Transcript The Living World

Reproduction
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1 Asexual and
Sexual Reproduction
In asexual reproduction,
the offspring are
genetically identical to
one parent
The process begins
with mitosis
Protists typically
divide by fission
Cnidarians typically
divide by budding
Fig. 1 Asexual reproduction in Euglena
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1 Asexual and
Sexual Reproduction
In sexual reproduction, a new individual is formed
by the union of two gametes (egg and sperm)
A zygote is formed
Develops by mitosis into a multicellular
organism
Haploid gametes are produced in the gonads
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Different Approaches to Sex
Parthenogenesis
Offspring are produced from unfertilized eggs
Common among arthropods
Some are exclusive
Others switch!
Common also in some lizard species
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Hermaphroditism
Hamlet bass
One individual has both
testes and ovaries
Tapeworms and
earthworms
Hermaphroditism may
be sequential
Switch sexual
roles!
Switch
sexes!
Fig. 2
Individuals change sex
Protogyny
From female to male
Protandry
From male to female
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Bluehead wrasse
Sex Determination
In some reptiles, sex is determined by
environmental changes
In mammals, it is determined early in embryonic
development
Embryonic gonads are indifferent
Y chromosome converts them to testes
Responsible gene is SRY
Sex-determining region of the Y
chromosome
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Fig. 3 Sex
determination
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2 Evolution of Reproduction
Among the Vertebrates
Vertebrate sexual reproduction evolved in the
ocean before vertebrates colonized land
Most marine bony fish use external fertilization
Male and female gametes are released into
the water where fertilization occurs
Most other vertebrates use internal fertilization
Male gametes are introduced into the female
reproductive tract
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There are three strategies for internal fertilization
1. Oviparity
Fertilized eggs are deposited outside mother’s body
to complete their development
2. Ovoviviparity
Fertilized eggs are retained within the mother to
complete their development
Young obtain nourishment from egg yolk
3. Viviparity
Fertilized eggs are retained within the mother to
complete their development
Young obtain nourishment from mother’s blood
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Fish and Amphibians
The eggs of most bony fish are fertilized externally
Eggs contain little yolk
Young fish must seek its food from the water
surrounding it
Thousands of eggs are fertilized, but only a
few of resulting individuals reach maturity
Fertilization in most cartilaginous fish is internal
Development of young is viviparous
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Fertilization is external
in most amphibians
Fig. 6
Eggs of most species
develop in water
Poison arrow frog
With some
Male!
interesting
exceptions
South American
marsupial frog
Female
Fig. 7
Development
is divided into
embryonic,
larval and
adult stages
Surinam frog
Darwin’s frog
Female
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Male!
Reptiles and Birds
Most reptiles are oviparous, laying amniotic eggs
Other species are ovoviviparous or viviparous
Fig. 8 How turtles do it 
Most male reptiles use
a penis to inject sperm
into females
This process is
called copulation
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All birds are oviparous,
laying amniotic eggs
As egg passes along
oviduct, glands secrete
albumin proteins and
the hard calcareous
shell
Fig. 9
Birds are
homeotherms
Incubate eggs to
keep them warm
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Mammals
Some mammals are seasonal breeders
Others have reproductive cycles
Periodic release of a mature ovum (ovulation)
Most female mammals have estrous cycles
Females sexually receptive to males only
around time of ovulation (estrus)
Apes and humans have menstrual cycles
Females bleed when shedding inner lining of
the uterus
Can copulate at any time in their cycle
Cats and rabbits are induced ovulators
Ovulation only after copulation due to LH secretion
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Three types of mammals
Monotremes are oviparous
Lay eggs
Young hatchlings obtain milk by
licking mammary glands (they lack
nipples)
Marsupials are viviparous
Give birth to incompletely
developed fetuses
Complete development in
mother’s pouch
Obtain food from nipples in
mammary glands
Placentals are viviparous
Retain young in uterus for long
periods of development
Fetuses are nourished by the
placenta
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Fig. 10 Reproduction in mammals
Duck-billed platypus
Kangaroo
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Deer
Review the Concepts
What are gametes?
Sex is determine by which chromosomes?
What is the union of the sperm and egg
called?
