Download Lecture 8: A i l Animal development development and cell fate

生物學
Lecture 8:
A i l
Animal
development
and cell fate
生物醫學系
羅時成老師
[email protected]
ext: 3295
Animal development and cell fate
• 學習目標:
• To understand
• 1. How a fertilized egg
b
becomes
an organism?
i ?
• 2.
2 Molecular
M l l mechanisms
h i
in
i
cell differentiation
Animal development
p
and cell fate
• 1. Features of sperms
p
and eggs
gg when they
y meet
together.
• 2.
2 E
Embryonic
b
i d
development:
l
t germ llayers, tissues
ti
and organ formation.
• 3. Evolution of animal development (patterns and
hox genes)
• 4. What are master genes to maintain cells in a
stem cell state.
Figure 47.2
EMBRYONIC DEVELOPMENT
Sperm
Zygote
Adult
frog
Egg
Metamorphosis
Blastula
Larval
stages
Gastrula
Tail-bud
embryo
Figure 32.11
Ct
Ctenophora
h
Eumeta
azoa
Me
etazoa
ANCESTRAL
COLONIAL
FLAGELLATE
Porifera
Cnidaria
Acoela
Bilateria
Chordata
Platyhelminthes
L
Lophotro
ochozoa
a Ecdysozoa
Deute
erostomiia
Echinodermata
Rotifera
Ectoprocta
Brachiopoda
Mollusca
Annelida
Nematoda
Arthropoda
Ontogeny
g y Recapitulates
p
Phylogeny
y g y (ORP)
重演論
我們的身體裡有一條魚
Recapitulation theory
• The theory of recapitulation, also called the biogenetic law or
embryological parallelism - and often expressed as "ontogeny
ontogeny
recapitulates phylogeny" (ORP)- is a hypothesis that in
developing from embryo to adult, animals go through stages
resembling
bli or representing
i successive
i stages in
i the
h evolution
l i off
their remote ancestors. In biology, there are several examples of
embryonic
y
stages
g showing
g features of ancestral organisms,
g
, but a
"strong" formulation of the concept has been discredited.
• In 1866, the German zoologist Ernst Haeckel proposed that the
embryonic
b
i d
development
l
off an iindividual
di id l organism
i (its
(i
ontogeny) followed the same path as the evolutionary history of
itss spec
species
es ((itss pphylogeny).
y oge y).
Ontogeny Recapitulates Phylogeny (ORP
ORP))
This idea is an extreme one. If it were strictly true, it
would predict, for example, that in the course of a
chick's development, it would go through the following
stages:
g
a single
g celled organism,
g
a multi-celled
invertebrate ancestor, a fish, a lizard-like reptile, an
ancestral bird,, and then finally,
y, a babyy chick.
Figure 32.3
Individual
choanoflagellate
Ch
Choanoflagellates
fl ll t
OTHER
EUKARYOTES
Sponges
An
nimals
Other animals
Collar cell
(choanocyte)
Figure 32.2-3
Zygote
Cleavage
Blastocoel
Cleavage
Eight-cell
stage
g
Blastula
Cross section
of blastula
Gastrulation
Blastocoel
Endoderm
Ectoderm
Archenteron
Cross section
of gastrula
Bl t
Blastopore
Figure 32.8
(a) Coelomate
Coelom
Digestive tract
(from endoderm)
Body covering
(f
(from
ectoderm)
d
)
Tissue layer
lining coelom
and
d suspending
di
internal organs
(from mesoderm)
(b) Pseudocoelomate
Body covering
(from ectoderm)
P d
Pseudocoelom
l
Digestive tract
(from endoderm)
Muscle layer
(from
mesoderm)
(c) Acoelomate
Body covering
(from ectoderm)
TissueTi
filled region
(from
mesoderm)
Wall of digestive cavity
(from endoderm)
Figure 32.9
Protostome development
(examples: molluscs,
annelids)
( ) Cleavage
(a)
Cl
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Eight-cell stage
Spiral and determinate
Radial and indeterminate
(b) Coelom formation
Coelom
Archenteron
Coelom
Mesoderm
Blastopore
Blastopore
Solid masses of mesoderm
split and form coelom.
