Nervous System Part 2IB-202-154-24-06

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 ligand-gated 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

Figure 48.19

Central nervoussystem (CNS)

Peripheral nervoussystem (PNS)

Brain

Spinal cordCranialnerves

GangliaoutsideCNSSpinalnerves

• 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

Whitematter

Ventricles

Figure 48.20

Grey matter is unmylinated axons, dendrites and nerve bodies.

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

Peripheralnervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision

Figure 48.21

Somatic largely voluntary control of muscle in response to external stimuli

Autonomic regulates the internal environment in an involuntary manner.

• The sympathetic and parasympathetic divisions– Have antagonistic effects on target organs

Parasympathetic division Sympathetic division

Action on target organs: Action on target organs:

Location ofpreganglionic neurons:brainstem and sacralsegments of spinal cord

Neurotransmitterreleased bypreganglionic neurons:acetylcholine

Location ofpostganglionic neurons:in ganglia close to orwithin target organs

Neurotransmitterreleased bypostganglionic neurons:acetylcholine

Constricts pupilof eye

Stimulates salivarygland secretion

Constrictsbronchi in lungs

Slows heart

Stimulates activityof stomach and

intestines

Stimulates activityof pancreas

Stimulatesgallbladder

Promotes emptyingof bladder

Promotes erectionof genitalia

Cervical

Thoracic

Lumbar

Synapse

Sympatheticganglia

Dilates pupilof eye

Inhibits salivary gland secretion

Relaxes bronchiin lungs

Accelerates heart

Inhibits activity of stomach and intestines

Inhibits activityof pancreas

Stimulates glucoserelease from liver;inhibits gallbladder

Stimulatesadrenal medulla

Inhibits emptyingof bladder

Promotes ejaculation and vaginal contractionsSacral

Location ofpreganglionic neurons:thoracic and lumbarsegments of spinal cord

Neurotransmitterreleased bypreganglionic neurons:acetylcholine

Location ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near spinal cord

Neurotransmitterreleased bypostganglionic neurons:norepinephrine

Figure 48.22

• 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

Figure 48.23a

Forebrain

Midbrain

Hindbrain

Midbrain Hindbrain

Forebrain

(a) Embryo at one month

Embryonic brain regions

• By the fifth week of human embryonic development– Five brain regions have formed from the three

embryonic regions

Figure 48.23b

Telencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

(b) Embryo at five weeks

MesencephalonMetencephalon

Myelencephalon

Spinal cord

Diencephalon

Telencephalon

Embryonic brain regions

• As a human brain develops further– The most profound change occurs in the

forebrain, which gives rise to the cerebrum

Figure 48.23c

Brain structures present in adult

Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

(c) Adult

Cerebral hemisphereDiencephalon:

Hypothalamus

ThalamusPineal gland(part of epithalamus)

Brainstem:

Midbrain

Pons

Medullaoblongata

Cerebellum

Central canal

Spinal cord

Pituitarygland

• 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

Temporal lobe Occipital lobe

Parietal lobe

Frontalassociationarea

Speech

Smell

Hearing

Auditoryassociationarea

Vision

Visualassociationarea

Somatosensoryassociationarea

Reading

Speech

TasteS

omat

osen

sory

cor

tex

Mot

or c

orte

x

Figure 48.27

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

Figure 48.31a, b

(a) Touching the siphon triggers a reflex thatcauses the gill to withdraw. If the tail isshocked just before the siphon is touched,the withdrawal reflex is stronger. Thisstrengthening of the reflex is a simple formof learning called sensitization.

(b) Sensitization involves interneurons thatmake synapses on the synaptic terminals ofthe siphon sensory neurons. When the tailis shocked, the interneurons releaseserotonin, which activates a signaltransduction pathway that closes K+

channels in the synaptic terminals ofthe siphon sensory neurons. As a result,action potentials in the siphon sensoryneurons produce a prolongeddepolarization of the terminals. That allowsmore 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 neuronsgenerate action potentials at a higher frequency,producing a more forceful gill withdrawal.

Siphon

Mantle

Gill

Tail

Head

Gill withdrawal pathway

Touchingthe siphon

Shockingthe tail Tail sensory

neuron

Interneuron

Sensitization pathway

Siphon sensoryneuron

Gill motorneuron

Gill

• In the vertebrate brain, a form of learning called long-term potentiation (LTP)– Involves an increase in the strength of

synaptic transmission

Figure 48.32

PRESYNAPTIC NEURON

NO

Glutamate

NMDAreceptor

Signal transduction pathways

NO

Ca2+

AMPA receptor

POSTSYNAPTIC NEURON

Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.

5

Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).

6

The presynapticneuron releases glutamate.1

Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.

2

Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.

3

Ca2+ diffuses into thepostsynaptic neuron.