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3 Males
Fig. 11 The
male
reproductive
organs
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The testis produces sperm and testosterone
Enclosed in a hanging sac called the scrotum
Sperm need cooler temperature to develop
Spermatogenesis occurs in the seminiferous tubules
Sperm are then
transferred to the
epididymis for storage
and maturation
From there to the
vas deferens
To the urethra
which empties
through the penis
Contains 23
chromosomes
Fig. 13 Human sperm cell
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Fig. 12 The testis and formation of sperm
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The penis contains
long cylinders of
spongy tissue
These get filled
with blood causing
an erection
Physical stimulation is
required for ejaculation
2-5 milliliters of semen
are ejected
Fig. 14
This volume contains several hundred million sperm
Plus secretions from the prostate and other glands
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4 Females
Fig. 15 The
female
reproductive
organs
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At birth, a female’s ovaries contains all the oocytes
she will ever produce
~ 2 million oocytes are arrested in prophase I of
the first meiotic division
At puberty, the release of FSH causes the
resumption of meiosis I in a few oocytes
However, only one becomes dominant and
is ovulated
Mature egg cells are called ova
(singular, ovum)
This cycle is repeated about every 28 days
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Fig. 16 The ovary and formation of an ovum
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Fertilization of the egg occurs high in the Fallopian
tubes (also called uterine tubes or oviducts)
The fertilized egg is now called a zygote
It is transported to the uterus
A muscular pear-shaped organ about the
size of a fist
It narrows to a muscular ring called the
cervix
Leads to the vagina
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Review the Concepts
Name the male and female reproductive
organs.
Name the two things produced by the
testes.
Where does spermatogenesis occurs
In which stage are the oocytes arrested?
How often does ovulation occurs?
Where does fertilization occurs?
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Fig. 17 A comparison of mammalian uteruses
Primates
Cats, dogs and cows
Rats, mice and rabbits
Marsupials, such as opossums, have two unconnected horns,
two cervices and two vaginas
Male marsupials have a forked penis!
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The fertilized egg is pushed down the oviducts by
the rhythmic contraction of its smooth muscles
The journey takes 5-7 days
The uterus is lined with a stratified epithelial
membrane called the endometrium
The zygote attaches to this layer and begins
embryonic development!
If the egg is not fertilized, the surface layer of the
endometrium is shed during menstruation
The underlying layer generates a new surface
layer during the next cycle
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5 Hormones Coordinate
the Reproductive Cycle
The female reproductive cycle is composed of two
distinct phases
Follicular phase
Egg reaches maturation and is ovulated
Luteal phase
Body continues to prepare for pregnancy
A family of hormones coordinates these two
phases
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Follicular Phase
Development of the egg within the ovary
The oocyte and its surrounding mass of tissue is
called the follicle
FSH secretion triggers the maturation of several
follicles and resumption of meiosis in their oocytes
But only one achieves full maturity
FSH also causes the ovary to secrete estrogen
Negative feedback by estrogen, causes the
hypothalamus to stop the pituitary’s FSH output
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Luteal Phase
The body is prepared for fertilization
Hypothalamus causes the anterior pituitary to
begins secreting luteinizing hormone (LH)
LH inhibits further estrogen production
It also causes the wall of the follicles to burst
Oocyte is ovulated into oviducts
LH directs the repair of the ruptured follicle,
which becomes the corpus luteum
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Fig. 18 The human menstrual cycle
Levels of
gonadotropic
hormones in blood
Pituitary
gland
LH
FSH
FSH
0
7
14
21
28 days
Ovarian cycle
Developing follicles
Ovulation
Corpus luteum
Luteal phase
Hormone blood levels
Estradiol
Progesterone
0
7
14
21
28 days
Endometrial changes
during menstrual cycle
Menstrual
phase
0
Proliferative
phase
7
Ovulation
14
Secretory
phase
21
28 days
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Luteal Phase
The corpus luteum begins to secrete the hormone
progesterone
Progesterone inhibits FSH
It also thickens the endometrium preparing for
fertilization
If fertilization does not occur, progesterone
production stops and the luteal phase ends
Thickened endometrial layer sloughs off
This causes the bleeding associated
with menstruation
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Fig. 18 The human menstrual cycle
Levels of
gonadotropic
hormones in blood
Pituitary
gland
LH
FSH
FSH
0
7
14
21
28 days
Ovarian cycle
Developing follicles
Ovulation
Corpus luteum
Luteal
regression
Luteal phase
Hormone blood levels
Estradiol
Progesterone
0
7
14
21
28 days
Endometrial changes
during menstrual cycle
Menstrual
phase
0
Proliferative
phase
7
Ovulation
14
Secretory
phase
21
Menstrual
phase
28 days
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Luteal Phase
If fertilization does occur, the corpus luteum is
maintained by human chorionic gonadotropin (hCG)
hCG is a hormone produced by the embryo
It is tested for in all pregnancy tests
Two other hormones are of importance
Prolactin
Stimulates milk production
Oxytocin
Initiates milk release
Induces labor
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Review the Concepts
What are the two phase of the female
reproductive cycle?
List the events of the Follicular phase.
