Download 04-24-06

Nervous System Part 2
IB-202-15
4-24-06
Chapt 48 pp 1022-1028, 1036 (memory),
1040-1041 (Alzheimer’s and Parkinson’s
disease)
Direct Synaptic Transmission
• The process of direct synaptic transmission
– Involves the binding of neurotransmitters to ligandgated ion channels
• Neurotransmitter binding
– Causes the ion channels to open, generating a
postsynaptic potential
• After its release from channel, the
neurotransmitter
– Diffuses out of the synaptic cleft
– May be taken up by surrounding cells and degraded
by enzymes
• Major neurotransmitters
Table 48.1
Acetylcholine
• Acetylcholine
– Is one of the most common neurotransmitters
in both vertebrates and invertebrates.
Transmitter for neuromuscular synapses in
vertebrates (skeletal muscle).
– Can be inhibitory or excitatory with other
types of muscle.
Biogenic Amines
• Biogenic amines
– Include epinephrine (adrenalin),
norepinephrine, dopamine, and serotonin
– Are active in the CNS and peripheral nervous
system (PNS)
• Various amino acids and peptides
– Are active in the brain
Gases
• Gases such as nitric oxide and carbon
monoxide
– Are local regulators in the PNS
• Concept 48.5: The vertebrate nervous
system is regionally specialized
• In all vertebrates, the nervous system
– Shows a high degree of cephalization and
distinct CNS and PNS components
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Figure 48.19
• The brain provides the integrative power
– That underlies the complex behavior of vertebrates
• The spinal cord integrates simple responses to
certain kinds of stimuli
– And conveys information to and from the brain
• The central canal of the spinal cord and the
four ventricles of the brain
– Are hollow, since they are derived from the
dorsal embryonic nerve cord
Gray matter
Grey matter is
unmylinated axons,
dendrites and nerve
bodies.
Figure 48.20
White
matter
Ventricles
Mylinated
axons
interconnecting
parts of brain
and nerve
tracks to spinal
cord
The Peripheral Nervous System
• The PNS transmits information to and from the
CNS
– And plays a large role in regulating a vertebrate’s
movement and internal environment
• The cranial nerves originate in the brain
– And terminate mostly in organs of the head and upper
body
• The spinal nerves originate in the spinal cord
– And extend to parts of the body below the head
• The PNS can be divided into two functional
components
– The somatic nervous system and the
autonomic nervous system
Peripheral
nervous system
Somatic
nervous
system
Somatic largely
voluntary control
of muscle in
response to
external stimuli
Figure 48.21
Autonomic
nervous
system
Sympathetic
division
Autonomic regulates the
internal environment in
an involuntary manner.
Parasympathetic
division
Enteric
division
• The sympathetic and parasympathetic
divisions
– Have antagonistic effects on target organs
Parasympathetic division
Sympathetic division
Action on target organs:
Location of
preganglionic neurons:
brainstem and sacral
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Action on target organs:
Dilates pupil
of eye
Constricts pupil
of eye
Inhibits salivary
gland secretion
Stimulates salivary
gland secretion
Constricts
bronchi in lungs
Sympathetic
ganglia
Cervical
Accelerates heart
Slows heart
Location of
postganglionic neurons:
in ganglia close to or
within target organs
Stimulates activity
of stomach and
intestines
Stimulates
gallbladder
Thoracic
Inhibits activity
of pancreas
Stimulates glucose
release from liver;
inhibits gallbladder
Promotes emptying
of bladder
Figure 48.22
Location of
postganglionic neurons:
some in ganglia close to
target organs; others in
a chain of ganglia near
spinal cord
Lumbar
Stimulates
adrenal medulla
Promotes erection
of genitalia
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Inhibits activity of
stomach and intestines
Stimulates activity
of pancreas
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Relaxes bronchi
in lungs
Location of
preganglionic neurons:
thoracic and lumbar
segments of spinal cord
Inhibits emptying
of bladder
Synapse
Sacral
Promotes ejaculation and
vaginal contractions