(c) Fate of the
blastopore
Mesoderm
Folds of archenteron
form coelom.
Anus
Mouth
Digestive tube
Key
Ectoderm
Mesoderm
Endoderm
Mouth
Mouth develops from blastopore.
Anus
Anus develops from blastopore.
Figure 47.3-5
Sperm
plasma
membrane
Sperm
nucleus
Basal body
(centriole)
p
Sperm
head
Acrosome
Jelly coat
Sperm binding
Sperm-binding
receptors
Fertilization
l
envelope
Acrosomal
process
Actin
filament
Cortical
Fused
granule
plasma
membranes
b
Hydrolytic enzymes
Perivitelline
space
Vitelline layer
Egg plasma membrane
EGG CYTOPLASM
The Cortical Reaction
• Fusion of egg and sperm also initiates the cortical
reaction
• Seconds after the sperm binds to the egg
egg, vesicles just
beneath the egg plasma membrane release their contents
and form a fertilization envelope
• Gamete contact and/or fusion depolarizes the egg cell
membrane
b
and
d sets
t up a fast
f t bl
block
k tto polyspermy
l
• The fertilization envelope acts as the slow block to
polyspermy
© 2011 Pearson Education, Inc.
• The cortical reaction requires a high concentration of
Ca2 ions in the egg
• The reaction is triggered by a change in Ca2
concentration
• Ca
C 2 spread
d across th
the egg correlates
l t with
ith the
th
appearance of the fertilization envelope
© 2011 Pearson Education, Inc.
FigureEXPERIMENT
47.4
10 sec after
fertilization
25 sec
35 sec
1 min
10 sec after
f ii i
fertilization
20 sec
30 sec
500 m
RESULTS
1 sec before
f ili i
fertilization
CONCLUSION
Point of sperm
nucleus
entry
Spreading
wave of Ca2
Fertilization
envelope
l
500 m
m
Egg Activation
• The rise in Ca2+ in the cytosol increases the rates of
cellular respiration and protein synthesis by the egg cell
• With these rapid changes in metabolism
metabolism, the egg is said
to be activated
• The
Th proteins
i andd mRNAs
RNA needed
d d ffor activation
i i are
already present in the egg
• The sperm nucleus merges with the egg nucleus and
cell division begins
© 2011 Pearson Education, Inc.
Fertilization in Mammals
• Fertilization in mammals and other terrestrial animals is
internal
• Secretions in the mammalian female reproductive tract
alter sperm motility and structure
• This
Thi is
i called
ll d capacitation,
it ti andd mustt occur before
b f
sperm
are able to fertilize an egg
© 2011 Pearson Education, Inc.
• Sperm travel through an outer layer of cells to reach the
zona pellucida, the extracellular matrix of the egg
• When the sperm binds a receptor in the zona pellucida
pellucida,
it triggers a slow block to polyspermy
• No
N ffastt block
bl k to
t polyspermy
l
has
h been
b
identified
id tifi d in
i
mammals
© 2011 Pearson Education, Inc.
Figure 47.5
Zona pellucida
Follicle cell
Sperm
basal body
Sperm
nucleus
Cortical
granules
• In mammals the first cell division occurs 12
1236
36 hours
after sperm binding
• The diploid nucleus forms after this first division of the
zygote
© 2011 Pearson Education, Inc.
Cleavage
• Fertilization is followed by cleavage,
cleavage a period of rapid
cell division without growth
• Cleavage partitions the cytoplasm of one large cell into
many smaller cells called blastomeres
• The
Th blastula
bl t l is
i a ball
b ll off cells
ll with
ith a fluid-filled
fl id fill d cavity
it
called a blastocoel
© 2011 Pearson Education, Inc.
Figure 47.6
50 m
(a) Fertilized egg
(b) Four-cell stage
(c) Early blastula
(d) Later blastula
Cleavage Patterns
• In frogs and many other animals
animals, the distribution of
yolk (stored nutrients) is a key factor influencing the
pattern of cleavage
• The vegetal pole has more yolk; the animal pole has
less yolk
• The difference in yolk distribution results in animal and
vegetal
t l hemispheres
h i h
that
th t differ
diff in
i appearance
© 2011 Pearson Education, Inc.
