Animal Development

1
Chapter 47
Animal Development
2
Overview A Body-Building Plan

• A human embryo at about 7 weeks after conception
  shows development of distinctive features

3
Figure 47.1
1 mm
4

• Development occurs at many points in the life
  cycle of an animal
• This includes metamorphosis and gamete
  production, as well as embryonic development

5
Figure 47.2
EMBRYONIC DEVELOPMENT
Sperm
Zygote
Adultfrog
Egg
FERTILIZATION
CLEAVAGE
Metamorphosis
Blastula
GASTRULATION
ORGANO-GENESIS
Larvalstages
Gastrula
Tail-budembryo
6

• Although animals display different body plans,
  they share many basic mechanisms of development
  and use a common set of regulatory genes
• Biologists use model organisms to study
  development, chosen for the ease with which they
  can be studied in the laboratory

7
Concept 47.1 Fertilization and cleavage initiate
embryonic development

• Fertilization is the formation of a diploid
  zygote from a haploid egg and sperm

8
Fertilization

• Molecules and events at the egg surface play a
  crucial role in each step of fertilization
• Sperm penetrate the protective layer around the
  egg
• Receptors on the egg surface bind to molecules on
  the sperm surface
• Changes at the egg surface prevent polyspermy,
  the entry of multiple sperm nuclei into the egg

9
The Acrosomal Reaction

• The acrosomal reaction is triggered when the
  sperm meets the egg
• The acrosome at the tip of the sperm releases
  hydrolytic enzymes that digest material
  surrounding the egg

10
Figure 47.3-1
Basal body(centriole)
Spermhead
Acrosome
Jelly coat
Vitelline layer
Sperm-bindingreceptors
Egg plasma membrane
11
Figure 47.3-2
Basal body(centriole)
Spermhead
Acrosome
Hydrolytic enzymes
Jelly coat
Vitelline layer
Sperm-bindingreceptors
Egg plasma membrane
12
Figure 47.3-3
Spermnucleus
Acrosomalprocess
Basal body(centriole)
Actinfilament
Spermhead
Acrosome
Hydrolytic enzymes
Jelly coat
Vitelline layer
Sperm-bindingreceptors
Egg plasma membrane
13
Figure 47.3-4
Spermplasmamembrane
Spermnucleus
Acrosomalprocess
Basal body(centriole)
Actinfilament
Spermhead
Fusedplasmamembranes
Acrosome
Hydrolytic enzymes
Jelly coat
Vitelline layer
Sperm-bindingreceptors
Egg plasma membrane
14
Figure 47.3-5
Spermplasmamembrane
Spermnucleus
Fertilizationenvelope
Acrosomalprocess
Basal body(centriole)
Actinfilament
Spermhead
Corticalgranule
Fusedplasmamembranes
Acrosome
Hydrolytic enzymes
Perivitellinespace
Jelly coat
Vitelline layer
Sperm-bindingreceptors
EGG CYTOPLASM
Egg plasma membrane
15

• Gamete contact and/or fusion depolarizes the egg
  cell membrane and sets up a fast block to
  polyspermy

16
The Cortical Reaction

• Fusion of egg and sperm also initiates the
  cortical reaction
• Seconds after the sperm binds to the egg,
  vesicles just beneath the egg plasma membrane
  release their contents and form a fertilization
  envelope
• The fertilization envelope acts as the slow block
  to polyspermy

17

• The cortical reaction requires a high
  concentration of Ca2? ions in the egg
• The reaction is triggered by a change in Ca2?
  concentration
• Ca2? spread across the egg correlates with the
  appearance of the fertilization envelope

