Transcript a. Sperm

CHAPTER 54
LECTURE
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Animal Development
Chapter 54
Fertilization
• In all sexually-reproducing animals, the
first step is fertilization – union of male and
female gametes
• Fertilization itself consists of three events
– Sperm penetration and membrane fusion
– Egg activation
– Fusion of nuclei
3
Fertilization
• Sperm penetration and membrane fusion
– Protective layers of egg include the jelly layer
and vitelline envelope in sea urchins, and the
zona pellucida in mammals
– Sperm’s acrosome contains digestive
enzymes that enable the sperm to tunnel its
way through to the egg’s cell membrane
– Membrane fusion permits sperm nucleus to
enter directly into egg’s cytoplasm
4
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Sperm
a.
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Sperm
a.
Jelly layer
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Sperm
Jelly layer
Plasma
membrane
a.
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Sperm
Jelly layer
Plasma
membrane
Vitelline
envelope
a.
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Sperm
Jelly layer
Plasma
membrane
Vitelline
envelope
Nucleus
of egg
a.
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Sperm
Jelly layer
Plasma
membrane
Vitelline
envelope
Cytoplasm
a.
Nucleus
of egg
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Sperm
Jelly layer
Plasma
membrane
Vitelline
envelope
Cytoplasm
Cortical granules
a.
Nucleus
of egg
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Sperm
Oocyte
Granulosa
cell
c.
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
1.2 µm
3.3 µm
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Sperm
Granulosa cell
Oocyte
Granulosa
cell
c.
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
1.2 µm
3.3 µm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sperm
Granulosa cell
Zona pellucida
Oocyte
Granulosa
cell
c.
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
1.2 µm
3.3 µm
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Sperm
Granulosa cell
Zona pellucida
Oocyte
Plasma
membrane
Granulosa
cell
c.
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
1.2 µm
3.3 µm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sperm
Granulosa cell
Zona pellucida
Oocyte
Plasma
membrane
Granulosa
cell
First
polar
body
b.
c.
d.
c-d: © David M. Phillips/Visuals Unlimited
1.2 µm
3.3 µm
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Sperm
Granulosa cell
Zona pellucida
Oocyte
Plasma
membrane
Granulosa
cell
First
polar
body
c.
1.2 µm
Cytoplasm
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
3.3 µm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Granulosa cell
Sperm
Zona pellucida
Oocyte
Plasma
membrane
Granulosa
cell
First
polar
body
Cortical granules
c.
1.2 µm
Cytoplasm
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
3.3 µm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sperm
Granulosa cell
Sperm
Jelly layer
Zona pellucida
Plasma
membrane
Oocyte
Plasma
membrane
Vitelline
envelope
Granulosa
cell
First
polar
body
Cytoplasm
1.2 µm
Nucleus
of egg
Cortical granules
a.
c.
Cortical granules
Cytoplasm
b.
d.
c-d: © David M. Phillips/Visuals Unlimited
3.3 µm
Fertilization
1. Sperm penetrates
2. Some of the zona
4. The sperm nucleus
3. Sperm and egg
between granulosa
pellucida is degraded
dissociates and
plasma membranes
cells.
by acrosomal enzymes.
enters cytoplasm.
fuse.
Plasma
membrane
Granulosa
cells
Zona
pellucida
Cortical
granules
6. Additional sperm
can no longer penetrate
the zona pellucida.
5. Cortical granules
release enzymes that
harden zona pellucida
and strip it of sperm
receptors. Hyalin
attracts water by osmosis.
7. Sperm and egg
pronuclei are
enclosed in a
nuclear envelope.