• The first two cleavage furrows in the frog form four
equally sized blastomeres
• The third cleavage is asymmetric,
asymmetric forming unequally
sized blastomeres
© 2011 Pearson Education, Inc.
• Holoblastic cleavage,
cleavage complete division of the egg,
egg
occurs in species whose eggs have little or moderate
amounts of yolk,
yolk such as sea urchins and frogs
• Meroblastic cleavage, incomplete division of the egg,
occurs in species with yolk-rich
yolk rich eggs
eggs, such as reptiles
and birds
© 2011 Pearson Education, Inc.
Figure 47.7
Zygote
22-cell
ll
stage
forming
Gray crescent
0.25 mm
8-cell stage (viewed
from the animal pole)
4-cell
stage
forming
8 cell
8-cell
stage
Animal
pole
0.25 mm
Blastula (at least 128 cells)
Vegetal pole
Blastula
(cross
section)
Blastocoel
Regulation of Cleavage
• Animal embryos complete cleavage when the ratio of
material in the nucleus relative to the cytoplasm is
sufficiently large
© 2011 Pearson Education, Inc.
Concept:
p Morphogenesis
p g
in animals
involves specific changes in cell
shape, position, and survival
• After cleavage, the rate of cell division slows and the
normal cell cycle is restored
• Morphogenesis, the process by which cells occupy
their appropriate locations, involves
– Gastrulation, the movement of cells from the blastula
surface to the interior of the embryo
– Organogenesis, the formation of organs
© 2011 Pearson Education, Inc.
Gastrulation
• Gastrulation rearranges the cells of a blastula into a
three-layered embryo, called a gastrula
© 2011 Pearson Education, Inc.
• The three layers produced by gastrulation are called
embryonic germ layers
– The ectoderm forms the outer layer
– The endoderm lines the digestive tract
– The
Th mesoderm
d
partly
tl fills
fill the
th space between
b t
the
th
endoderm and ectoderm
• E
Eachh germ layer
l
contributes
ib
to specific
ifi structures in
i the
h
adult animal
© 2011 Pearson Education, Inc.
Figure 47.8
ECTODERM (outer layer of embryo)
• Epidermis of skin and its derivatives (including sweat glands,
hair follicles)
• Nervous and sensory systems
• Pituitary gland,
gland adrenal medulla
• Jaws and teeth
• Germ cells
MESODERM (middle layer of embryo)
• Skeletal and muscular systems
• Circulatory and lymphatic systems
• Excretory and reproductive systems (except germ cells)
• Dermis of skin
• Adrenal cortex
ENDODERM (inner layer of embryo)
• Epithelial lining of digestive tract and associated organs
(liver, pancreas)
• Epithelial lining of respiratory, excretory, and reproductive tracts
and ducts
• Thymus, thyroid, and parathyroid glands
Figure 47.10
1
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
Dorsal
lip of
blastopore
Early
gastrula
Vegetal pole
Blastopore
Blastocoel
shrinking
2
3
Dorsal
lip of
blastopore
Archenteron
Blastocoel
remnant
Ectoderm
Mesoderm
Endoderm
Blastopore
Bl
t
Yolk plug
Archenteron
Key
Future ectoderm
Future mesoderm
Future endoderm
Late
gastrula
Blastopore
Figure 47.11
Fertilized egg
Primitive
streak
Embryo
Yolk
Primitive streak
Epiblast
p
Future
ectoderm
Bl t
Blastocoel
l
Migrating
cells
(mesoderm)
Endoderm
YOLK
Hypoblast
Gastrulation in Humans
• Human eggs have very little yolk
• A blastocyst is the human equivalent of the blastula
• The
Th inner
i
cell
ll mass is
i a cluster
l
off cells
ll at one endd off
the blastocyst
• The trophoblast is the outer epithelial layer of the
blastocyst and does not contribute to the embryo, but
instead initiates implantation
© 2011 Pearson Education, Inc.
• Following implantation
implantation, the trophoblast continues to
expand and a set of extraembryonic membranes is
formed
• These enclose specialized structures outside of the
embryo
• Gastrulation involves the inward movement from the
epiblast,
ibl t through
th
h a primitive
i iti streak,
t k similar
i il to
t the
th
chick embryo
© 2011 Pearson Education, Inc.