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
• The sympathetic division
– Correlates with the “fight-or-flight” response
• The parasympathetic division
– Promotes a return to self-maintenance functions
• The enteric division
– Controls the activity of the digestive tract, pancreas,
and gallbladder
Embryonic Development of the Brain
• In all vertebrates
– The brain develops from three embryonic regions: the
forebrain, the midbrain, and the hindbrain
Embryonic brain regions
Forebrain
Midbrain
Hindbrain
Midbrain
Hindbrain
Forebrain
Figure 48.23a
(a) Embryo at one month
• By the fifth week of human embryonic
development
– Five brain regions have formed from the three
embryonic regions
Embryonic brain regions
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Mesencephalon
Metencephalon
Diencephalon
Myelencephalon
Spinal cord
Telencephalon
Figure 48.23b
(b) Embryo at five weeks
• As a human brain develops further
– The most profound change occurs in the
forebrain, which gives rise to the cerebrum
Brain structures present in adult
Cerebrum (cerebral hemispheres; includes cerebral
cortex, white matter, basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem),
cerebellum
Medulla oblongata (part of brainstem)
Cerebral hemisphere
Diencephalon:
Hypothalamus
Thalamus
Pineal gland
(part of epithalamus)
Brainstem:
Midbrain
Pons
Pituitary
gland
Spinal cord
Cerebellum
Central canal
Figure 48.23c
(c) Adult
Medulla
oblongata
• In humans, the largest and most complex
part of the brain
– Is the cerebral cortex, where sensory
information is analyzed, motor commands are
issued, and language is generated
• Concept 48.6: The cerebral cortex controls
voluntary movement and cognitive
functions
• Each side of the cerebral cortex has four
lobes
– Frontal, parietal, temporal, and occipital
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Taste
Speech
Smell
Somatosensory
association
area
Reading
Hearing
Auditory
association
area
Visual
association
area
Vision
Figure 48.27
Temporal lobe
Occipital lobe
The Diencephalon
• The embryonic diencephalon develops into
three adult brain regions
– The epithalamus, thalamus, and hypothalamus
• The hypothalamus regulates
– Homeostasis
– Basic survival behaviors such as feeding,
fighting, fleeing, and reproducing
Memory and Learning
• The frontal lobes
– Are a site of short-term memory
– Interact with the hippocampus and amygdala
to consolidate long-term memory
• Many sensory and motor association areas
of the cerebral cortex
– Are involved in storing and retrieving words
and images
• Many sensory and motor association areas
of the cerebral cortex
– Are involved in storing and retrieving words
and images
Cellular Mechanisms of Learning
• Experiments on invertebrates
–
Have revealed the cellular basis of some types of learning
(a) Touching the siphon triggers a reflex that
causes the gill to withdraw. If the tail is
shocked just before the siphon is touched,
the withdrawal reflex is stronger. This
strengthening of the reflex is a simple form
of learning called sensitization.
Siphon
Mantle
Gill
Tail
Head
Figure 48.31a, b
(b) Sensitization involves interneurons that
make synapses on the synaptic terminals of
the siphon sensory neurons. When the tail
is shocked, the interneurons release
serotonin, which activates a signal
transduction pathway that closes K+
channels in the synaptic terminals of
the siphon sensory neurons. As a result,
action potentials in the siphon sensory
neurons produce a prolonged
depolarization of the terminals. That allows
more Ca2+ to diffuse into the terminals,
which causes the terminals to release more
of their excitatory neurotransmitter onto the gill
motor neurons. In response, the motor neurons
generate action potentials at a higher frequency,
producing a more forceful gill withdrawal.
Gill withdrawal pathway
Touching
the siphon
Siphon sensory
neuron
Gill motor
neuron
Sensitization pathway
Shocking
the tail
Interneuron
Tail sensory
neuron
Gill
• In the vertebrate brain, a form of learning
called long-term potentiation (LTP)
– Involves an increase in the strength of
synaptic transmission
1 The presynaptic
neuron releases glutamate.