20
Fertilization
• Membrane fusion
– Egg activation
• Dramatic increase in the levels of free intracellular
Ca2+ ions in the egg shortly after the sperm makes
contact with the egg’s plasma membrane
• Act as second messengers to initiate changes
– Block to polyspermy
• Rapid transient change in membrane potential
• Cortical granules remove sperm receptors
• Vitelline envelope lifts off – fertilization envelope
21
Fertilization
• Sperm penetration has three other effects
– Triggers the egg to complete meiosis
– Triggers a cytoplasmic rearrangement
– Causes a sharp increase in protein synthesis and
metabolic activity in general
Primary Oocyte
First Metaphase of Meiosis Second Metaphase of Meiosis
Diploid nucleus
Meiosis Complete
Polar
bodies
Polar
body
Female
pronucleus
(haploid)
• Roundworms (Ascaris)
• Polychaete worms (Myzostoma)
• Clam worms (Nereis)
• Clams (Spisula)
• Nemertean worms (Cerebratulus)
• Polychaete worms (Chaetopterus)
• Mollusks (Dentalium)
• Many insects
• Sea stars
• Lancelets (Branchiostoma)
• Amphibians
• Mammals
• Fish
• Cnidarians
• Sea urchins
22
Fertilization
• Fusion of nuclei
– 3rd and final stage of fertilization
– Haploid sperm and haploid egg nuclei fuse to
form diploid nucleus of the zygote
23
Cleavage
• Rapid division of the zygote into a larger
and larger number of smaller and smaller
cells (blastomeres)
• Not accompanied by an increase in the
overall size of the embryo
• Animal pole
– Forms external tissues
• Vegetal pole
– Forms internal tissues
24
Cleavage
• Blastula
– Hollow ball of cells
– Blastocoel – fluid-filled cavity
• Cleavage patterns are quite diverse
– Relative amount of nutritive yolk in the egg is
the characteristic that most affects the
cleavage pattern of an animal embryo
– Vertebrates exhibit a variety of reproductive
strategies involving different patterns of yolk
utilization
25
Cleavage Patterns
• Eggs with little or no yolk
– Holoblastic cleavage
– Invertebrates, amphibians, mammals
• Eggs with large amounts of yolk
– Meroblastic cleavage
– Embryo forms thin cap on yolk
Sea Urchin
Frog
Chicken
Animal pole
Nucleus
Cytoplasm
Cytoplasm
Shell
Nucleus
Air
bubble
Nucleus
Plasma
membrane
Albumen
Yolk
Yolk
Vegetal pole
a.
b.
Yolk
26
c.
Holoblastic cleavage
Meroblastic cleavage
27
28
Cleavage Patterns
• Mammalian eggs contain very little yolk
– Undergo holoblastic cleavage
– Form a blastocyst composed of
• Trophoblast
– Outer layer of cells
– Contributes to the placenta
• Blastocoel
– Central fluid-filled cavity
• Inner cell mass
– Located at one pole
– Forms the developing embryo
29
Cleavage Patterns
ICM
Blastocoel
Blastodisc
Yolk
Trophoblast
30
Fate of Blastomeres
• In many animals removal of committed cells
results in embryos deficient in tissues that would
have developed from those tissues
• In mammals, early blastomeres do not appear to
be committed to a particular fate
– Cell removed for preimplantation genetic diagnosis
– Split embryos form identical twins
• In mammals, body form determined primarily by
cell-to-cell interactions
31
Gastrulation
• Process involving a complex series of cell shape
changes and cell movements that occurs in the
blastula
• Establishes the basic body plan and creates the
three primary germ layers
– Ectoderm – Exterior
• Epidermis of skin, nervous system, sense organs
– Mesoderm – Middle
• Skeleton, muscles, blood vessels, heart, blood, gonads,
kidneys, dermis of skin
– Endoderm – Inner
• Lining of digestive and respiratory tracts, liver, pancreas,
thymus, thyroid
32
Gastrulation
• Cells move during gastrulation using a
variety of cell shape changes
– Cells that are tightly attached to each other
via junctions will move as cell sheets
– Invagination – Cell sheet dents inward
– Involution – Cell sheet rolls inward
– Delamination – Cell sheet splits in two
– Ingression – Cells break away from cell sheet
and migrate as individual cells
33
Gastrulation Patterns
• Vary according to the amount of yolk
• Gastrulation in sea urchins
– Develop from relatively yolk-poor eggs
– Form hollow symmetrical blastulas
– Deuterostome – anus develops first and
mouth second
34
Animal pole
Ectoderm
Future
ectoderm
Ectoderm
Blastocoel
Primary
mesenchyme
cells (PMC’s)
Vegetal pole
a.
Filopodia
Archenteron
PMC
Future
endoderm
Blastopore
b.
Anus
c.
35
Gastrulation Patterns
• Gastrulation in frogs
– Asymmetrical yolk distribution
– Yolk-laden cells of the vegetal pole are less
numerous but much larger than the yolk-free
cells of the animal pole
• Makes gastrulation more complex
36
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Animal pole
Dorsal lip
Ectoderm
Ectoderm
Mesoderm
Archenteron
Endoderm
Ectoderm
Archenteron
Blastocoel
Blastocoel
Mesoderm
Vegetal pole
a.