18
Figure 47.4
EXPERIMENT
10 sec afterfertilization
25 sec
35 sec
1 min
500 ?m
RESULTS
1 sec beforefertilization
30 sec
20 sec
10 sec afterfertilization
500 ?m
CONCLUSION
Fertilizationenvelope
Spreadingwave of Ca2?
Point of spermnucleusentry
19
Figure 47.4a
EXPERIMENT
10 sec afterfertilization
25 sec
35 sec
1 min
500 ?m
RESULTS
1 sec beforefertilization
30 sec
20 sec
10 sec afterfertilization
500 ?m
20
Figure 47.4b
CONCLUSION
Fertilizationenvelope
Spreadingwave of Ca2?
Point of spermnucleusentry
21
Figure 47.4c
10 sec afterfertilization
22
Figure 47.4d
25 sec
23
Figure 47.4e
35 sec
24
Figure 47.4f
1 min
25
Figure 47.4g
1 sec beforefertilization
26
Figure 47.4h
10 sec afterfertilization
27
Figure 47.4i
20 sec
28
Figure 47.4j
30 sec
29
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, the egg
  is said to be activated
• The proteins and mRNAs needed for activation are
  already present in the egg
• The sperm nucleus merges with the egg nucleus and
  cell division begins

30
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 is called capacitation and must occur before
  sperm are able to fertilize an egg

31

• 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, it triggers a slow block to polyspermy
• No fast block to polyspermy has been identified
  in mammals

32
Figure 47.5
Zona pellucida
Follicle cell
Corticalgranules
Spermnucleus
Spermbasal body
33

• In mammals the first cell division occurs 12?36
  hours after sperm binding
• The diploid nucleus forms after this first
  division of the zygote

34
Cleavage

• Fertilization is followed by cleavage, a period
  of rapid cell division without growth
• Cleavage partitions the cytoplasm of one large
  cell into many smaller cells called blastomeres
• The blastula is a ball of cells with a
  fluid-filled cavity called a blastocoel

35
Figure 47.6
50 ?m
(a) Fertilized egg
(d) Later blastula
(b) Four-cell stage
(c) Early blastula
36
Figure 47.6a
(a) Fertilized egg
37
Figure 47.6b
(b) Four-cell stage
38
Figure 47.6c
(c) Early blastula
39
Figure 47.6d
(d) Later blastula
40
Cleavage Patterns

• In frogs and many other 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 hemispheres that differ in
  appearance

41

• The first two cleavage furrows in the frog form
  four equally sized blastomeres
• The third cleavage is asymmetric, forming
  unequally sized blastomeres

42

• Holoblastic cleavage, complete division of the
  egg, occurs in species whose eggs have little or
  moderate amounts of yolk, such as sea urchins and
  frogs
• Meroblastic cleavage, incomplete division of the
  egg, occurs in species with yolk-rich eggs, such
  as reptiles and birds

43
Figure 47.7
Zygote
2-cellstageforming
Gray crescent
0.25 mm
8-cell stage (viewedfrom the animal pole)
4-cellstageforming
Animalpole
8-cellstage
0.25 mm
Blastula (at least 128 cells)
Vegetal pole
Blastocoel
Blastula(crosssection)
44
Figure 47.7a-1
Zygote
45
Figure 47.7a-2
Gray crescent
2-cell stageforming
Zygote
46
Figure 47.7a-3
Gray crescent
2-cell stageforming
4-cell stageforming
Zygote
47
Figure 47.7a-4
Animal pole
Gray crescent
Vegetal pole
2-cell stageforming
4-cell stageforming
Zygote
8-cell stage
48
Figure 47.7a-5
Animal pole
Blastocoel
Gray crescent
Vegetal pole
2-cell stageforming
4-cell stageforming
Zygote
8-cell stage
Blastula(cross section)
49
Figure 47.7b
0.25 mm
Animalpole
8-cell stage (viewedfrom the animal pole)
50
Figure 47.7c
0.25 mm
Blastocoel
Blastula (at least 128 cells)
51
Regulation of Cleavage

• Animal embryos complete cleavage when the ratio
  of material in the nucleus relative to the
  cytoplasm is sufficiently large

52
Concept 47.2 Morphogenesis 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

53
Gastrulation

• Gastrulation rearranges the cells of a blastula
  into a three-layered embryo, called a gastrula