2 Glutamate binds to AMPA
receptors, opening the AMPAreceptor channel and depolarizing
the postsynaptic membrane.
PRESYNAPTIC NEURON
7 NO diffuses into the
presynaptic neuron, causing
it to release more glutamate.
NO
6 Ca2+ stimulates the
postsynaptic neuron to
produce nitric oxide (NO).
Glutamate
AMPA receptor
NO
Figure 48.32
5 Ca2+ initiates the phosphorylation of AMPA receptors,
making them more responsive.
Ca2+ also causes more AMPA
receptors to appear in the
postsynaptic membrane.
NMDA
receptor
3 Glutamate also binds to NMDA
receptors. If the postsynaptic
membrane is simultaneously
depolarized, the NMDA-receptor
channel opens.
P
Ca2+
Signal transduction pathways
POSTSYNAPTIC NEURON
4 Ca2+ diffuses into the
postsynaptic neuron.
Alzheimer’s Disease
• Alzheimer’s disease (AD)
– Is a mental deterioration characterized by
confusion, memory loss, and other symptoms
• AD is caused by the formation of
– Neurofibrillary tangles and senile plaques of
protein in the brain
Senile plaque
Figure 48.35
Neurofibrillary tangle
20 m
Parkinson’s Disease
• Parkinson’s disease is a motor disorder
– Caused by the death of dopamine-secreting
neurons in the mid-brain. It is characterized
by difficulty in initiating movements,
slowness of movement, and rigidity
– Transplantation of stem cells that appear to
transform into dopamine-secreting cells
alleviate the symptoms but thus far no success
in humans
Sensory and Motor Mechanisms
• Chapt 49 (pp 1063-1074)
• Concept 49.5: Animal skeletons function in
support, protection, and movement
• The various types of animal movements
– All result from muscles working against some
type of skeleton
Types of Skeletons
• The three main functions of a skeleton are
– Support, protection, and movement
• The three main types of skeletons are
– Hydrostatic skeletons, exoskeletons, and
endoskeletons
Endoskeletons
• An endoskeleton consists of hard supporting
elements
– Such as bones, buried within the soft tissue of an
animal
• Endoskeletons
– Are found in sponges, echinoderms, and chordates
• The mammalian skeleton is built from more
than 200 bones
– Some fused together and others connected at
joints by ligaments that allow freedom of
movement
key
Axial skeleton
Appendicular
skeleton
Skull
Examples
of joints
Head of
humerus
Scapula
1
• The human skeleton
Shoulder
girdle
Clavicle
Scapula
Sternum
Rib
Humerus
2
Vertebra
3
Radius
Ulna
Pelvic
girdle
1 Ball-and-socket joints, where the humerus contacts
the shoulder girdle and where the femur contacts the
pelvic girdle, enable us to rotate our arms and
legs and move them in several planes.
Humerus
Carpals
Phalanges
Ulna
Metacarpals
Femur
Patella
2 Hinge joints, such as between the humerus and
the head of the ulna, restrict movement to a single
plane.
Tibia
Fibula
Ulna
Figure 49.26
Tarsals
Metatarsals
Phalanges
Radius
3 Pivot joints allow us to rotate our forearm at the
elbow and to move our head from side to side.