4

NO diffuses into thepresynaptic neuron, causing it to release more glutamate.

7

P

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

Figure 48.35

Senile plaque Neurofibrillary tangle20 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

• The human skeleton

Figure 49.26

1 Ball-and-socket joints, where the humerus contactsthe shoulder girdle and where the femur contacts thepelvic girdle, enable us to rotate our arms andlegs and move them in several planes.

2 Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.

3 Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.

keyAxial skeletonAppendicularskeleton

Skull

Shouldergirdle

Clavicle

Scapula

Sternum

RibHumerus

Vertebra

RadiusUlnaPelvicgirdle

Carpals

Phalanges

Metacarpals

Femur

Patella

Tibia

Fibula

TarsalsMetatarsalsPhalanges

1

Examplesof joints

2

3

Head ofhumerus

Scapula

Humerus

Ulna

UlnaRadius

• 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

Figure 49.27

Human Grasshopper

Bicepscontracts

Tricepsrelaxes

Forearmflexes

Bicepsrelaxes

Tricepscontracts

Forearmextends

Extensormusclerelaxes

Flexormusclecontracts

Tibiaflexes

Extensormusclecontracts

Flexormusclerelaxes

Tibiaextends

Vertebrate Skeletal Muscle• Vertebrate skeletal muscle

– Is characterized by a hierarchy of smaller and smaller units

Figure 49.28

Muscle

Bundle ofmuscle fibers

Single muscle fiber(cell)

Plasma membrane

Myofibril

Lightband Dark band

Z line

Sarcomere

TEM 0.5 mI band A band I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zoneSarcomere

Z lineZ line

Nuclei

Sarcomere

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

Figure 49.29a–c

(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.

(b) Contracting muscle fiber. During contraction, the thick andthin filaments slide past each other, reducing the width of theI bands and H zone and shortening the sarcomere.

(c) Fully contracted muscle fiber. In a fully contracted musclefiber, the sarcomere is shorter still. The thin filaments overlap,eliminating the H zone. The I bands disappear as the ends ofthe thick filaments contact the Z lines.

0.5 m

Z HA

Sarcomere

Correlation of structure as seen with the electron microscope and function.

• 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

Figure 49.30

Thick filament

Thin filaments

Thin filament

ATPATP

ADPADP

ADP

P i P i

P i

Cross-bridge

Myosin head (low-energy configuration)

Myosin head (high-energy configuration)

+

Myosin head (low-energy configuration)

Thin filament moves toward center of sarcomere.

Thick filament

ActinCross-bridge binding site

1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.

2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.

P

1 The myosin head binds toactin, forming a cross-bridge.

3

4 Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.

P

5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.

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

Figure 49.31a

ActinTropomyosin Ca2+-binding sites

Troponin complex

(a) Myosin-binding sites blocked

• For a muscle fiber to contract the myosin-binding sites must be uncovered

• This occurs when calcium ions (Ca2+) bind to another set of regulatory proteins, the troponin complex

Figure 49.31b

Ca2+

Myosin-binding site

(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

Figure 49.32

Motorneuron axon

Mitochondrion

Synapticterminal

T tubule

Sarcoplasmicreticulum

Myofibril

Plasma membraneof muscle fiber

Sarcomere

Ca2+ releasedfrom sarcoplasmicreticulum

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

ACh

Synapticterminalof motorneuron

Synaptic cleft T TUBULEPLASMA MEMBRANE

SR

ADP

CYTOSOL

Ca2

Ca2

P2

Cytosolic Ca2+ is removed by active transport into SR after action potential ends.

6

Calcium as a regulator of muscle contraction!

Figure 49.33

Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.

1

Action potential is propa-gated along plasmamembrane and downT tubules.

2

Action potentialtriggers Ca2+

release from sarco-plasmic reticulum(SR).

3

Myosin cross-bridges alternately attachto actin and detach, pulling actinfilaments toward center of sarcomere;ATP powers sliding of filaments.

5

Calcium ions bind to troponin;troponin changes shape,removing blocking actionof tropomyosin; myosin-bindingsites exposed.

4

Tropomyosin blockage of myosin-binding sites is restored; contractionends, and muscle fiber relaxes.

7

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

Figure 49.34

Spinal cord

Nerve

Motor neuroncell body

Motorunit 1

Motorunit 2

Motor neuronaxon

Muscle

Tendon

Synaptic terminals

Muscle fibers

• 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

Figure 49.35

Actionpotential Pair of

actionpotentials

Series of action potentials at

high frequency

Time

Ten

sion

Singletwitch

Summation of two twitches

Tetanus

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

CONCLUSIONFor animals of a given

body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.

FlyingRunning

Swimming

10–3 103 1061

10–1

10

102

1

Body mass(g)

En

erg

y co

st (

J/K

g/m

)CONCLUSION

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.