Figure 47.12
1 Blastocyst reaches uterus.
Uterus
Endometrial epithelium
(uterine lining)
Inner cell mass
Trophoblast
Blastocoel
2 Blastocyst implants
(7 days after fertilization).
Expanding region of
trophoblast
Maternal
blood
vessel
Epiblast
Hypoblast
Trophoblast
3 Extraembryonic membranes
start to form (10–11 days),
and gastrulation begins
(13 days).
Expanding region of
trophoblast
Amniotic cavity
Epiblast
Hypoblast
Yolk sac (from hypoblast)
Extraembryonic mesoderm cells
(from epiblast)
(f
ibl t)
Chorion (from trophoblast)
4 Gastrulation has produced a
three-layered
three
layered embryo with
four extraembryonic
membranes.
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic mesoderm
Allantois
Figure 47.1 A “homunculus” inside the head of a human sperm
生物的形狀是如何決定的
先成說 ?
還是誘導說?
看青蛙胚胎發育實驗結果如何?
• Roux’s experiment
Concept: Cytoplasmic determinants
and
d inductive
i d i signals
i l contribute
ib to
cell fate specification
• Determination is the term used to describe the process
by which a cell or group of cells becomes committed to
a particular fate
p
in
• Differentiation refers to the resultingg specialization
structure and function
© 2011 Pearson Education, Inc.
• Cells in a multicellular organism share the same
genome
• Differences in cell types is the result of the expression
of different sets of genes
© 2011 Pearson Education, Inc.
Fate Mapping
• Fate maps are diagrams showing organs and other
structures that arise from each region of an embryo
• Classic studies using frogs indicated that cell lineage in
ggerm layers
y is traceable to blastula cells
© 2011 Pearson Education, Inc.
Figure 47.17
Epidermis
Central
nervous
system
Notochord
Epidermis
Mesoderm
Endoderm
g
Neural tube stage
(transverse section)
Blastula
(a) Fate map of a frog embryo
64-cell embryos
Blastomeres
injected with dye
Larvae
(b) Cell lineage analysis in a tunicate
Figure 47.22-2
EXPERIMENT
Control egg
(dorsal view)
Experimental egg
(side view)
1a Control
1b Experimental
group
group
g
p
Gray
crescent
Gray
crescent
Thread
2
RESULTS
Normal
Belly piece
Normal
Restricting Developmental Potential
• H
Hans S
Spemann performed
f
d experiments
i
t to
t determine
d t
i a
cell’s developmental potential (range of structures to
which
hi h it can give
i rise)
i )
• Embryonic fates are affected by distribution of
determinants and the pattern of cleavage
• The first two blastomeres of the frog embryo are
totipotent (can develop into all the possible cell types)
© 2011 Pearson Education, Inc.
Figure 47.22 The “organizer” of Spemann and Mangold
The “Organizer”
Organizer of Spemann and
Mangold
• Spemann and Mangold transplanted tissues between
earlyy ggastrulas and found that the transplanted
p
dorsal lip
p
triggered a second gastrulation in the host
• The dorsal lip functions as an organizer of the embryo
body plan, inducing changes in surrounding tissues to
form notochord, neural tube, and so on
© 2011 Pearson Education, Inc.
Cell Fate Determination and Pattern
Formation by Inductive Signals
• As embryonic
y
cells acquire
q
distinct fates,, they
y influence
each other’s fates by induction
© 2011 Pearson Education, Inc.
Formation of the Vertebrate Limb
• Inductive signals play a major role in pattern
formation, development of spatial organization
• The molecular cues that control pattern formation are
called positional information
• This
Thi information
i f
ti tells
t ll a cell
ll where
h it is
i with
ith respectt to
t
the body axes
• It determines how the cell and its descendents respond
to future molecular signals
© 2011 Pearson Education, Inc.
• The wings and legs of chicks, like all vertebrate limbs,
begin as bumps of tissue called limb buds
© 2011 Pearson Education, Inc.
