Download CHAPTER 27 Reproduction and Embryonic Development

CHAPTER 27
Reproduction and Embryonic
Development
Overview:
Animal Reproduction
Asexual & Sexual
Human Reproductive System
Female
Male
Female Reproductive Cycle
Fertilization
Pregnancy
• Other sets of multiple births soon followed
• What accounted for the sudden rash of multiple
births?
• All of these multiple births were by women who
had taken fertility drugs because they couldn’t
become pregnant naturally
• Couples turn to fertility drugs to overcome their
natural reproductive limitations
UNIFYING CONCEPTS OF ANIMAL
REPRODUCTION
• Reproduction is the creation of new individuals
from existing ones
•
ASEXUAL AND SEXUAL
REPRODUCTION
Sexual and asexual reproduction are both
common among animals
Asexual Reproduction
• In asexual reproduction
– One parent produces genetically identical offspring
• Binary fission
– Is the simplest type of asexual reproduction
– Involves a single parent cell splitting through mitosis
into two genetically identical offspring cells
• Some multicellular organisms reproduce by a
similar means called fission, in which one
organism splits into two or more individuals
• Fragmentation is the breaking of a parent body
into several pieces
• Regeneration, which follows fragmentation, is
the regrowth of a whole animal from the pieces
• Budding
– Is the splitting off of
new individuals from
existing ones
• Asexual reproduction has a number of
advantages
– It allows a species to perpetuate itself if its individual
members are sessile or isolated from one another
– It allows organisms to multiply quickly
• One potential disadvantage of asexual
reproduction is that it produces genetically
uniform populations
Sexual Reproduction
• Sexual reproduction
– Involves the fusion of gametes (sperm and egg) from
two parents
– Increases the genetic variability among offspring
• Advantages of sexual reproduction
– Increases genetic variation
– Enhances reproductive success in changing
environments
• Disadvantage of sexual reproduction
– Locating a mate
• Some animals can
reproduce both
sexually and
asexually
• Some species are hermaphrodites with both male
and female reproductive systems
• The mechanics of fertilization play an important
part in sexual reproduction
• Many organisms use external fertilization, in
which parents discharge their gametes into the
water, where fertilization occurs
• Other organisms use internal fertilization, which
occurs within the female’s body
• Internal fertilization requires copulation, or
sexual intercourse
HUMAN REPRODUCTION
• Both sexes of humans have
– A pair of gonads, the organs that produce gametes
– Ducts to store and deliver the gametes
– Structures to facilitate copulation
Female Reproductive Anatomy
• The ovaries
– Are the site of gamete production in human females
• The ovaries contain follicles
– Each follicle consists of a single developing egg
cell surrounded by layers of cells that nourish and
protect it
– The follicles also produce estrogen, the female sex
hormone
•Oviducts
–Convey eggs
to the uterus
• Uterus
- Actual site of
pregnancy
– Development
of fertilized egg
– Opens into
the vagina
• Vagina
– Receives
penis during
intercourse
– Forms the
birth canal
• Ovulation
– Is the process by which an egg cell is ejected from the follicle
– An egg cell is released from a follicle at the surface
of an ovary
– The orange mass below the ejected oocyte is part of
the ovary
Male Reproductive Anatomy
• The penis
– Contains erectile tissue
• The testes
– Are the male gonads, enclosed in a sac called the
scrotum
– Produce sperm
• Several glands
– Contribute to the formation of the fluid that carries,
nourishes, and protects sperm
• Semen
– Consists of this fluid and sperm
• There are two stages of ejaculation
– First stage of ejaculation
– Second stage of ejaculation (expulsion stage)
• Androgens stimulate sperm production
– They also maintain homeostasis by a negative
feedback mechanism that inhibits the secretion of
FSH (follicle-stimulating hormone) and LH
(luteinizing hormone)
The formation of sperm and ova requires
meiosis
• Gametogenesis
– Is the production of gametes
• Human gametes
– Are haploid cells that develop by meiosis
• Spermatogenesis
– Produces sperm in the male
• Oogenesis
– Produces ova in the female
• Spermatogenesis
– Increases genetic variation