54

• The three layers produced by gastrulation are
  called embryonic germ layers
• The ectoderm forms the outer layer
• The endoderm lines the digestive tract
• The mesoderm partly fills the space between the
  endoderm and ectoderm
• Each germ layer contributes to specific
  structures in the adult animal

Video Sea Urchin Embryonic Development
55
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, 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
56
Gastrulation in Sea Urchins

• Gastrulation begins at the vegetal pole of the
  blastula
• Mesenchyme cells migrate into the blastocoel
• The vegetal plate forms from the remaining cells
  of the vegetal pole and buckles inward through
  invagination

57

• The newly formed cavity is called the archenteron
• This opens through the blastopore, which will
  become the anus

58
Figure 47.9
Animalpole
Blastocoel
Mesenchymecells
Vegetal plate
Vegetalpole
Blastocoel
Filopodia
Mesenchymecells
Archenteron
Blastopore
50 ?m
Blastocoel
Ectoderm
Archenteron
Blastopore
Key
Mouth
Future ectoderm
Mesenchyme(mesoderm formsfuture skeleton)
Digestive tube (endoderm)
Future mesoderm
Anus (from blastopore)
Future endoderm
59
Figure 47.9a-1
Animalpole
Blastocoel
Mesenchymecells
Vegetalpole
Vegetalplate
Key
Future ectoderm
Future mesoderm
Future endoderm
60
Figure 47.9a-2
Animalpole
Blastocoel
Mesenchymecells
Vegetalpole
Vegetalplate
Key
Future ectoderm
Future mesoderm
Future endoderm
61
Figure 47.9a-3
Animalpole
Blastocoel
Mesenchymecells
Filopodia
Vegetalpole
Vegetalplate
Archenteron
Key
Future ectoderm
Future mesoderm
Future endoderm
62
Figure 47.9a-4
Animalpole
Blastocoel
Mesenchymecells
Filopodia
Vegetalpole
Archenteron
Vegetalplate
Blastocoel
Archenteron
Blastopore
Key
Future ectoderm
Future mesoderm
Future endoderm
63
Figure 47.9a-5
Animalpole
Blastocoel
Mesenchymecells
Filopodia
Vegetalpole
Archenteron
Vegetalplate
Blastocoel
Digestive tube(endoderm)
Archenteron
Ectoderm
Blastopore
Mouth
Key
Mesenchyme(mesoderm formsfuture skeleton)
Future ectoderm
Future mesoderm
Anus(from blastopore)
Future endoderm
64
Figure 47.9b
Blastocoel
Filopodia
Mesenchymecells
Archenteron
Blastopore
50 ?m
65
Gastrulation in Frogs

• Frog gastrulation begins when a group of cells on
  the dorsal side of the blastula begins to
  invaginate
• This forms a crease along the region where the
  gray crescent formed
• The part above the crease is called the dorsal
  lip of the blastopore

66

• Cells continue to move from the embryo surface
  into the embryo by involution
• These cells become the endoderm and mesoderm
• Cells on the embryo surface will form the
  ectoderm

67
Figure 47.10
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
Dorsal lip ofblasto-pore
Dorsal lip ofblastopore
Blastopore
Earlygastrula
Vegetal pole
Blastocoelshrinking
Archenteron
Ectoderm
Blastocoelremnant
Mesoderm
Endoderm
Key
Future ectoderm
Blastopore
Lategastrula
Future mesoderm
Yolk plug
Archenteron
Blastopore
Future endoderm
68
Figure 47.10a
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
Key
Dorsal lip ofblasto-pore
Future ectoderm
Future mesoderm
Dorsal lip ofblastopore
Blastopore
Earlygastrula
Future endoderm
Vegetal pole
69
Figure 47.10b
Blastocoelshrinking
Archenteron
Key
Future ectoderm
Future mesoderm
Future endoderm
70
Figure 47.10c
Ectoderm
Blastocoelremnant
Mesoderm
Endoderm
Key
Future ectoderm
Future mesoderm
Blastopore
Lategastrula
Future endoderm
Yolk plug
Archenteron
Blastopore
71
Gastrulation in Chicks