Figure 47.24
Anterior
Limb bud
AER
ZPA
Posterior
Limb buds
50 m
2
Digits
Apical
ectodermal
ridge (AER)
Anterior
3
4
Ventral
Proximal
Distal
Dorsal
Posterior
(a) Organizer regions
(b) Wing of chick embryo
• The embryonic cells in a limb bud respond to positional
information indicating location along three axes
– Proximal
Proximal-distal
distal axis
– Anterior-posterior axis
– Dorsal-ventral
D
l
t l axis
i
© 2011 Pearson Education, Inc.
• One limb-bud
limb bud regulating region is the apical
ectodermal ridge (AER)
• The AER is thickened ectoderm at the bud
bud’ss tip
• The second region is the zone of polarizing activity
(ZPA)
• The ZPA is mesodermal tissue under the ectoderm
where the posterior side of the bud is attached to the
body
© 2011 Pearson Education, Inc.
• Tissue transplantation experiments support the
hypothesis that the ZPA produces an inductive signal
that conveys positional information indicating
“posterior”
© 2011 Pearson Education, Inc.
Figure 47.25
EXPERIMENT
Anterior
New
ZPA
Donor
limb
bud
Host
limb
bud
ZPA
Posterior
RESULTS
4
3
2
2
4
3
Cytoplasmic Determinants and
Inductive Signals
• An egg’s cytoplasm contains RNA, proteins, and other
substances that are distributed unevenly in the
unfertilized egg
• Cytoplasmic
C
l
i determinants
d
i
are maternall substances
b
in
i
the egg that influence early development
• As the zygote divides by mitosis, cells contain different
cytoplasmic determinants, which lead to different gene
expression
© 2011 Pearson Education, Inc.
Figure 18.17
((a)) Cytoplasmic
y p
determinants in the egg
gg
((b)) Induction by
y nearby
y cells
Unfertilized egg
Sperm
Fertilization
Early embryo
(32 cells)
N l
Nucleus
Molecules of two
different cytoplasmic
determinants
NUCLEUS
Zygote
(fertilized egg)
Mitotic
cell division
Two-celled
embryo
y
Signal
transduction
pathway
Signal
receptor
Signaling
Si
li
molecule
(inducer)
Figure 18.17a
(a) Cytoplasmic determinants in the egg
Unfertilized egg
Sperm
Fertilization
Zygote
(fertilized egg)
Mitotic
cell division
Two-celled
embryo
b
Nucleus
Molecules
M
l
l off two
t
different cytoplasmic
determinants
• The other important source of developmental
information is the environment around the cell,
especially signals from nearby embryonic cells
• In the process called induction, signal molecules from
embryonic cells cause transcriptional changes in nearby
target cells
• Thus,
Th interactions
i t
ti
between
b t
cells
ll induce
i d
differentiation
diff
ti ti
of specialized cell types
© 2011 Pearson Education, Inc.
Figure 18.17b (b) Induction by nearby cells
Early embryo
(32 cells)
ll )
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signaling
molecule
(i d
(inducer)
)
Sequential Regulation of Gene
Expression During Cellular
Differentiation
• Determination commits a cell to its final fate
• Determination precedes differentiation
• Cell
C ll differentiation
diff
ti ti is
i marked
k d by
b the
th production
d ti off
tissue-specific proteins
© 2011 Pearson Education, Inc.
• Myoblasts produce muscle-specific
muscle specific proteins and form
skeletal muscle cells
• MyoD is one of several “master
master regulatory genes”
genes that
produce proteins that commit the cell to becoming
skeletal muscle
• The MyoD protein is a transcription factor that binds to
enhancers
h
off various
i
target
t
t genes
© 2011 Pearson Education, Inc.
Figure 18.18-1
Nucleus
Embryonic
precursor cell
Master regulatory
gene myoD
Other muscle-specific genes
DNA
OFF
OFF
Figure 18.18-2
Nucleus
Embryonic
precursor cell
Myoblast
(determined)
Master regulatory
gene myoD
Other muscle-specific genes
DNA
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
Figure 18.18-3
Nucleus
Embryonic
precursor cell
Master regulatory
gene myoD
Other muscle-specific genes
DNA
Myoblast
(determined)
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
mRNA
Part of a muscle fiber
(fully differentiated cell)
MyoD
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell cycle
cycle–
blocking proteins
Pattern Formation: Setting Up the
Body Plan
• Pattern formation is the development of a spatial
organization of tissues and organs
• In animals, pattern formation begins with the
establishment
bli h
off the
h major
j axes
• Positional information, the molecular cues that control
pattern formation, tells a cell its location relative to the
body axes and to neighboring cells
© 2011 Pearson Education, Inc.