• The action of a muscle is always to contract
• Skeletal muscles are attached to the skeleton in
antagonistic pairs even with exoskeletons
– With each member of the pair working against each
other
Human
Grasshopper
Extensor
muscle
relaxes
Biceps
contracts
Triceps
relaxes
Extensor
muscle
contracts
Forearm
extends
Triceps
contracts
Flexor
muscle
contracts
Forearm
flexes
Biceps
relaxes
Figure 49.27
Tibia
flexes
Tibia
extends
Flexor
muscle
relaxes
Vertebrate Skeletal Muscle
• Vertebrate skeletal muscle
– Is characterized by a hierarchy of smaller and smaller
units
Muscle
Bundle of
muscle fibers
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Sarcomere
Z line
Light
band
Dark band
Sarcomere
0.5 m
TEM
I band
A band
I band
M line
Thick
filaments
(myosin)
Figure 49.28
Thin
filaments
(actin)
Z line
H zone
Sarcomere
Z line
Muscle fiber
composed of
many individual
embryonic muscle
cells fused end to
end. Note many
nuclei.
• A skeletal muscle consists of a bundle of long
fibers
– Running parallel to the length of the muscle
• A muscle fiber
– Is itself a bundle of smaller myofibrils arranged
longitudinally
• The myofibrils are composed to two kinds of
myofilaments
– Thin filaments, consisting of two strands of actin
and one strand of regulatory protein
– Thick filaments, staggered arrays of myosin
molecules
• Skeletal muscle is also called striated
muscle
– Because the regular arrangement of the
myofilaments creates a pattern of light and
dark bands
The Sliding-Filament Model of
Muscle Contraction
• According to the sliding-filament model of
muscle contraction
– The filaments slide past each other
longitudinally, producing more overlap
between the thin and thick filaments
• As a result of this sliding
– The I band and the H zone shrink
Correlation of structure as
seen with the electron
microscope and function.
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands
and H zone are relatively wide.
(b) Contracting muscle fiber. During contraction, the thick and
thin filaments slide past each other, reducing the width of the
I bands and H zone and shortening the sarcomere.
Figure 49.29a–c
(c) Fully contracted muscle fiber. In a fully contracted muscle
fiber, the sarcomere is shorter still. The thin filaments overlap,
eliminating the H zone. The I bands disappear as the ends of
the thick filaments contact the Z lines.
0.5 m
Z
H
A
Sarcomere
• The sliding of filaments is based on
– The interaction between the actin and myosin
molecules of the thick and thin filaments
• The “head” of a myosin molecule binds to an
actin filament
– Forming a cross-bridge and pulling the thin
filament toward the center of the sarcomere
• Myosin-actin interactions underlying
muscle fiber contraction
Thick filament
1 Starting here, the myosin head is
bound to ATP and is in its lowenergy confinguration.
Thin filaments
5 Binding of a new molecule of ATP releases the
myosin head from actin,
and a new cycle begins.
Thin filament
Myosin head (lowenergy configuration)
ATP
ATP
Thick
filament
Thin filament moves
toward center of sarcomere.
Figure 49.30
+
4 Releasing ADP and ( P i), myosin
relaxes to its low-energy configuration,
sliding the thin filament.
Actin
Pi
ADP
Pi
Cross-bridge
binding site
ADP
Myosin head (lowenergy configuration)
ADP
2 The myosin head hydrolyzes
ATP to ADP and inorganic
phosphate ( P I ) and is in its
high-energy configuration.
Pi
Cross-bridge
Myosin head (highenergy configuration)
13 The myosin head binds to
actin, forming a crossbridge.
The Role of Calcium and Regulatory
Proteins
• A skeletal muscle fiber contracts only when
stimulated by a motor neuron
• When a muscle is at rest the myosin-binding sites
on the thin filament are blocked by the regulatory
protein tropomyosin
Tropomyosin
Actin
Figure 49.31a
Ca2+-binding sites
(a) Myosin-binding sites blocked
Troponin complex
• For a muscle fiber to contract the myosinbinding sites must be uncovered
• This occurs when calcium ions (Ca2+) bind
to another set of regulatory proteins, the
troponin complex
Ca2+
Myosinbinding site
Figure 49.31b
(b) Myosin-binding sites exposed
• The stimulus leading to the contraction of a
skeletal muscle fiber
– Is an action potential in a motor neuron that
makes a synapse with the muscle fiber
Motor
neuron axon
Mitochondrion
Synaptic
terminal
T tubule
Sarcoplasmic
reticulum
Ca2+ released
from sarcoplasmic
reticulum
Myofibril
Figure 49.32
Plasma membrane
of muscle fiber
Sarcomere
Skip to figure!