Blastocoel
Yolk plug
Dorsal lip of
blastopore
Ventral lip
b.
c.
Neural plate Neural fold
Neural plate
37
d.
e.
Gastrulation Patterns
• Gastrulation in birds
– At the end of cleavage in a bird or reptile, the
developing embryo is a small cap of cells
called the blastoderm
• Sits on top of the large ball of yolk
– Upper layer of the blastoderm gives rise to all
three germ layers
38
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Blastoderm
Yolk
Blastocoel
Yolk
Primitive streak
Mesoderm
Ectoderm
Endoderm
Yolk
39
Gastrulation Patterns
• Gastrulation in mammals
– Proceeds similarly to that in birds
– Embryo develops from inner cell mass
– Embryo gastrulates as if it was sitting in a ball
of yolk
• Embryo obtains nutrition from placenta
40
Gastrulation Patterns
Inner cell mass
Primitive streak
Amniotic cavity
Ectoderm
Ectoderm
Mesoderm
Formation of
yolk sac
Trophoblast
a.
b.
Endoderm
c.
Endoderm
d.
41
Extraembryonic Membranes
• Adaptation to life on dry land
– Reptiles, birds, and mammals
– Amniotic species developed
• Extraembryonic membranes
– Amnion, chorion, yolk sac, and allantois
• Nourish and protect the developing
embryo
42
Extraembryonic Membranes
• Amnion
– Encloses amniotic fluid
• Chorion
– Located near eggshell in birds
– Contributes to the placenta in mammals
• Yolk sac
– Food source in bird embryos
– Found in mammals, but it is not nutritive
• Allantois
– Unites with chorion in birds, forming a structure used for gas
exchange
– In mammals, it contributes blood vessels to the developing
umbilical cord
43
Extraembryonic Membranes
Chick Embryo
Mammal Embryo
Chorion
Amnion
Chorion
Yolk sac
Amnion
Umbilical
blood
vessels
Yolk sac
Villus of chorion
frondosum
Allantois
Maternal blood
a.
b.
44
Organogenesis
• Formation of organs in their proper
locations
• Occurs by interaction of cells within and
between the three germ layers
• Thus, it follows rapidly on the heels of
gastrulation
– In many animals it begins before gastrulation
is complete
45
Organogenesis
• To a large degree, a cell’s location in the
developing embryo determines its fate
• At some stage, every cell’s ultimate fate
becomes fixed – cell determination
• A cell’s fate can be established by
– Inheritance of cytoplasmic determinants
– Interactions with neighboring cells
• Induction
46
Organogenesis in Drosophila
• Salivary gland development
– Sex combs reduced (scr) gene is a homeotic gene in
the Antennapedia complex
• Prior to organogenesis, it is expressed in an anterior band of
cells
– At the same time, Decapentaplegic protein (Dpp) is
released from dorsal cells
• Forms a gradient in the dorsal–ventral direction
• Determines ventral position of salivary glands
– During organogenesis, salivary glands develop in
areas where Scr is expressed and Dpp is absent
47
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Prior to Organogenesis
Dpp
a.
During Organogenesis
Salivary
gland
Labium
b.
48
Organogenesis in Vertebrates
• Begins with the formation of two structures
unique to chordates
– Notochord
– Dorsal nerve cord – neurulation
49
Development of Neural Tube
• Notochord
– Forms from mesoderm
– Region of dorsal ectodermal cells situated
above notochord thickens to form the neural
plate
– Cells of the neural plate fold together to form
a long hollow cylinder, the neural tube
– Will become brain and spinal cord
50
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Neural plate
Amniotic
cavity
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
a.
Neural groove
Neural fold
Ectoderm
Notochord
Mesoderm
Endoderm
b.
Neural tube
Ectoderm
Neural crest
Mesoderm
Endoderm
Somite
51
c.
