1. Animal Development Chapter 47 1 mm Figure 47.1
Figure 47.1 How did a single cell develop into this intricately detailed embryo?
Chapter 47
1 mm

2. Development occurs at many points in the life cycle of an animal
This includes metamorphosis and gamete production, as well as embryonic development
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3. EMBRYONIC DEVELOPMENT Sperm
Figure 47.2
EMBRYONIC DEVELOPMENT
Sperm
Zygote
Adult frog
Egg
FERTILIZATION
CLEAVAGE
Metamorphosis
Blastula
GASTRULATION
Figure 47.2 Developmental events in the life cycle of a frog.
ORGANO- GENESIS
Larval stages
Gastrula
Tail-bud embryo

4. Animals display different body plans
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
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5. Concept 47.1: Fertilization and cleavage initiate embryonic development
Fertilization is the formation of a diploid zygote from a haploid egg and sperm
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6. 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
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
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
Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy
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7. Fertilization envelope
Figure
Sperm plasma membrane
Sperm nucleus
Fertilization envelope
Acrosomal process
Basal body (centriole)
Actin filament
Sperm head
Cortical granule
Fused plasma membranes
Figure 47.3 The acrosomal and cortical reactions during sea urchin fertilization.
Acrosome
Hydrolytic enzymes
Perivitelline space
Jelly coat
Vitelline layer
Sperm-binding receptors
EGG CYTOPLASM
Egg plasma membrane

8. 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 reaction is triggered by a change in Ca2 concentration
The fertilization envelope acts as the slow block to polyspermy
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9. 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 sperm nucleus merges with the egg nucleus and cell division begins
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10. 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
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
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11. Zona pellucida Follicle cell Sperm nucleus Cortical granules
Figure 47.5
Zona pellucida
Follicle cell
Figure 47.5 Fertilization in mammals.
Sperm nucleus
Cortical granules
Sperm basal body

12. Cleavage
Fertilization is followed by cleavage, a period of rapid cell division without growth
In mammals the first cell division occurs 1236 hours after sperm binding
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
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13. (a) Fertilized egg (b) Four-cell stage (c) Early blastula
Figure 47.6
50 m
(a) Fertilized egg
(b) Four-cell stage
(c) Early blastula
(d) Later blastula
Figure 47.6 Cleavage in an echinoderm embryo.

14. 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 first two cleavage furrows in the frog form four equally sized blastomeres
The third cleavage is asymmetric, forming unequally sized blastomeres
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15. 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
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16. 8-cell stage (viewed from the animal pole)
Figure 47.7
Zygote
2-cell stage forming
Gray crescent
0.25 mm
8-cell stage (viewed from the animal pole)
4-cell stage forming
Animal pole
8-cell stage
Figure 47.7 Cleavage in a frog embryo.
0.25 mm
Blastula (at least 128 cells)
Vegetal pole
Blastocoel
Blastula (cross section)

17. 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
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18. Gastrulation
Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula
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
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19. ECTODERM (outer layer of embryo)
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
Figure 47.8 Major derivatives of the three embryonic germ layers in vertebrates.
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

20. 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
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21. 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 chick embryo
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22. Endometrial epithelium (uterine lining)
Figure 47.12a
Endometrial epithelium (uterine lining)
Inner cell mass
Uterus
Trophoblast
Blastocoel
Figure Four stages in the early embryonic development of a human. 1
Blastocyst reaches uterus.

23. Expanding region of trophoblast
Figure 47.12b
Expanding region of trophoblast
Maternal blood vessel
Epiblast
Hypoblast
Trophoblast
Figure Four stages in the early embryonic development of a human. 2
Blastocyst implants (7 days after fertilization).

24. Expanding region of trophoblast
Figure 47.12c
Expanding region of trophoblast
Amniotic cavity
Epiblast
Hypoblast
Yolk sac (from hypoblast)
Extraembryonic mesoderm cells (from epiblast)
Chorion (from trophoblast)
Figure Four stages in the early embryonic development of a human. 3
Extraembryonic membranes start to form (10–11 days), and gastrulation begins (13 days).

25. Extraembryonic mesoderm
Figure 47.12d
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic mesoderm
Allantois
Figure Four stages in the early embryonic development of a human. 4
Gastrulation has produced a three-layered embryo with four extraembryonic membranes.

26. 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
In both adaptations, embryos are surrounded by fluid in a sac called the amnion
This protects the embryo from desiccation and allows reproduction on land
Mammals and reptiles including birds are called amniotes for this reason
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27. 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
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28. 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
The neural plate soon curves inward, forming the neural tube
The neural tube will become the central nervous system (brain and spinal cord)
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29. Figure 47.13
Neural folds
Eye
Somites
Tail bud
Neural fold
Neural plate
SEM
1 mm
1 mm
Neural tube
Neural crest cells
Neural fold
Neural plate
Notochord
Neural crest cells
Coelom
Notochord
Ectoderm
Somite
Figure Neurulation in a frog embryo.
Mesoderm
Outer layer of ectoderm
Archenteron (digestive cavity)
Endoderm
Neural crest cells
(c) Somites
Archenteron
(a) Neural plate formation
Neural tube
(b) Neural tube formation

30. Mesoderm lateral to the notochord forms blocks called somites
Neural crest cells develop along the neural tube of vertebrates and form various parts of the embryo (nerves, parts of teeth, skull bones, etc)
Mesoderm lateral to the notochord forms blocks called somites
Lateral to the somites, the mesoderm splits to form the coelom (body cavity)
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31. Archenteron (digestive cavity)
Figure 47.13c
Eye
Somites
Tail bud
SEM
1 mm
Neural tube
Neural crest cells
Notochord
Coelom
Figure Neurulation in a frog embryo.
Somite
Archenteron (digestive cavity)
(c) Somites

32. (b) Late organogenesis
Figure 47.14b
Eye
Forebrain
Heart
Blood vessels
Somites
Figure Organogenesis in a chick embryo.
Neural tube
(b) Late organogenesis

33. 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
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34. Mechanisms of Morphogenesis
Morphogenesis in animals involves movement of cells
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
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35. Ectoderm Neural plate Microtubules Actin filaments Neural tube
Figure
Ectoderm
Neural plate
Microtubules
Actin filaments
Figure Change in cell shape during morphogenesis.
Neural tube

36. 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
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37. 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
Differences in cell types are the result of the expression of different sets of genes
Differentiation refers to the resulting specialization in structure and function
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38. 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
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39. Blastomeres injected with dye
Figure 47.17
Epidermis
Epidermis
Central nervous system
Notochord
Mesoderm
Endoderm
Blastula
Neural tube stage (transverse section)
(a) Fate map of a frog embryo
64-cell embryos
Figure Fate mapping for two chordates.
Blastomeres injected with dye
Larvae
(b) Cell lineage analysis in a tunicate

40. 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
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41. 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
The anterior-posterior axis of the frog embryo is determined during oogenesis
The animal-vegetal asymmetry indicates where the anterior-posterior axis forms
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
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42. Restricting Developmental Potential
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)
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
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43. Cell Fate Determination and Pattern Formation by Inductive Signals
As embryonic cells acquire distinct fates, they influence each other’s fates by induction
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
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