• The synaptic terminal of the motor neuron
– Releases the neurotransmitter acetylcholine,
depolarizing the muscle and causing it to
produce an action potential
• Action potentials travel to the interior of the
muscle fiber
– Along infoldings of the plasma membrane called
transverse (T) tubules
• The action potential along the T tubules
– Causes the sarcoplasmic reticulum to release Ca2+
• The Ca2+ binds to the troponin-tropomyosin
complex on the thin filaments
– Exposing the myosin-binding sites and allowing the
cross-bridge cycle to proceed
Calcium as a regulator of muscle contraction!
Synaptic
terminal
of motor
neuron
1 Acetylcholine (ACh) released by synaptic terminal diffuses across synaptic
cleft and binds to receptor proteins on muscle fiber’s plasma membrane,
triggering an action potential in muscle fiber.
Synaptic cleft
ACh
2 Action potential is propagated along plasma
membrane and down
T tubules.
SR
3 Action potential
triggers Ca2+
release from sarcoplasmic reticulum
(SR).
Ca2
7 Tropomyosin blockage of myosinbinding sites is restored; contraction
ends, and muscle fiber relaxes.
Ca2
CYTOSOL
ADP
P2
Figure 49.33
PLASMA MEMBRANE
T TUBULE
4 Calcium ions bind to troponin;
troponin changes shape,
removing blocking action
of tropomyosin; myosin-binding
sites exposed.
2+
6 Cytosolic Ca is
removed by active
transport into
SR after action
potential ends.
5 Myosin cross-bridges alternately attach
to actin and detach, pulling actin
filaments toward center of sarcomere;
ATP powers sliding of filaments.
Neural Control of Muscle
Tension
• Contraction of a whole muscle is graded
– Which means that we can voluntarily alter the extent
and strength of its contraction
• There are two basic mechanisms by which the
nervous system produces graded contractions of
whole muscles
– By varying the number of fibers that contract
– By varying the rate at which muscle fibers are
stimulated
• In a vertebrate skeletal muscle
– Each branched muscle fiber is innervated by
only one motor neuron
• Each motor neuron
– May synapse with multiple muscle fibers
Motor
unit 1
Spinal cord
Motor
unit 2
Synaptic terminals
Nerve
Motor neuron
cell body
Motor neuron
axon
Muscle
Muscle fibers
Figure 49.34
Tendon
• A motor unit
– Consists of a single motor neuron and all the
muscle fibers it controls
• Recruitment of multiple motor neurons
– Results in stronger contractions
• A muscle twitch results from a single action
potential in a motor neuron
• More rapidly delivered action potentials
produce a graded contraction by summation
• Tetanus is a state of smooth and sustained
contraction produced when motor neurons
deliver a volley of action potentials
Tension
Tetanus
Summation of
two twitches
Single
twitch
Action
potential
Figure 49.35
Time
Pair of
action
potentials
Series of action
potentials at
high frequency
Types of Muscle Fibers
• Skeletal muscle fibers are classified as slow
oxidative, fast oxidative, and fast glycolytic
– Based on their contraction speed and major
pathway for producing ATP
• Types of skeletal muscles
Other Types of Muscle
• Cardiac muscle, found only in the heart
– Consists of striated cells that are electrically
connected by intercalated discs
– Can generate action potentials without neural
input
• In smooth muscle, found mainly in the walls of
hollow organs
– The contractions are relatively slow and may be
initiated by the muscles themselves
• In addition, contractions may be caused by
– Stimulation from neurons in the autonomic nervous
system
• Concept 49.7: Locomotion requires energy to overcome
friction and gravity
• Movement is a hallmark of all animals
– And usually necessary for finding food or evading predator
• Overcoming friction is a major problem for swimmers
• Overcoming gravity is less of a problem for swimmers
than for animals that move on land or fly
Locomotion on Land
• Walking, running, hopping, or crawling on
land
– Requires an animal to support itself and move
against gravity
• Diverse adaptations for traveling on land
– Have evolved in various vertebrates
Figure 49.36
Comparing Costs of Locomotion
•The energy cost of locomotion
–Depends on the mode of locomotion and the
environment
EXPERIMENT
Physiologists typically determine an animal’s rate of energy use during locomotion by measuring
its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a
wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that
collects the air the bird exhales as it flies.