• Prior to gastrulation, the embryo is composed of
  an upper and lower layer, the epiblast and
  hypoblast, respectively
• During gastrulation, epiblast cells move toward
  the midline of the blastoderm and then into the
  embryo toward the yolk

72

• The midline thickens and is called the primitive
  streak
• The hypoblast cells contribute to the sac that
  surrounds the yolk and a connection between the
  yolk and the embryo, but do not contribute to the
  embryo itself

73
Figure 47.11
Fertilized egg
Primitivestreak
Embryo
Yolk
Primitive streak
Epiblast
Future ectoderm
Blastocoel
Endoderm
Migratingcells(mesoderm)
Hypoblast
YOLK
74
Gastrulation in Humans

• Human eggs have very little yolk
• A blastocyst is the human equivalent of the
  blastula
• The inner cell mass is a cluster of cells at one
  end of the blastocyst
• The trophoblast is the outer epithelial layer of
  the blastocyst and does not contribute to the
  embryo, but instead initiates implantation

75

• Following 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, through a primitive streak, similar
  to the chick embryo

76
Figure 47.12
Endometrial epithelium(uterine lining)
Blastocyst reaches uterus.
Inner cell mass
Uterus
Trophoblast
Blastocoel
Blastocyst implants(7 days after fertilization).
Expanding region oftrophoblast
Maternal bloodvessel
Epiblast
Hypoblast
Trophoblast
Expanding region oftrophoblast
Extraembryonic membranesstart to form (1011
days),and gastrulation begins(13 days).
Amniotic cavity
Epiblast
Hypoblast
Yolk sac (from hypoblast)
Extraembryonic mesoderm cells(from epiblast)
Chorion (from trophoblast)
Gastrulation has produced athree-layered embryo
withfour extraembryonicmembranes.
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic mesoderm
Allantois
77
Figure 47.12a
Endometrial epithelium(uterine lining)
Inner cell mass
Uterus
Trophoblast
Blastocoel
Blastocyst reaches uterus.
78
Figure 47.12b
Expanding region oftrophoblast
Maternal bloodvessel
Epiblast
Hypoblast
Trophoblast
Blastocyst implants(7 days after fertilization).
79
Figure 47.12c
Expanding region oftrophoblast
Amniotic cavity
Epiblast
Hypoblast
Yolk sac (from hypoblast)
Extraembryonic mesoderm cells (from epiblast)
Chorion (from trophoblast)
Extraembryonic membranesstart to form (1011
days),and gastrulation begins(13 days).
80
Figure 47.12d
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic mesoderm
Allantois
Gastrulation has produced athree-layered embryo
withfour extraembryonicmembranes.
81
Developmental Adaptations of Amniotes

• The colonization of land by vertebrates was made
  possible only after the evolution of
• The shelled egg of birds and other reptiles as
  well as monotremes (egg-laying mammals)
• The uterus of marsupial and eutherian mammals

82

• In both adaptations, embryos are surrounded by
  fluid in a sac called the amnion
• This protects the embryo from desiccation and
  allows reproduction on dry land
• Mammals and reptiles including birds are called
  amniotes for this reason

83

• The four extraembryonic membranes that form
  around the embryo
• The chorion functions in gas exchange
• The amnion encloses the amniotic fluid
• The yolk sac encloses the yolk
• The allantois disposes of waste products and
  contributes to gas exchange

84
Organogenesis

• During organogenesis, various regions of the germ
  layers develop into rudimentary organs
• Early in vertebrate organogenesis, the notochord
  forms from mesoderm, and the neural plate forms
  from ectoderm

85
Figure 47.13
Eye
Somites
Tail bud
Neural folds
Neural fold
Neural plate
SEM
1 mm
1 mm
Neural tube
Neuralcrestcells
Neural plate
Neural fold
Notochord
Coelom
Neural crest cells
Notochord
Somite
Ectoderm
Outer layerof ectoderm
Mesoderm
Archenteron(digestivecavity)
Neural crest cells
Endoderm
(c) Somites
Archenteron
(a) Neural plate formation
Neural tube
(b) Neural tube formation
86
Figure 47.13a
Neural folds
1 mm
Neural fold
Neural plate
Notochord
Ectoderm
Mesoderm
Endoderm
Archenteron
(a) Neural plate formation
87