Generation of Somites
• While the neural tube is forming from dorsal ectoderm,
the rest of the basic architecture of the body is being
rapidly established by changes in the mesoderm
• Sheets of mesoderm on either side of the developing
notochord separate into a series of rounded regions
called somitomeres
• Somitomeres then separate into segmented blocks
called somites
• Cells at the presumptive boundary regions in the
presomitic mesoderm instruct cells anterior to them to
condense and separate into somites at certain times
– Contact-mediated signaling
52
Generation of Somites
• Mesoderm in the head region remains
connected as somitomeres
– Form muscles of the face, jaws, and throat
• Some body organs develop within a strip of
mesoderm lateral to each row of somites
– Kidneys, adrenal glands, and gonads
• Remainder of mesoderm moves out to surround
the endoderm completely
• Mesoderm separates into two layers
– Coelom forms in between
53
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Chordamesoderm
Notochord
Intermediate
mesoderm
Kidney
Gonads
Circulatory
system
Lateral plate
mesoderm
Linings of
body cavities
Extraembryonic
Head
Paraxial
mesoderm
Somite
Cartilage
Skeletal
muscle
Dermis
54
Neural Crest Cells
• Neurulation occurs in all chordates
– In vertebrates it is accompanied by an
additional step
• Just before the neural groove closes to
form the neural tube, its edges pinch off,
forming a small cluster of cells called the
neural crest
• These cells migrate to colonize many
different regions of developing embryo
55
Neural Crest Cells
• Neural crest cells migrate along pathways
– Cranial neural crest cells are anterior cells that
migrate into the head and neck
• Contribute to skeletal and connective tissues of the face and
skull, as well as differentiating into nerve and glial cells of the
nervous system, and melanocyte pigment cells
– Trunk neural crest cells are posterior cells that
migrate in two pathways
• Ventral pathway cells differentiate into sensory neurons and
Schwann cells
• Lateral pathway cells differentiate into melanocytes of the
skin
56
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Epidermis
Neural tube
Posterior
Lateral Pathway
Cells take a
dorsolateral
route between
the epidermis
and somites
Neural
crest cells
Anterior Aorta Notochord
a.
Posterior
somite
Anterior somite
Ventral Pathway
Cells travel
ventrally
through the
anterior half
of each somite
Ventral Pathway Cell Fates Lateral Pathway Cell Fates
Dorsal root
ganglia
Ventral root
Schwann
cells
Melanocytes
Sympathetic
ganglia
Adrenal
medulla
b.
57
Neural Crest Cells
• Many of the unique vertebrate adaptations that
contribute to their varied ecological roles involve
neural crest derivatives
• For example, gill chambers provided a greatly
improved means of gas exchange
• Allowed transition from filter feeding to active
predation (higher metabolic rate)
• Other changes
– Better prey detection, and rapid response to sensory
information
58
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Chordates
Vertebrates
Zygote
Pharynx
Lining of
respiratory
tract
Lining of
digestive
tract
Endoderm
Blastula
Gastrula
Ectoderm
Neural
crest
Gill arches,
sensory ganglia,
Schwann cells,
adrenal medulla
Liver
Mesoderm
Outer covering
of internal
organs
Lining of
thoracic and
abdominal
cavities
Dorsal
nerve cord
Epidermis, skin, hair,
epithelium, inner
ear, lens of eye
Major
glands
Pancreas
Brain,
spinal cord,
spinal nerves
Notochord
Circulatory
system
Integuments
Blood
Heart
Vessels
Somites
Gonads
Kidney
Dermis
Skeleton
Striated
muscles
59
Vertebrate Axis Formation
• Hans Spemann and Hilde Mangold transplanted
cells of the dorsal lip of one embryo into the
future belly region of another
• Some of the embryos developed two
notochords: a normal dorsal one, and a second
one along the belly
• Moreover, a complete set of dorsal axial
structures formed at the ventral transplantation
site in most embryos
• Transplanted donor cells acted as organizers
60
Vertebrate Axis Formation
Donor embryo
Recipient embryo
Primary
neural fold
Primary notochord, somites,
and neural development
Dorsal lip
Secondary
neural fold
Secondary
notochord, somites,
and neural
development
Primary embryo
Secondary embryo
61
Organizers
• An organizer is a cluster of cells that release
diffusible signal molecules which convey
positional information to other cells
– The closer a cell is to an organizer, the higher the
concentration of the signal molecule (morphogen) it
experiences
– Organizers and the diffusible morphogens that they
release are thought to be part of a widespread
mechanism for determining relative position and cell
fates during vertebrate development
62
Induction
• Primary induction occurs between the
three primary germ layers
– Differentiation of the central nervous system
during neurulation
• Secondary induction occurs between
tissues that have already been specified to
develop along a particular pathway
– Development of the lens of the vertebrate eye
63
Wall of forebrain
Ectoderm
Optic cup
Lens
vesicle
Neural
cavity
Optic stalk
Lens
invagination
Lens
Optic nerve
Lens
Sensory
layer
Pigment
layer
• An extension of the optic stalk grows until it
contacts the surface ectoderm, where it induces
a section of the ectoderm to pinch off and form
the lens
• Other structures of the eye develop from the
optic stalk, with lens cells reciprocally inducing
the formation of photoreceptors in the optic cup
64
Retina
Human Development
• Human development from fertilization to
birth takes an average of 266 days, or
about 9 months
– Commonly divided into three periods called
trimesters
65
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or display.
