Chapter 47 Study Guide
Overview
Vocabulary
1. Cytoplasmic Determinants – Material substances in the egg that influence the course of
early development – regulate gene expression
2. Cell differentiation – Specialization of cell function/structure. Caused by gene
expression
3. Morphogenesis – Process by which an animal takes shape. Cells end up in appropriate
locations
Notes
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Epigenesis – An animal emerges gradually from a relatively formless egg. This is the right
explanation for zygote becoming an animal.
Preformation – egg or sperm contains an embryo that becomes larger during
development. Not the right explanation for zygote becoming an animal
Organism’s development – determined by genome of zygote, differences between early
embryonic cells  allows the expression of different genes in different cells
o Cells are different because  Cytoplasmic determinants, location in embryo, or
combination of the 2.
47.1 – After Fertilization, Embryonic Development Proceeds Through
Cleavage, Gastrulation, and Organogenesis
Vocabulary
1. Acrosomal Reaction – discharge of a sperm’s acrosome when it is near the egg
2. Acrosome – Vesicle at the tip of sperm, helps sperm penetrate the egg
3. Fast Block to Polyspermy – depolarization of egg membrane after sperm binds to
vitelline layer. Prevents more sperm from entering
4. Cortical Reaction – exocytosis of enzymes from cortical granules in the egg cytoplasm
during fertilization – help separate vitelline layer and plasma membrane
5. Cortical Granules – in eggs cortex – release enzymes during cortical reaction
6. Fertilization Envelope – the changed vitelline layer – prevents other sperm from
entering the egg
7. Slow Block to Polyspermy – Formation of fertilization envelope and other changes,
opposite of Fast block, lasts longer
8. Zona Pellucida – extracellular matrix of egg, has sperm receptors.
9. Cleavage – Rapid Cell divisions after Fertilization. S phase (DNA synthesis) and M phase
(mitosis). Skips protein synthesis
10. Blastomeres – smaller cells that the embryo divides into
11. Morula – cluster of cells after the first 5-7 divisions
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12. Blastocoel – a fluid filled cavity. Begins to form in morula, fully formed in blastula
13. Blastula – hollow ball of cells
14. Yolk – stored nutrients – distributed differently in all embryos
15. Vegetal Pole – The pole that the yolk is most concentrated
16. Animal Pole – Opposite pole, very little yolk
17. Gray Cresent – Light grey region of cytoplasm located near the equator of the egg on
the side opposite the sperm entry
18. Meroblastic Cleavage – Incomplete division of yolk-rich egg. In birds
19. Holoblastic Cleavage – Complete division of eggs having little yolk. In brids
20. Blastoderm – Cap of cells produced by bird embryos during early cleavage
21. Gastrulation – Morphogenetic phase. Drastic rearrangement of the cells of the blastula.
Forms a three-layered embryo with a primitive gut.