RESULTS

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.

EXPERIMENT

•The energy cost of locomotion

–Depends on the mode of locomotion and the environment

Figure 49.37

Comparing Costs of Locomotion

• Animals that are specialized for swimming– Expend less energy per meter traveled than

equivalently sized animals specialized for flying or running

Chapter 47

Animal DevelopmentRead 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

Figure 47.1 1 mm

Head, with eye plaque, internal organs and tail.

• 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

Figure 47.2

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.

• 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!

• The acrosomal reaction

Spermnucleus

Sperm plasmamembrane

Hydrolytic enzymes

Corticalgranule

Cortical granulemembrane

EGG CYTOPLASM

Basal body(centriole)

Spermhead

Acrosomalprocess

Actin

Acrosome

Jelly coatEgg plasmamembrane

Vitelline layer

Fused plasmamembranes

Perivitellinespace

Fertilizationenvelope

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 thevitelline layer, and clipping off sperm-binding receptors. The resulting fertilization envelope is the slow block to polyspermy.

5 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.

3 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, bindingto receptors in the egg cell membrane that extend through the vitelline layer.

2 Contact. The sperm cell contacts the egg’s jelly coat, triggering exocytosis from the sperm’s acrosome.

1

Sperm-bindingreceptors

Entry of sperm nucleus.4

Figure 47.3

You will be able to see the fertilization envelope in lab.

• 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

Figure 47.4

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.

EXPERIMENT

RESULTS

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.

CONCLUSION

30 sec20 sec10 sec afterfertilization

1 sec beforefertilization

Point ofspermentry

Spreading waveof calcium ions

500 m

• 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

Figure 47.5

Binding of sperm to egg

Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)

Increased intracellular calcium level

Cortical reaction begins (slow block to polyspermy)

Formation of fertilization envelope complete

Increased intracellular pH

Increased protein synthesis

Fusion of egg and sperm nuclei complete

Onset of DNA synthesis

First cell division

1

2

34

6

8

10

20

30

4050

1

2

345

10

20

30

40

60

Sec

onds

Mi n

utes

90

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

Figure 47.7a–d

Fertilized egg. Shown here is thezygote shortly before the first cleavage division, surrounded by the fertilization envelope. The nucleus is visible in the center.

(a) Four-cell stage. Remnants of the mitotic spindle can be seen between the two cells that have just completed the second cleavage division.

(b) Morula. After further cleavage divisions, the embryo is a multicellular ball that is stillsurrounded by the fertilization envelope. The blastocoel cavityhas begun to form.

(c) 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.

(d)

Gastrulation• The morphogenetic process called gastrulation

rearranges the cells of a blastula into a three-layered embryo, called a gastrula, that has a primitive gut. Three germ layers develope.

Figure 47.11

Digestive tube (endoderm)

Key

Future ectodermFuture mesodermFuture endoderm

BlastocoelMesenchymecells

Vegetalplate

Animalpole

Vegetalpole

Filopodiapullingarchenterontip

Archenteron

Blastocoel

Blastopore

50 µm

Blastopore

Archenteron

Blastocoel

Mouth

Ectoderm

Mesenchyme:(mesodermforms future skeleton) Anus (from blastopore)

Mesenchymecells

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.

1

2 The vegetal plate invaginates (buckles inward). Mesenchyme cells migrate throughout the blastocoel.2

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).

3

Contraction of these filopodia then drags the archenteron across the blastocoel.4

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.

5

Sea urchin is a deuterostome so blastopore forms the anus. New opening for mouth. Mesoderm buds off from endoderm.

• 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

Figure 47.9

Zygote

2-cellstageforming

4-cellstageforming

8-cellstage

Eight-cell stage (viewed from the animal pole). The largeamount 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).

0.25 mm0.25 mm

Vegetal pole

Blastula(crosssection)

Animal poleBlasto-coel

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. Vegetal pole

Blastula(crosssection)

Animal poleBlasto-coel

0.25 mm

0.25 mm

Because of large amount of yolk the animal pole cells smaller!

• 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

Figure 47.10 Epiblast Hypoblast

BLASTODERMBlastocoel

YOLK MASS

Fertilized eggDisk ofcytoplasm

Zygote. Most of the cell’s volume is yolk, with a small disk of cytoplasm located at the animal pole.

Four-cell stage. Early cell divisions are meroblastic (incomplete). The cleavage furrow extends through the cytoplasm but not through the yolk.

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 epiblastand hypoblast, that enclose a fluid-filled cavity, theblastocoel.

3

1

2

In birds embryo forms on top of huge yolk.

• Gastrulation in the chick– Is affected by the large amounts of yolk in the egg

Figure 47.13

Epiblast

Futureectoderm

Migratingcells(mesoderm)

Endoderm

Hypoblast

YOLK

Primitivestreak