List the events of the Luteal phase.
List the hormones involved in the Follicular
phase
List the hormones involved in the Luteal
phase
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6 Embryonic Development
The vertebrate embryo develops in three stages
Cleavage
A hollow ball of cell forms
Gastrulation
Cells move to the interior, forming the primary
tissues
Neurulation
The organs of the body form
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Cleavage: Setting the Stage for Development
During cleavage, zygote rapidly divides into larger
and larger numbers of smaller and smaller cells
A morula forms
A tightly packed mass of about 32 blastomeres
Further division results in a hollow ball of 5002,000 cells called the blastocyst
Contains a fluid-filled cavity, the blastocoel
Inner cell mass  Forms the embryo
Trophoblast  Becomes the placenta
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Cleavage: Setting the Stage for Development
Embryo reaches the uterus on day 6
It penetrates the endometrial lining
Initiates membrane formation
Amnion
Encloses embryo
Chorion
Forms from the trophoblast
Interacts with uterine tissue to form the
placenta
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Gastrulation: Onset of Developmental Change
Certain groups of cells move inwards from the inner
cell mass at about 10-11 days after fertilization
This process of gastrulation results in the three
primary germ layers
Endoderm
Ectoderm
Mesoderm
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Review the Concepts
List the three stages of the vertebrate embryo
development.
List the three primary germ cells and their
functions.
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Neurulation: Determination of Body Architecture
In the third week, the three primary germ layers
begin development into body tissues and organs
First, the notochord develops from the mesoderm
The neural tube develops from the ectoderm
The gut develops from the endoderm
On either side of the notochord blocks of tissue form
These somites give rise to muscles, vertebrae and
connective tissues developing notochord
By the end of the third week, the embryo is about
2 mm (< 0.1 inches) long
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7 Fetal Development
Fourth week
Formation of body
organs, or
organogenesis
Critical time in
development
Fig. 20a
Alcohol use may
cause fetal
alcohol syndrome
Embryo reaches
about 5 mm
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7 Fetal Development
Second month
Great changes in
morphology occur
Limbs assume
adult shape
Major internal
organs are evident
Embryo reaches
about 25 mm
Fig. 20b
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7 Fetal Development
Third month
Development is
essentially complete
except for lungs and
brain
Fig. 20c
Developing human
is now called a fetus
It carries out
primitive reflexes
like sucking
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7 Fetal Development
Second trimester
A time of growth
Bone formation
occurs
Hair and body are
covered with fine
hair called lanugo
By the end of the 6th
month, the fetus is
30 cm (1 foot) long
Fig. 20d
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7 Fetal Development
Third trimester
Pace of growth
accelerates
Weight of fetus
more than doubles
Nutrients
provided by
mother’s blood
via the placenta
Fig. 21
Most major nerve
tracts are formed
in the brain
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Postnatal development
Babies typically
double birth weight
within a few months
Different body parts
grow at different rates
Allometric growth
Nerve cells produced
at an average rate of
> 250,000 per minute
At 6 months, neuron
production ceases
permanently
Fig. 22
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8 Sexually Transmitted Diseases and
Contraception
Sexually transmitted diseases are transmitted
during sexual intercourse or sexual activities
Contraception, or birth control, is the prevention of
pregnancy
Several different methods are available
These differ in their effectiveness and
acceptability to different couples
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Sexually Transmitted Diseases
STDs are diseases that are
spread from one person to
another through sexual contact
They include
Acquired Immune
Deficiency Syndrome
(AIDS)
Caused by the
human
immunodeficiency
virus (HIV)
Gonorrhea
Caused by the
bacterium Neisseria
gonorrhoeae
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Sexually Transmitted Diseases
Chlamydia
Caused by the bacterium
Chlamydia trachomatis
Syphilis
Caused by the bacterium
Treponema pallidum
Genital Herpes
Caused by the herpes
simplex virus type 2
(HSV-2)
Most common STD in the
US
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Birth Control
Abstinence
Simplest and most reliable way
Natural family planning, or the
rhythm method
Prevention of egg maturation
Birth-control pills
Estrogen and progesterone
Shut down production
of the pituitary
hormones FSH and LH
Birth-control injections
Birth-control patches
The Ring
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Birth Control (cont.)
Prevention of embryo
implantation
Intrauterine devices (IUDs)
RU486 (“morning after pill”)
Sperm blockage
Condoms
Diaphragms
Sperm destruction
Spermicidal jellies
Foams
Sterilization
Vasectomy in males
Tubal ligation in females
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Effectiveness of
Contraceptives
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Review the Concepts
List some of the most common STD’s
Which is the most effective method of
contraception?
Which is the least effective method of
contraception?
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