• The neural plate soon curves inward, forming the
  neural tube
• The neural tube will become the central nervous
  system (brain and spinal cord)

Video Frog Embryo Development
88
Figure 47.13b-1
Neural fold
Neural plate
(b) Neural tube formation
89
Figure 47.13b-2
Neural fold
Neural plate
Neural crest cells
(b) Neural tube formation
90
Figure 47.13b-3
Neural fold
Neural plate
Neural crest cells
Outer layerof ectoderm
Neural crest cells
Neural tube
(b) Neural tube formation
91

• Neural crest cells develop along the neural tube
  of vertebrates and form various parts of the
  embryo (nerves, parts of teeth, skull bones, and
  so on)
• Mesoderm lateral to the notochord forms blocks
  called somites
• Lateral to the somites, the mesoderm splits to
  form the coelom (body cavity)

92
Figure 47.13c
Eye
Somites
Tail bud
SEM
1 mm
Neural tube
Neuralcrestcells
Notochord
Coelom
Somite
Archenteron(digestivecavity)
(c) Somites
93
Figure 47.13d
Neural folds
1 mm
94
Figure 47.13e
Eye
Somites
Tail bud
SEM
1 mm
95

• Organogenesis in the chick is quite similar to
  that in the frog

96
Figure 47.14
Neural tube
Eye
Notochord
Forebrain
Somite
Archenteron
Coelom
Heart
Lateral fold
Endoderm
Bloodvessels
Mesoderm
Ectoderm
Somites
Yolk stalk
Yolk sac
These layersform extraembryonicmembranes.
Neuraltube
YOLK
(a) Early organogenesis
(b) Late organogenesis
97
Figure 47.14a
Neural tube
Notochord
Somite
Archenteron
Coelom
Lateral fold
Endoderm
Mesoderm
Ectoderm
Yolk stalk
Yolk sac
These layersform extraembryonicmembranes.
YOLK
(a) Early organogenesis
98
Figure 47.14b
Eye
Forebrain
Heart
Bloodvessels
Somites
Neuraltube
(b) Late organogenesis
99

• The mechanisms of organogenesis in invertebrates
  are similar, but the body plan is very different
• For example, the neural tube develops along the
  ventral side of the embryo in invertebrates,
  rather than dorsally as occurs in vertebrates

100
Mechanisms of Morphogenesis

• Morphogenesis in animals but not plants involves
  movement of cells

101
The Cytoskeleton in Morphogenesis

• Reorganization of the cytoskeleton is a major
  force in changing cell shape during development
• For example, in neurulation, microtubules
  oriented from dorsal to ventral in a sheet of
  ectodermal cells help lengthen the cells along
  that axis

102
Figure 47.15-1
Ectoderm
103
Figure 47.15-2
Ectoderm
Neuralplate
Microtubules
104
Figure 47.15-3
Ectoderm
Neuralplate
Microtubules
Actinfilaments
105
Figure 47.15-4
Ectoderm
Neuralplate
Microtubules
Actinfilaments
106
Figure 47.15-5
Ectoderm
Neuralplate
Microtubules
Actinfilaments
Neural tube
107

• The cytoskeleton promotes elongation of the
  archenteron in the sea urchin embryo
• This is convergent extension, the rearrangement
  of cells of a tissue that cause it to become
  narrower (converge) and longer (extend)
• Convergent extension occurs in other
  developmental processes
• The cytoskeleton also directs cell migration

108
Figure 47.16
Convergence
Extension
109
Programmed Cell Death

• Programmed cell death is also called apoptosis
• At various times during development, individual
  cells, sets of cells, or whole tissues stop
  developing and are engulfed by neighboring cells
• For example, many more neurons are produced in
  developing embryos than will be needed
• Extra neurons are removed by apoptosis