RESULTS
This graph compares the energy cost, in joules per kilogram of
body mass per meter traveled, for animals specialized for running, flying, and
swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.
CONCLUSION
Flying
Energy cost (J/Kg/m)
For animals of a given
body
mass, swimming is the most energyCONCLUSION
efficient and running the least energyefficient mode of locomotion. In any mode,
a small animal expends more energy per
kilogram of body mass than a large animal.
102
10
1
Swimming
10–1
10–3
Figure 49.37
Running
1
103
Body mass(g)
106
• Animals that are specialized for swimming
– Expend less energy per meter traveled than
equivalently sized animals specialized for
flying or running
Chapter 47
Animal Development
Read pages 987-992 and 994-995 for
information on sea urchin fertilization and
development.
It is difficult to imagine that each of us began life as a
single cell, a zygote
• A human embryo at approximately 6–8 weeks
after conception
– Shows the development of distinctive features
Head, with eye
plaque, internal
organs and tail.
Figure 47.1
1 mm
• The question of how a zygote becomes an
animal has been asked for centuries
• As recently as the 18th century
– The prevailing theory was a notion called
preformation
• Preformation is the idea that the egg or
sperm contains an embryo
– A preformed miniature infant, or
“homunculus,” that simply becomes larger
during development
We now know that animals
emerge gradually from a
formless egg in a series of
progressive steps as
determined by the genome
of the zygote.
Figure 47.2
• An organism’s development is determined by the
genome of the zygote and by differences that
arise between early embryonic cells. Two terms!
• Cell differentiation
– Is the specialization of cells in their structure and
function (ectodermal, endodermal and mesodermal
cells give rise to specific tissues and organs)
• Morphogenesis
– Is the process by which an animal takes shape
• Concept 47.1: After fertilization, embryonic
development proceeds through cleavage, gastrulation,
and organogenesis
• Important events regulating development
– Occur during fertilization and each of the three
successive stages that build the animal’s body
– Next week’s lab we will look at fertilization and
early development in the sea urchin.
Fertilization
• The main function of fertilization
– Is to bring the haploid nuclei of sperm and egg
together to form a diploid zygote
• Contact of the sperm with the egg’s surface
– Initiates metabolic reactions within the egg that
trigger the onset of embryonic development
Rapid events occur when sperm contacts
the egg!
1 Contact. The
sperm cell
contacts the
egg’s jelly coat,
triggering
exocytosis from the
sperm’s acrosome.
2 Acrosomal reaction. Hydrolytic
enzymes released from the
acrosome make a hole in the
jelly coat, while growing actin
filaments form the acrosomal
process. This structure protrudes
from the sperm head and
penetrates the jelly coat, binding
to receptors in the egg cell
membrane that extend through
the vitelline layer.
3 Contact and fusion of sperm
and egg membranes. A hole
is made in the vitelline layer,
allowing contact and fusion of
the gamete plasma membranes.
The membrane becomes
depolarized, resulting in the
fast block to polyspermy.
4 Entry of
sperm nucleus.
• The acrosomal reaction
Sperm plasma
membrane
You will be
able to see the
fertilization
envelope in lab.