22. Gastrula – 3 layered Embryo
23. Germ Layers – The 3 layers produced
24. Ectoderm – Outer layer
25. Endoderm – Inner Layer
26. Mesoderm – Partly fills space between Ectoderm & Endoderm
27. Invagination – When cells fold inward
28. Archenteron – Primitive Gut. An endoderm-lined cavity
29. Blastopore – Opening in the archenteron, develops into the anus.
30. Dorsal Lip – The Dorsal side of the blastopore
31. Involution – When future endoderm and mesoderm cells on the surface roll over edge
of the lip into the interior of the embryo
32. Yolk Plug – Large food-laden endodermal cells surrounded by blastopore
33. Primitive Streak – groove on surface of an early bird embryo along the future long axis
of the body
34. Organogenesis – When regions of the three-layered embryo develop into fundamental
organs
35. Notochord – Formed from dorsal mesoderm
36. Neural Tube – when neural plate curves inward – rolling into itself
37. Neural Crest – band of cells along border of Neural tube
38. Somites – Paired blocks of mesoderm lateral to notochord
39. Amniotes – Animals that have an amniotic egg containing specialized membranes that
protect the embryo
40. Extraembryonic Membranes – Membranes located outside the embryo. In chicks
41. Blastocyst – mammalian version of a blastula
42. Inner Cell Mass – Group of cells clustered at one end of blastocyst cavity
43. Trophoblast – The outer epithelium of the blastocyst. Initiates implantation
44. Chorion – The outermost of 4 extraembryonic membranes. Contributes to forming the
placenta
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45. Amnion – The inner most of 4 extraembryonic membranes. Encloses a fluid-filled sac in
which the embryo is suspended
46. Yolk Sac – 1 of 4 extraembryonic membranes that support embryonic development
47. Allantois – 1 of 4 exraembryonic membranes. Serves as a repository for embryo’s
nitrogenous waste.
Notes
Fertilization (Vocab. 1-8)
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Sperm and egg unite – combining haploid sets of chromosomes, forming a single diploid
cell
Activation of the egg also happens – contact between egg and sperm initiates metabolic
reactions in the egg, triggering onset of embryonic development
Model – Sea Urchin
o Eggs fertilized externally – sperm head comes into contact with jelly coat of egg,
molecules present in egg’s coat trigger acrosomal reaction, which starts when
the acrosome discharges hydrolytic enzymes
o Jelly coat is digested by enzymes – allows the elongated sperm structure (the
acrosomal process) to penetrate jelly surface, proteins on tip of acrosomal
process adheres to molecules of a specific receptor protein on egg surface
o Vitelline layer – meshwork of extracellular matrix molecules under jelly coat.
Protein receptors for the sperm extend from plasma membrane and through the
layer
o Contact of tip of acrosome with egg leads to fusion of sperm and egg plasma
membranes and the entry of the sperm nucleus into the cytoplasm of the egg –
ion channels open in egg’s plasma membrane, Na+ flows into egg, changing
membrane potential (depolarization)
o Depolarization prevents additional sperm from fusing with the egg’s plasma
membrane (fast block) which only lasts a few minutes (short-term block), but
fusion of membranes triggers series of changes in the egg creating a longerlasting block
o Sperm binding – activates signal transduction pathway – causing Ca+ to be
released from the ER into the cytosol – high concentration of Ca+ starts cortical
reaction. Cortical granules release contents into perivitelline space (between
plasma membrane and vitelline layer)
o Enzymes (from granules) degrade proteins holding vitelline layer to plasma
membrane, H2O is drawn into perivitelline space, swelling it – pushes vitelline
layer away from plasma membrane – fertilization envelope formed which is a
slow block
Model – Mammals
o Internal – secretions in female reproductive tract alter certain molecules on
surface of sperm cell, and increase sperm motility.
o Egg is cloaked by follicle cells released with egg during ovulation. Sperm must
migrate though the layer to reach zona pellucida. Hydrolytic enzymes from
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acrosome enable the sperm to penetrate zona pellucida and reach the plasma
membrane
Binding of sperm and egg causes changes in egg, leading to cortical reaction,
released enzymes catalyze alterations of zona pellucida (becomes slow block)
Egg and sperm membranes fuse, the whole sperm is taken into the egg, a
centrosome forms around the centriole that acted as the basal body of the
sperm’s flagellum
Centrosome includes a 2nd centiole, duplicates forming 2 centrosomes in the
zygote – generated the mitotic spindle for the 1st cell division
Haploid nuclei doesn’t fuse immediately – envelopes of both nuclei disperse, the
2 sets of chromosomes share a common spindle apparatus. After 1 st division
diploid nuclei formed in the 2 daughter cells
Cleavage (Vocab. 9-20)
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Follows fertilization – rapid division of cells.