110
Concept 47.3 Cytoplasmic determinants and
inductive signals contribute 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
• Differentiation refers to the resulting
  specialization in structure and function

111

• Cells in a multicellular organism share the same
  genome
• Differences in cell types are the result of the
  expression of different sets of genes

112
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 germ layers is traceable to blastula
  cells

113
Figure 47.17
Epidermis
Epidermis
Centralnervoussystem
Notochord
Mesoderm
Endoderm
Blastula
Neural tube stage(transverse section)
(a) Fate map of a frog embryo
64-cell embryos
Blastomeresinjected with dye
Larvae
(b) Cell lineage analysis in a tunicate
114
Figure 47.17a
Epidermis
Epidermis
Centralnervoussystem
Notochord
Mesoderm
Endoderm
Blastula
Neural tube stage(transverse section)
(a) Fate map of a frog embryo
115
Figure 47.17b
64-cell embryos
Blastomeresinjected with dye
Larvae
(b) Cell lineage analysis in a tunicate
116
Figure 47.17c
117
Figure 47.17d
118

• 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

119
Figure 47.18
Zygote
0
First cell division
Nervoussystem,outer skin,muscula-ture
Muscula-ture, gonads
Outer skin,nervous system
Germ line(futuregametes)
Time after fertilization (hours)
Musculature
10
Hatching
Intestine
Intestine
Anus
Mouth
Eggs
Vulva
ANTERIOR
POSTERIOR
1.2 mm
120
Figure 47.18a
121

• 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 C. elegans, such complexes are called P
  granules, persist throughout development, and can
  be detected in germ cells of the adult worm

122
Figure 47.19
100 ?m
123

• 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 cells
• P granules act as cytoplasmic determinants,
  fixing germ cell fate at the earliest stage of
  development

124
Figure 47.20
20 ?m
Newly fertilized egg
Zygote prior to first division
Two-cell embryo
Four-cell embryo
125
Figure 47.20a
20 ?m
Newly fertilized egg
126
Figure 47.20b
20 ?m
Zygote prior to first division
127
Figure 47.20c
20 ?m
Two-cell embryo
128
Figure 47.20d
20 ?m
Four-cell embryo
129
Axis Formation

• A body plan with bilateral symmetry is found
  across a range of animals
• This body plan exhibits asymmetry across the
  dorsal-ventral and anterior-posterior axes
• The right-left axis is largely symmetrical

130

• The anterior-posterior axis of the frog embryo is
  determined during oogenesis
• The animal-vegetal asymmetry indicates where the
  anterior-posterior axis forms
• The dorsal-ventral axis is not determined until
  fertilization

131

• Upon fusion of the egg and sperm, the egg surface
  rotates with respect to the inner cytoplasm
• This cortical rotation brings molecules from one
  area of the inner cytoplasm of the animal
  hemisphere to interact with molecules in the
  vegetal cortex
• This leads to expression of dorsal- and
  ventral-specific gene expression

132
Figure 47.21
Dorsal
Right
Anterior
Posterior
Left
Ventral
(a) The three axes of the fully developed embryo
First cleavage
Animal pole
Pigmentedcortex
Animalhemisphere
Point ofspermnucleusentry
Futuredorsalside
Vegetalhemisphere
Gray crescent
Vegetal pole
(b) Establishing the axes
133

• In chicks, gravity is involved in establishing
  the anterior-posterior axis
• Later, pH differences between the two sides of
  the blastoderm establish the dorsal-ventral axis
• In mammals, experiments suggest that orientation
  of the egg and sperm nuclei before fusion may
  help establish embryonic axes

134
Restricting Developmental Potential

• Hans Spemann performed experiments to determine a
  cells developmental potential (range of
  structures to which it can give rise)
• 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)

135
Figure 47.22-1
EXPERIMENT
Control egg(dorsal view)
Experimental egg(side view)
Experimentalgroup
Controlgroup
Graycrescent
Graycrescent
Thread
136
Figure 47.22-2
EXPERIMENT
Control egg(dorsal view)
Experimental egg(side view)
Experimentalgroup
Controlgroup
Graycrescent
Graycrescent
Thread
RESULTS
Normal
Normal
Belly piece
137