Sperm
nucleus
Acrosomal
process
Basal body
(centriole)
Fertilization
envelope
Sperm
head
Actin
Acrosome
Jelly coat
Sperm-binding
receptors
Figure 47.3
5 Cortical reaction. Fusion of the
gamete membranes triggers an
increase of Ca2+ in the egg’s
cytosol, causing cortical granules
in the egg to fuse with the plasma
membrane and discharge their
contents. This leads to swelling of the
perivitelline space, hardening of the
vitelline layer, and clipping off
sperm-binding receptors. The resulting
fertilization envelope is the slow block
to polyspermy.
Fused plasma
Cortical membranes
granule
Perivitelline
Hydrolytic enzymes
space
Cortical granule
membrane
Vitelline layer
Egg plasma
membrane
EGG CYTOPLASM
• Gamete contact and/or fusion
– Depolarizes the egg cell membrane and sets up
a fast block to polyspermy (prevents other
sperm from entering egg).
The Cortical Reaction
• Fusion of egg and sperm also initiates the
cortical reaction inducing a rise in Ca2+ that
stimulates cortical granules to release their
contents outside the egg plasma membrane
EXPERIMENT
A fluorescent dye that glows when it binds free Ca2+ was injected into unfertilized sea urchin eggs. After sea urchin
sperm were added, researchers observed the eggs in a fluorescence microscope.
500 m
RESULTS
10 sec after
fertilization
1 sec before
fertilization
Point of
sperm
entry
Figure 47.4
20 sec
30 sec
Spreading wave
of calcium ions
CONCLUSION The release of Ca2+ from the endoplasmic reticulum into the cytosol at the site of sperm entry triggers the release
of more and more Ca2+ in a wave that spreads to the other side of the cell. The entire process takes about 30 seconds.
• These changes cause the formation of a
fertilization envelope
– That functions as a slow block to polyspermy
Activation of the Egg
• Another outcome of the sharp rise in Ca2+ in the
egg’s cytosol
– Is a substantial increase in the rates of cellular
respiration and protein synthesis by the egg cell
• With these rapid changes in metabolism
– The egg is said to be activated
• In a fertilized egg of a sea urchin, a model
organism
– Many events occur in the activated egg
1
Binding of sperm to egg
2
Acrosomal reaction: plasma membrane
depolarization (fast block to polyspermy)
3
4
6
8
10
Increased intracellular calcium level
20
Cortical reaction begins (slow block to polyspermy)
30
40
50
1
Formation of fertilization envelope complete
2
Increased intracellular pH
3
4
5
Increased protein synthesis
10
20
Fusion of egg and sperm nuclei complete
30
40
Onset of DNA synthesis
60
Figure 47.5
90
First cell division
Cleavage
• Fertilization is followed by cleavage
– A period of rapid cell division without growth
shown in the next slide.
Fertilization is followed by cleavage
-- rapid cell division without growth
• Cleavage partitions the cytoplasm of one large cell
– Into many smaller cells called blastomeres
(a) Fertilized egg. Shown here is the (b) Four-cell stage. Remnants of the (c) Morula. After further cleavage
mitotic spindle can be seen
divisions, the embryo is a
zygote shortly before the first
between
the
two
cells
that
have
multicellular ball that is still
cleavage division, surrounded
just
completed
the
second
surrounded by the fertilization
by the fertilization envelope.
cleavage
division.
envelope. The blastocoel cavity
The nucleus is visible in the
has begun to form.
center.
Figure 47.7a–d
(d) Blastula. A single layer of cells
surrounds a large blastocoel
cavity. Although not visible here,
the fertilization envelope is still
present. The blastula will next
undergo gastrulation.
Gastrulation
• The morphogenetic process called gastrulation
rearranges the cells of a blastula into a threelayered embryo, called a gastrula, that has a
primitive gut. Three germ layers develope.
Key
Future ectoderm
Future mesoderm
Future endoderm
Blastocoel
Mesenchyme
cells
Vegetal
plate
Sea urchin is a
deuterostome so
blastopore forms
the anus. New
opening for
mouth. Mesoderm
buds off from
endoderm.