One large cell  Blastomeres  Morula and Blastocoel  Blastula
Eggs and Zygotes of sea urchins and other animals (except mammals) have definite
polarity, having poles (Vegetal and Animal). Polarity determined by uneven distribution
of substances in cytoplasm
3 Body Axes – 1)anterior/posterior 2)ventral/dorsal 3) left/right – best studied in frogs
o Embryo  Animal hemisphere is deep grey, Vegetal hemisphere is yellow.
o After fusion of egg and sperm, rearrangement of egg cytoplasm establishes 1
body axes
o Cytoplasmic determinants form – initiate development of dorsal structures.
Cortical Rotation establishes dorsal-ventral (back) axis of zygote. Sometimes
exposes Grey Crescent
Model – Frogs
o 1st 2 divisions are meridional (vertical) = 4 blastomers of equal size
o 3rd division is equatorial (horizontal) = 4 small blastomers (animal hemisphere)
and 4 big blastomers (vegetal hemisphere) – all because uneven distribution of
yolk
o Continuous division produces a morula then a blastula, with the blastocoel in the
animal hemisphere
Model – Bird Egg
o Zygote mostly yolk w/ a small disk of cytoplasm at animal pole
o Early divisions = Meroblastic -- Cleavage is though cytoplasm and not yolk
o Many divisions produce Blastoderm
o Blastomeres become 2 layers – Epiblast (upper) and Hypoblast (lower)
Gastrulation (Vocab. 21-33)
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After Cleavage – driven by changes in cell motility/shape, and changes in cellular
adhesion to other cells and to molecules of the extracellular matrix
Cells at surface of blastula move to interior and form 3 cell layers – Gastrula – Layers
called germ layers – layers = Ectoderm, endoderm, and mesoderm
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Model – Sea Urchin
o Blastula = 1 layer of ciliated cells surrounding the blastocoel. Mesenchyme cells
(migratory cells) move from vegetal pole into the blastocoel
o Vegetal plate invaginates, and mesenchyme cells migrate throughout the
blastocoel
o Endoderm cells form archenteron. New mesenchyme cells at the tip of the tube
begin to send out thin extensions (filopodia) toward the ectoderm cells of the
blastocoel wall
o Contraction of filopodia drag archenteron across blastocoel.
o Fusion of archenteron with the blastocoel wall completes formation of the
digestive tube with a mouth and Blastopore. The gastrula has 3 germ layers and
is covered with cilia (function in swimming and feeding)
Model – Frog
o Gastrulation – begins when a small indented crease, the dorsal lip, appears on
one side of the blastula. Crease is formed by cells changing shape and pushing
inward from the surface (invagination). Involution happens and cells move into
the interior and form mesoderm and endoderm. Cells of the animal pole (future
ectoderm) change shape and spread over the outer surface
o Blastopore lip grows on both sides of embryo, more cells invaginate. When sides
of the lip meet, the blastopore forms a circle that becomes smaller as ectoderm
spreads downward over the surface. Inside – involution continues, expanding
the endoderm and mesoderm, archenteron begins to form and blastocoel
becomes smaller
o Late gastrulation – endoderm-lined archenteron has completely replaced the
blastocoel and the 3 germ layers are in place. The circular blastopore surrounds
the yolk plug
Model – Bird
o Just like frog – cells move inward, but the movement is affected by mass of yolk
pressing against the bottom of the embryo.