• In mammals, embryonic cells remain totipotent
  until the 8-cell stage, much longer than other
  organisms
• Progressive restriction of developmental
  potential is a general feature of development in
  all animals
• In general tissue-specific fates of cells are
  fixed by the late gastrula stage

138
Cell Fate Determination and Pattern Formation by
Inductive Signals

• As embryonic cells acquire distinct fates, they
  influence each others fates by induction

139
The Organizer of Spemann and Mangold

• Spemann and Mangold transplanted tissues between
  early gastrulas and found that the transplanted
  dorsal lip 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

140
Figure 47.23
EXPERIMENT
RESULTS
Primary embryo
Dorsal lip ofblastopore
Secondary (induced) embryo
Pigmentedgastrula(donor embryo)
Primary structures
Neural tube
Nonpigmentedgastrula(recipient embryo)
Notochord
Secondary structures
Notochord (pigmented cells)
Neural tube(mostly nonpigmented cells)
141
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 information tells a cell where it is with
  respect to the body axes
• It determines how the cell and its descendents
  respond to future molecular signals

142

• The wings and legs of chicks, like all vertebrate
  limbs, begin as bumps of tissue called limb buds

143
Figure 47.24
Anterior
Limb bud
AER
ZPA
Limb buds
2
Posterior
50 ?m
Digits
Apicalectodermalridge (AER)
3
4
Anterior
Ventral
Proximal
Distal
Dorsal
Posterior
(a) Organizer regions
(b) Wing of chick embryo
144
Figure 47.24a
Anterior
Limb bud
AER
ZPA
Limb buds
Posterior
50 ?m
Apicalectodermalridge (AER)
(a) Organizer regions
145

• The embryonic cells in a limb bud respond to
  positional information indicating location along
  three axes
• Proximal-distal axis
• Anterior-posterior axis
• Dorsal-ventral axis

146
Figure 47.24b
2
Digits
3
4
Anterior
Ventral
Distal
Proximal
Dorsal
Posterior
(b) Wing of chick embryo
147
Figure 47.24c
50 ?m
Apicalectodermalridge (AER)
148

• One limb budregulating region is the apical
  ectodermal ridge (AER)
• The AER is thickened ectoderm at the buds 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

149

• Tissue transplantation experiments support the
  hypothesis that the ZPA produces an inductive
  signal that conveys positional information
  indicating posterior

150
Figure 47.25
EXPERIMENT
Anterior
New ZPA
Donorlimbbud
Hostlimbbud
ZPA
Posterior
RESULTS
4
3
2
2
3
4
151
Figure 47.25a
EXPERIMENT
Anterior
New ZPA
Donorlimbbud
Hostlimbbud
ZPA
Posterior
152
Figure 47.25b
RESULTS
4
3
2
2
3
4
153

• Sonic hedgehog is an inductive signal for limb
  development
• Hox genes also play roles during limb pattern
  formation

154
Cilia and Cell Fate

• Ciliary function is essential for proper
  specification of cell fate in the human embryo
• Motile cilia play roles in left-right
  specification
• Monocilia (nonmotile cilia) play roles in normal
  kidney development

155
Figure 47.26
Lungs
Heart
Liver
Spleen
Stomach
Large intestine
Normal locationof internal organs
Location insitus inversus
156
Figure 47.UN01
Sperm-egg fusion and depolarizationof egg
membrane (fast block topolyspermy)
Cortical granule release(cortical reaction)
Formation of fertilization envelope(slow block
to polyspermy)
157
Figure 47.UN02
2-cellstage forming
Animal pole
8-cellstage
Vegetal pole
Blastocoel
Blastula
158
Figure 47.UN03
159
Figure 47.UN04
Neural tube
Neural tube
Notochord
Notochord
Coelom
Coelom
160
Figure 47.UN05
Species
Stage
161
Figure 47.UN06