• Pattern formation has been extensively studied in the
fruit fly Drosophila melanogaster
• Combining anatomical
anatomical, genetic,
genetic and biochemical
approaches, researchers have discovered developmental
principles common to many other species,
species including
humans
© 2011 Pearson Education, Inc.
The Life Cycle of Drosophila
• In Drosophila, cytoplasmic determinants in the
unfertilized egg determine the axes before fertilization
• After fertilization,
fertilization the embryo develops into a
segmented larva with three larval stages
© 2011 Pearson Education, Inc.
Figure
18.19
Thorax Abdomen
Head
1 Egg
developing within
ovarian follicle
Follicle cell
Nucleus
Egg
gg
0.5 mm
Nurse cell
Dorsal
BODY
AXES
Anterior
Left
Ventral
Right
Posterior
2 Unfertilized egg
Depleted
nurse cells
(a) Adult
Egg
shell
Fertilization
Laying of egg
3 Fertilized egg
Embryonic
development
4 Segmented
embryo
0.1 mm
Body
segments
5 Larval stage
g
(b) Development from egg to larva
Hatching
Figure 18.19a
Head Thorax
Abdomen
0 5 mm
0.5
Dorsal
BODY
AXES
Anterior
Left
(a) Adult
Ventral
Right
Posterior
Figure 18.19b
Follicle cell
1 Egg
developing within
ovarian follicle
Nucleus
Egg
Nurse cell
2 Unfertilized egg
Depleted
nurse cells
Egg
shell
Fertilization
Laying of egg
3 Fertilized egg
Embryonic
development
4 Segmented
embryo
Body segments
0.1 mm
Hatching
5 Larval stage
(b) Development from egg to larva
Genetic Analysis of Early
Development: Scientific Inquiry
• Edward B. Lewis, Christiane Nüsslein-Volhard, and
Eric Wieschaus won a Nobel 1995 Prize for decoding
pattern formation in Drosophila
• Lewis
L i discovered
di
d the
h homeotic
h
i genes, which
hi h controll
pattern formation in late embryo, larva, and adult
stages
© 2011 Pearson Education, Inc.
Figure 18.20
Eye
y
Leg
Antenna
Wild type
Mutant
• Nüsslein-Volhard
Nüsslein Volhard and Wieschaus studied segment
formation
• They created mutants,
mutants conducted breeding experiments
experiments,
and looked for corresponding genes
• Many of the identified mutations were embryonic
lethals, causing death during embryogenesis
• They
The found
fo nd 120 genes essential for normal
segmentation
© 2011 Pearson Education, Inc.
Axis Establishment
• Maternal effect genes encode for cytoplasmic
determinants that initially establish the axes of the body
of Drosophila
• These maternal effect genes are also called eggpolarity genes because they control orientation of the
egg and consequently the fly
© 2011 Pearson Education, Inc.
Bicoid: A Morphogen Determining Head
Structures
• One maternal effect gene, the bicoid gene, affects the
front half of the body
• An embryo whose mother has no functional bicoid gene
lacks the front half of its body and has duplicate
posterior structures at both ends
© 2011 Pearson Education, Inc.
Figure 18.21
Head
Tail
A8
T1 T2 T3
A1
A2 A3 A4 A5
A6
Wild-type larva
A7
250 m
Tail
Tail
A8
A8
A7
A6
A7
Mutant larva (bicoid)
• This phenotype suggests that the product of the
mother’s bicoid gene is concentrated at the future
anterior end
• This hypothesis is an example of the morphogen
gradient hypothesis
hypothesis, in which gradients of substances
called morphogens establish an embryo’s axes and
other features
© 2011 Pearson Education, Inc.