– Primary spermatocytes are produced throughout a
male’s reproductive years
– Diploid cells undergo meiosis to form four haploid
sperm
• Oogenesis
– Most of the process occurs within the ovaries
– Lifetime supply of primary oocytes is present at birth
– One primary oocyte matures each month to form a
secondary oocyte
– If the secondary oocyte is fertilized, it completes
meiosis and becomes a haploid ovum
The Female Reproductive Cycle
• Human females have a reproductive cycle, a
recurring series of events that produces gametes,
makes them available for fertilization, and
prepares the body for pregnancy
• The female reproductive cycle involves two sets
of changes
– The ovarian cycle controls the growth and release of
an ovum
– The menstrual cycle prepares the uterus for possible
implantation of an embryo
• Hormones
– Synchronize cyclical changes in the ovaries and
uterus
The human sexual response occurs in four
phases
• Excitement
–
–
–
–
–
Sexual passion builds
Penis and clitoris become erect
Testes, labia, nipples swell
Vagina secretes lubricating fluid
Muscles of arms and legs tighten
• Plateau
– Continuation of excitement responses
– Increase in breathing and heart rates
• Orgasm
– Rhythmic contraction of the reproductive structures
– Extreme pleasure
– Ejaculation by the male
• Resolution
–
–
–
–
Reverse previous phase responses
Structures return to normal size
Muscles relax
Passion subsides
REPRODUCTIVE HEALTH
• Two issues of human reproductive health
– Contraception
– Transmission of disease
Contraception
• Contraception
– Is the deliberate prevention of pregnancy
• Contraception prevents pregnancy in one of three
ways
– Blocking the
release of
gametes
– Preventing
fertilization
– Preventing
implantation
• There are many
forms of
contraception, each
with varying
degrees of
reliability
• Contraceptive methods and their effectiveness
Sexually Transmitted Diseases
• Sexually transmitted diseases (STDs)
– Are contagious diseases spread by sexual contact
• Viral STDs, such as AIDS, genital herpes, and
genital warts, cannot be cured but can be
controlled by medications
REPRODUCTIVE TECHNOLOGIES
• Reproductive technologies
– Can help solve problems related to the inability to
conceive a child
• Infertility
Infertility
– Is the inability to have children after one year of
trying
– Is most often due to problems in the man, such as
underproduction of sperm or impotence
• Female infertility can result from a lack of eggs
or a failure to ovulate
• There are technologies available to help treat the
many forms of infertility
In Vitro Fertilization
• In vitro fertilization (IVF)
– Happens under artificial, laboratory conditions
• IVF
– Begins with the surgical removal of eggs and the
collection of sperm
– Involves fertilization of eggs in a petri dish
• IVF
– Offers choices that nature does not
– Raises many moral and legal issues
PRINCIPLES OF EMBRYONIC
DEVELOPMENT
Fertilization results in a zygote and triggers embryonic
development
• The shape of a human sperm cell is adapted to its
function
• Embryonic development
– Begins with fertilization, the union of sperm and egg
to form a zygote
Fertilization
• Copulation releases hundreds of millions of
sperm into the vagina, but only a few hundred
survive the trip to the egg, and only one will
fertilize it
Basic Concepts of Embryonic
Development
• The key to development in all organisms is that
each stage of development takes place in a
highly organized fashion
• Development begins with cleavage, a series of
rapid cell divisions that results in a multicellular
ball
• Cleavage continues as the embryo moves down
the oviduct toward the uterus
• About 6–7 days after fertilization, the embryo
has reached the uterus as a fluid-filled hollow
ball of about 100 cells called a blastocyst
• The next stage of development is gastrulation, a
process that produces the three embryonic tissue
layers
Gastrulation produces a three-layered embryo
• Gastrulation is the second major phase of
embryonic development
– It adds more cells to the embryo
– It sorts all cells into three distinct cell layers
– The embryo is transformed from the blastula into the
gastrula
• The three layers produced in gastrulation
– Ectoderm, the outer layer
– Endoderm, an embryonic digestive tract
– Mesoderm, which partly fills the space between the
ectoderm and endoderm