o Cells from the epiblast form the embryo. Epiblast cells move toward the midline
of the blastoderm, then detach and move inward toward the yolik. Primitive
streak forms. The epiblast cells form the ectoderm, mesoderm, and endoderm
Organogenesis (Vocab. 34-38)
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After Gastrulation – involves more localized morphogenetic changes in tissue and cell
shape – 1st evidence of organ building is the appearance of folds, splits, dense clustering
of cells
Model – Frog
o 1st organs that begin to take form are the Notochord and the Neural tube
o Dorsal mesoderm forms the notochord
o Signals from the notochord start the formation of the Neural Plate which is
thickened dorsal ectoderm. The neural folds are the 2 ridges that form the
lateral edges of the plate
o In folding and pinching off of the neural plate generates the neural tube. At the
pinch off, the Neural plate forms
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o Lateral mesoderm separates into 2 tissue layers that line the coelom (lining of
the body cavity). Somites form and border the notochord
47.2 – Morphogenesis in Animals Involves Specific Changes in Cell
Shape, Position, and Adhesion
Vocabulary
1. Convergent Extension – morphogenetic movement – cells of tissue layer
rearrange, sheets become narrow (converge) and become longer (extend)
2. Cell Adhesion Molecules (CAMs) – glycoproteins – help cell migration and stable
tissue structure
3. Cadherins – important cell-to-cell adhesion molecule
Notes
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Morphogenesis – major aspect of development in animals – involves the movement of
cells – Movement causes changes in cell shape and can enable a cell to migrate. Changes
in shape and position are involves in cleavage, gastrulation, and organogensis
Changes in shape – involve reorganization of the cytoskeleton
o Cytoskeleton drives cell migration
o Invagination (during gastrulation) is initiated by the wedging of cells on the
surface of the blastula. –Movement deeper into embryo involves the extension
of filopodia by cells.
o Cells that move 1st drag the rest with them
Convergent extension – cell crawling is involved – cells wedge between each other,
tissue extends dramatically
Extracellular Matrix (ECM) – mixture of secreted glycoproteins – outside the plasma
membranes of cells – trigger/guide cell movement
o Some ECMs promote cell migration by providing specific molecular anchorage
for moving cells. Others inhibit migration in certain directions
CAMs are involved in cell migration and stable tissue structure. Cadherins require
calcium ions for proper function
47.3 – The Developmental Fate of Cells Depends on Their History and
on Inductive Signals
Vocabulary
1. Induction – Ability of one group of embryonic cells to influence the development
of another
2. Fate Maps – diagram of embryonic development – reveals future development
of individual cells/tissues
3. Totipotent – describes a cell that can become any part of an organism. Only the
Zygote
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4. Pattern Formation – development of an animal’s spatial organization,
arrangement of organs/tissues – influenced by inductive signals
5. Positional Information – Molecular cues – control pattern formation
6. Apical Ectodermal Ridge (AER) – 1 organizer – thickened area of ectoderm at the
tip of the limb bud
7. Zone of Polarizing Activity (ZPA) – other organizer – block of mesodermal tissue
located underneath ectoderm – posterior side of the bud is attached to body
Notes
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Development requires a combo of morphogenetic changes and the timely
differentiation of cells in specific location
2 general principles
o Early cleavage divisions – Embryonic cells must become different from each
other
o Once initial cells asymmetries are set up, subsequent interactions among the
embryonic cells influence their fate – usually causing changes in gene expression
 Induction brings about the differentiation.
Fate Maps – researchers worked out the developmental history of every cell in a roundworm – from 1st cleavage division to the zygote
o A cell’s fate can be changed by moving the cell to a new location
o 2 Important conclusions
 Specific tissues of the older embryo can be attributed to certain early
“founder cells”
 As development proceeds a cell’s developmental Potential becomes
restricted
Establishing basic body plan is 1st step in morphogenesis – a prerequisite for the
development of tissues/organs
Zygote’s cells
o Totipotent
o Zygote’s pattern of cleavage affects the fate of cells
o Progressive restriction of potency is a feature of development in all animals. The
tissue-specific fates of cells in late gastrula are fixed
Cell division creates cells that differ from each other  the cells then influence each
other’s fate (induction)
o Pattern Formation induction plays huge role. Positional Information is involved
Limbs begin as bumps of tissue called Limb buds which consist of a core of mesoderm
tissue covered by a layer of ectoderm – 2 organizer locations affect limb’s development
o Apical Ectodermal Ridge (AER) and Zone of Polarizing Activity (ZPA)
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