Figure 18.22
100 m
RESULTS
Anterior end
Fertilization,
t
translation
l ti off
bicoid mRNA
Bicoid mRNA in mature
unfertilized egg
Bicoid
Bi
id mRNA
RNA iin mature
t
unfertilized egg
Bicoid protein in
early embryo
Bicoid
Bi
id protein
i in
i
early embryo
• The bicoid research is important for three reasons
– It identified a specific protein required for some early
steps in pattern formation
– It increased understanding of the mother’s role in
embryo development
– It demonstrated a key developmental principle that a
gradient of molecules can determine polarity and
position in the embryo
© 2011 Pearson Education, Inc.
• Later studies of C. elegans used the ablation
(destruction) of single cells to determine the structures
that normally arise from each cell
• The researchers were able to determine the lineage of
each of the 959 somatic cells in the worm
© 2011 Pearson Education, Inc.
Time affter fertilization (h
hours)
Figure 47.18
0
Zygote
First cell division
Nervous
system,
outer skin,,
musculature
10
Musculature, gonads
Outer skin,
nervous system
Germ line
(future
gametes))
g
Musculature
Hatching
g
Intestine
Intestine
Anus
Mouth
Eggs
ANTERIOR
Vulva
1.2 mm
POSTERIOR
• Germ cells are the specialized cells that give rise to
sperm or eggs
• Complexes of RNA and protein are involved in the
specification of germ cell fate
• In
I C.
C elegans,
l
suchh complexes
l
are called
ll d P granules,
l
persist throughout development, and can be detected in
germ cells
ll off the
th adult
d lt worm
© 2011 Pearson Education, Inc.
• P granules are distributed throughout the newly
fertilized egg and move to the posterior end before the
first cleavage division
• With each subsequent cleavage, the P granules are
partitioned into the posterior-most
posterior most cells
• P granules act as cytoplasmic determinants, fixing germ
cell
ll fate
f t att the
th earliest
li t stage
t
off development
d l
t
© 2011 Pearson Education, Inc.
Figure 47.20
20 m
1 Newly fertilized egg
2 Zygote prior to first division
3 Two-cell embryo
4 Four-cell embryo
Concept:
p Cloningg organisms
g
may
y
lead to production of stem cells for
research and other applications
• Organismal cloning produces one or more organisms
genetically identical to the “parent” that donated the
single cell
© 2011 Pearson Education, Inc.
Cloning Plants: Single-Cell
C lt
Cultures
• One experimental approach for testing genomic
equivalence is to see whether a differentiated cell can
generate a whole organism
• A totipotent cell is one that can generate a complete
new organism
• Plant
Pl t cloning
l i is
i usedd extensively
t i l in
i agriculture
i lt
© 2011 Pearson Education, Inc.
Figure 20.17
Cross
section of
carrot root
2-mg
fragments
Fragments were
cultured in nutrient medium;
stirring caused
single cells to
shear off into
the liquid.
Single cells
free in
suspension
began to
divide.
Embryonic
plant developed
from a cultured
single cell.
Plantlet was
cultured on
agar medium.
Later it was
planted in soil.
Adult
plant
Cloning Animals: Nuclear
Transplantation
a sp a tat o
• In nuclear transplantation, the nucleus of an unfertilized
egg cell or zygote is replaced with the nucleus of a
differentiated cell
• Experiments with frog embryos have shown that a
transplanted nucleus can often support normal
development of the egg
• However,
H
th
the older
ld the
th donor
d
nucleus,
l
the
th lower
l
the
th
percentage of normally developing tadpoles
© 2011 Pearson Education, Inc.
Figure 20.18
EXPERIMENT Frog embryo
Frog egg cell
Frog tadpole
UV
Less differentiated cell
Fully differentiated
(intestinal) cell
Donor
D
nucleus
transplanted
Donor
D
nucleus
transplanted
l t d
Enucleated
egg cell
Egg with donor nucleus
activated to begin
development
RESULTS
Most develop
into tadpoles.
Most stop developing
before tadpole stage.