• Development of
frog gastrula
Organs start to form after gastrulation
• Embryonic tissue layers begin to differentiate
into specific tissues and organ systems
• In chordates
– the notochord develops from the mesoderm
– the neural tube develops from the ectoderm
• The neural tube becomes the brain and spinal
cord
• Somites are
blocks of
mesoderm that
will give rise to
segmental
structures
• The body cavity,
or coelom, also
develops from
the mesoderm
Changes in cell shape, cell migration, and
programmed cell death give form to the
developing animal
• Tissues and organs
take shape in a
developing embryo
as a result of
– cell shape changes
– cell migration
Embryonic induction initiates organ formation
• Induction is the mechanism by which one group
of cells influences the development of tissues and
organs from ectoderm, endoderm, and mesoderm
– Adjacent cells and cell layers use chemical signals to
influence differentiation
– Chemical signals turn on a set of genes whose
expression makes the receiving cells differentiate into
a specific tissue
Pattern formation organizes the animal body
• Pattern formation is the emergence of a body
form with structures in their correct relative
positions
– It involves the response of genes to spatial variations
of chemicals in the embryo
Pregnancy and Early Development
• Pregnancy, or gestation
– Is the carrying of developing young within the
female reproductive tract
– Is measured as 40 weeks from the start of the last
menstrual cycle in humans
• Gestation is pregnancy
– It begins at conception and continues until birth
– Human gestation is 266 days
(38 weeks or 9 months)
– Mouse gestation is 1 month
– Elephant gestation is 22 months
• About one week after conception
– The embryo, which has become a blastocyst,
implants itself in the uterine wall
– The outer cell layer, the trophoblast, becomes part of
the placenta
• Four structures develop that assist the developing
embryo
– The amnion, a fluid filled sac that encloses and
protects the embryo
– The yolk sac, which produces the embryo’s first
blood and germ cells
– The allantois, which forms part of the umbilical cord
– The chorion, which becomes part of the placenta
• The placenta allows for a variety of substances to
pass from mother to fetus
–
–
–
–
–
–
Protective antibodies
German measles virus
HIV
Drugs (prescription and nonprescription)
Alcohol
Chemicals in tobacco smoke
The Stages of Pregnancy
• Pregnancy is divided into three trimesters
The First Trimester
• A human embryo about 5 weeks after
fertilization
• A human embryo, now called a fetus, about 9
weeks after fertilization
• By the end of the first trimester
– The fetus looks like a miniature human being
– The sex of the fetus can be determined by ultrasound
The Second Trimester
• The main developmental changes during the
second and third trimesters involve an increase
in size and general refinement of the human
features
• A fetus at 14
weeks, 2 weeks
into the second
trimester
• At 20 weeks, the fetus
– Is about 19 cm
(7.6 in.) long
and weighs
about half a
kilogram (1 lb)
– Has the face of
an infant
The Third Trimester
• The third trimester
– Is a time of rapid growth
– Includes many important physical changes
• At birth
– A typical baby is
about 50 cm (20 in.)
long and weighs
2.7–4.5 kg (6–10
lb)
Childbirth
• The birth of a child is brought about by a series
of strong, rhythmic contractions of the uterus
called labor
• Hormones play a key role in inducing labor
– Estrogen, oxytocin, and prostaglandins are all
involved
• There are three
stages of labor
• Dilation of the cervix
is the first stage
– Cervix reaches full
dilation at 10cm
– Longest stage of labor
(6-12 hours or longer)
• Expulsion is the second stage
– Period from full dilation
of the cervix to delivery
of the infant
– Uterine contractions occur
every 2-3 minutes
– Mother feels urge to push
down with her abdominal
muscles
– Infant is forced down and
out of uterus and vagina
within a period of 20
minutes
• The delivery of the
placenta is the final
stage of labor
– Usually occurs within
15 minutes after the
birth of the baby
• Hormones continue to be important after the
baby and placenta are delivered
– Decreasing progesterone and estrogen levels allow
the uterus to return to its pre-pregnancy state
– Oxytocin and prolactin stimulate milk secretion