First Trimester
• First month
a.
© Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell Publishing
Company
– Zygote undergoes its first cleavage about 30
hr after fertilization
– By the time the embryo reaches the uterus,
6–7 days after fertilization, it has differentiated
into a blastocyst
– Trophoblast cells digest their way into the
endometrium in the process known as
implantation
66
First Trimester
• First month
– During the second week, the developing chorion and
mother’s endometrium engage to form the placenta
• Mom and baby’s blood come into close proximity, but do not
mix – gases are exchanged
• One hormone released by the placenta is human chorionic
gonadotropin (hCG)
–
–
–
–
Gastrulation occurs in the second week
Neurulation occurs in the third week
Organogenesis begins in the fourth week
Embryo is 5 mm in length
67
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Chorion
Amnion
Yolk sac
Umbilical
cord
Chorionic
frondosum
(fetal)
Decidua
basalis
(maternal)
Placenta
Umbilical artery
Umbilical vein
Uterine wall
a.
68
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reproduction or display.
First Trimester
• Second month
– Organogenesis continues
– Miniature limbs assume adult shape
– All major organs in the body established
– Embryo grows to about 25 mm in length
– Weighs about 1 g, and looks distinctly human
– 9th week marks the transition from embryo to
fetus
b.
© Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell Publishing
Company
69
Copyright © The McGraw-Hill Companies, Inc. Permission
required for reproduction or display.
First Trimester
• Third month
– Nervous system develops
c.
– Limbs start to move
– Secretion of hCG by the placenta declines,
and so corpus luteum degenerates
– Placenta takes over hormone secretion
© Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is
Born, Dell Publishing Company
70
Increasing Hormone Concentration
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hCG
Estrogen
Progesterone
0
1
2
3
4
5
6
Months of Pregnancy
7
8
9
71
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reproduction or display.
Second Trimester
d.
© Lennart Nilsson/Albert Bonniers Förlag AB, A Child Is Born, Dell
Publishing Company
• The basic body plan develops further
• Bones actively enlarge in fourth month
• Rapid fetal heartbeat can be heard by a
stethoscope
• By the end of the sixth month, fetus is over
300 mm long, and weighs 600 g
72
Third Trimester
• A period of growth and organ maturation
• Weight of the fetus doubles several times
• Most of the major nerve tracts in the brain
are formed
• Brain continues to develop and produce
neurons for months after birth
73
Birth
• Estrogen stimulates mother’s uterus to
release prostaglandins, and produce more
oxytocin receptors
– Prostaglandins begin uterine contractions
– Sensory feedback from uterus stimulates
oxytocin release from posterior pituitary
• Oxytocin and prostaglandins further
stimulate uterine contractions
74
Birth
• Strong contractions, aided by the mother’s
voluntary pushing, expel the fetus
• Now called a newborn baby, or neonate
• After birth, continuing uterine contractions
expel the placenta and associated
membranes
– Collectively called the afterbirth
75
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Intestine
Placenta
Umbilical
cord
Wall of
uterus
Cervix
Vagina
76
Nursing
• Milk production (lactation) occurs in alveoli
of mammary glands when stimulated by
the anterior pituitary hormone prolactin
• During pregnancy, the mammary glands
are prepared for, but prevented from,
lactating
77
Nursing
• After birth
– Prolactin – stimulates the mammary alveoli to
produce milk
– Suckling triggers posterior pituitary to release
oxytocin
• Stimulates contraction of smooth muscles
surrounding alveolar ducts
• Milk is ejected (milk let-down reflex)
• The first milk produced after birth, colostrum, is
rich in nutrients and maternal antibodies
78
Postnatal Development
• Growth of the infant continues rapidly after
birth
• Babies typically double their birth weight
within 2 months
• Different components grow at different
rates
– Allometric growth
79
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