Reproductive
R
d i Cloning
Cl i off
Mammals
• In 1997
1997, Scottish researchers announced the birth of
Dolly, a lamb cloned from an adult sheep by nuclear
transplantation from a differentiated mammary cell
• Dolly’s premature death in 2003, as well as her
arthritis led to speculation that her cells were not as
arthritis,
healthy as those of a normal sheep, possibly reflecting
incomplete reprogramming of the original transplanted
nucleus
© 2011 Pearson Education, Inc.
Figure TECHNIQUE
20.19
Mammary
cell donor
Egg cell
donor
1
Cultured
y
mammary
cells
2
Egg
cell from
ovary
3 Cells fused
4 Grown in culture
Nucleus
removed
Nucleus from
mammary cell
ll
Early embryo
5 Implanted in uterus
of a third sheep
Surrogate
mother
6 Embryonic
development
RESULTS
Lamb (“Dolly”) genetically
identical to mammary cell donor
• Since 1997
1997, cloning has been demonstrated in many
mammals, including mice, cats, cows, horses, mules,
ppigs,
g , and dogs
g
• CC (for Carbon Copy) was the first cat cloned;
however,, CC differed somewhat from her female
“parent”
y look or behave exactlyy
• Cloned animals do not always
the same
© 2011 Pearson Education, Inc.
P bl
Problems
Associated
A
i t d with
ith Animal
A i l
Cloning
• IIn mostt nuclear
l transplantation
t
l t ti studies,
t di only
l a small
ll
percentage of cloned embryos have developed normally
to birth
birth, and man
many cloned animals exhibit
e hibit defects
• Many epigenetic changes, such as acetylation of
hi
histones
or methylation
h l i off DNA, must be
b reversedd in
i
the nucleus from a donor animal in order for genes to
b expressedd or repressed
be
d appropriately
i l for
f early
l stages
of development
© 2011 Pearson Education, Inc.
Stem Cells of Animals
• A stem cell is a relatively unspecialized cell that can
reproduce itself indefinitely and differentiate into
specialized cells of one or more types
• Stem cells isolated from early embryos at the blastocyst
stage are called embryonic stem (ES) cells; these are
able to differentiate into all cell types
• The
Th adult
d lt body
b d also
l has
h stem
t cells,
ll which
hi h replace
l
nonreproducing specialized cells
© 2011 Pearson Education, Inc.
iPS, iPSC and four genes
Reprogramming of somatic cells to
pluripotent
p
stem cells
induced p
(iPSCs) can be high efficiency upon
inducible
d bl expression
i off Oct4,
O 4 Klf4,
Klf4 cMyc and Sox2
Myc,
John Gurdon (1958)—nuclear transplant
I Wilmut
Ian
Wil t (1997)--Dolly
(1997) D ll
Shinya Yamanaka (2006)—reprogramming genes for iPSC
The Nobel Prize in Physiology or
Medicine 2012
•
•
Sir John B. Gurdon
Shinya Yamanaka
Figure 20.21
Embryonic
stem cells
Adult
stem cells
Cells generating
some cell types
Cells generating
all embryonic
cell types
Cultured
stem cells
Different
culture
conditions
Different
types of
differentiated
cells
Liver
cells
ll
Nerve
cells
ll
Blood
ll
cells
• Researchers can transform skin cells into ES cells by
using viruses to introduce stem cell master regulatory
genes
• These transformed cells are called iPS cells (induced
pluripotent cells)
• These cells can be used to treat some diseases and to
replace
l
nonfunctional
f ti l tissues
ti
© 2011 Pearson Education, Inc.
Figure 20.22
1 R
Remove skin
ki cells
ll
from patient.
2 Reprogram skin cells
so the cells become
i d
induced
d pluripotent
l i t t
stem (iPS) cells.
Patient with
damaged heart
tissue or other
disease
3 Treat iPS cells so
that they differentiate
into a specific
cell type
type.
4 Return cells to
patient, where
p
they can repair
damaged tissue.
Human Gene Therapy
• Gene therapy is the alteration of an afflicted
individual’s genes
• Gene therapy holds great potential for treating
disorders traceable to a single defective gene
• Vectors are used for delivery of genes into specific
types of cells, for example bone marrow
• Gene therapy provokes both technical and ethical
questions
© 2011 Pearson Education, Inc.