ectoderm

– Fold inward during neural tube formation

Ectoderm

Figure 27.13A


• Programmed cell death, or apoptosis

– Is a key developmental process in which cells die

Figure 27.13B

Apoptosis

Dead cellengulfed anddigested byadjacent cell


27.14 Embryonic induction initiates organ formation

• In a process called induction

– Adjacent cells and cell layers influence each other’s differentiation via chemical signals


Futurebrain Optic

vesicle

Lens ectoderm

Opticstalk

Optic cup

Cornea

Lens

Futureretina

Figure 27.14

• Induction during eye development

1 2 3 4


27.15 Pattern formation organizes the animal body

• Pattern formation

– Is the emergence of the parts of a structure in their correct relative positions


HUMAN DEVELOPMENT

27.16 The embryo and placenta take shape during the first month of pregnancy

• Pregnancy, or gestation

– Is the carrying of developing young within the female reproductive tract


An Overview of Developmental Events

• Human development

– Begins with fertilization in the oviduct Cleavage starts

Fertilizationof ovum

Oviduct

Secondaryoocyte

Ovulation

Ovary

Blastocyst(implanted)

Endometrium

Uterus

Figure 27.16A


• Cleavage produces a blastocyst

– Whose inner cell mass becomes the embryo

Endometrium

Inner cell mass

Cavity

Trophoblast

Figure 27.16B


• The blastocyst’s outer layer, the trophoblast

– Implants in the uterine wall

EndometriumFuture embryo

Futureyolk sac

Blood vessel(maternal)

Multiplying cellsof trophoblast

Trophoblast

Uterine cavity

Figure 27.16C


• Meanwhile, the four extraembryonic membranes develop

– The amnion, the chorion, the yolk sac, and the allantois

Amnioticcavity Amnion

Mesodermcells Chorion

Yolk sac

Chorion

Amnion

Allantois

Yolk sac

Chorionic villi

Embryo:

EctodermMesodermEndoderm

Figure 27.16D, E


Roles of the Extraembryonic Membranes

• About a month after conception

– The extraembryonic membranes are fully formed

Mother’s bloodvessels

Allantois

Yolk sac

Placenta

Amnioticcavity

Amnion

Embryo

Chorion

Chorionicvilli

Figure 27.16F


• The embryo floats in a fluid-filled amniotic cavity

– Surrounded by the amnion

• The chorion and mesoderm cells from the yolk sac

– Form the embryo’s part of the placenta


• The allantois

– Forms part of the umbilical cord


The Placenta

• The placenta’s chorionic villi

– Absorb food and oxygen from the mother’s blood to nourish the embryo


27.17 Human development from conception to birth is divided into three trimesters

• Human development is divided into three trimesters

– Each about three months in length


The First Trimester

• The most rapid changes

– Occur during the first trimester

Figure 27.17A


• By 9 weeks

– The embryo is called a fetus

Figure 27.17B


Second Trimester

• The second trimester

– Involves an increase in size and general refinement of the human features

Figure 27.17C


• At 20 weeks

– The fetus is about 19 centimeters long, and weighs half a kilogram

Figure 27.17D


Third Trimester

• The third trimester

– Is a time of rapid growth, which ends in birth

Figure 27.17E


27.18 Childbirth is hormonally induced and occurs in three stages

• The birth of a child

– Is brought about by a series of strong, rhythmic contractions of the uterus, called labor


• Estrogen makes the uterus more sensitive to oxytocin

– Which acts with prostaglandins to initiate labor

Estrogen Oxytocin

fromovaries

from fetusand pituitary

Induces oxytocinreceptors on uterus

Stimulates uterusto contract

Stimulatesplacenta to make

Prostaglandins

Stimulate morecontractionsof uterus

Po

sitiv

e f

ee

db

ack

Figure 27.18A


Placenta

Umbilicalcord

Uterus

Cervix

Uterus

Placenta(detaching)

Umbilicalcord

Figure 27.18B

• Labor occurs in three stages

– Dilation, expulsion, and delivery of the placenta

Dilation of the cervix1

Expulsion: delivery of the infant2

Delivery of the placenta3


CONNECTION

27.19 Reproductive technology increases our reproductive options

• New techniques

– Can provide help to infertile couples

Figure 27.19


• Some of these methods

– Raise important ethical and legal questions

chapter 27 reproduction and embryonic development

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