Figure 47.11
1 The blastula consists of a single layer of ciliated cells surrounding the
blastocoel. Gastrulation begins with the migration of mesenchyme cells
from the vegetal pole into the blastocoel.
Animal
pole
Vegetal
pole
Blastocoel
2 The vegetal plate invaginates (buckles inward). Mesenchyme cells
migrate throughout the blastocoel.
Filopodia
pulling
archenteron
tip
3 Endoderm cells form the archenteron (future digestive tube). New
mesenchyme cells at the tip of the tube begin to send out thin
extensions (filopodia) toward the ectoderm cells of the blastocoel
wall (inset, LM).
Archenteron
Blastopore
Mesenchyme
cells
Blastocoel
50 µm
Archenteron
Ectoderm
Mesenchyme: Mouth
(mesoderm
forms future
skeleton)
Blastopore
Digestive tube (endoderm)
Anus (from blastopore)
4 Contraction of these filopodia then drags the archenteron across
the blastocoel.
5 Fusion of the archenteron with the blastocoel wall completes
formation of the digestive tube with a mouth and an anus. The
gastrula has three germ layers and is covered with cilia, which
function in swimming and feeding.
• The three layers produced by gastrulation
– Are called embryonic germ layers
• The ectoderm
– Forms the outer layer of the gastrula
• The endoderm
– Lines the embryonic digestive tract
• The mesoderm
– Partly fills the space between the endoderm and
ectoderm
• The eggs and zygotes of many animals, except
mammals
– Have a definite polarity
• The polarity is defined by the distribution of
yolk
– With the vegetal pole having the most yolk and the
animal pole having the least
• Holoblastic cleavage, the complete division
of the egg
– Occurs in species whose eggs have little or
moderate amounts of yolk, such as sea urchins
and frogs
• Cleavage planes usually follow a specific
pattern (Radial cleavage)
– That is relative to the animal and vegetal poles
of the zygote
Zygote
0.25 mm
2-cell
stage
forming
Because of large
amount of yolk the
animal pole cells
smaller!
Eight-cell stage (viewed from the animal pole). The large
amount of yolk displaces the third cleavage toward the animal pole,
forming two tiers of cells. The four cells near the animal pole
(closer, in this view) are smaller than the other four cells (SEM).
4-cell
stage
forming
8-cell
stage
0.25 mm
Animal pole
Figure 47.9
Blastula
(cross
section)
Vegetal pole
Blastocoel
Blastula (at least 128 cells). As cleavage continues, a fluid-filled
cavity, the blastocoel, forms within the embryo. Because of unequal
cell division due to the large amount of yolk in the vegetal
hemisphere, the blastocoel is located in the animal hemisphere, as
shown in the cross section. The SEM shows the outside of a
blastula with about 4,000 cells, looking at the animal pole.
• Meroblastic cleavage, incomplete division of the
egg. Occurs on the surface of the yolk!
– Occurs in species with yolk-rich eggs, such as reptiles
and birds
Fertilized egg
Disk of
cytoplasm
1 Zygote. Most of the cell’s volume is yolk, with a small disk
of cytoplasm located at the animal pole.
2 Four-cell stage. Early cell divisions are meroblastic
(incomplete). The cleavage furrow extends through the
cytoplasm but not through the yolk.
3 Blastoderm. The many cleavage divisions produce the
blastoderm, a mass of cells that rests on top of the yolk mass.
Cutaway view of the blastoderm. The cells of the
blastoderm are arranged in two layers, the epiblast
and hypoblast, that enclose a fluid-filled cavity, the
blastocoel.
Blastocoel
BLASTODERM
Figure 47.10
YOLK MASS
Epiblast
Hypoblast
In birds embryo forms on top of
huge yolk.
• Gastrulation
in the chick
Epiblast
– Is affected by the large amounts of yolk in the egg
Future
ectoderm
Primitive
streak
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Figure 47.13