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Developmental Zoology
(ZOO 228.1.0)
Gatrulation
1
Developmental Stages
Ø Early Development
•  Fertilization
•  Cleavage
•  Gastrulation
•  Neurulation
Ø Later Development
•  Organogenesis
•  Larval molts
•  Metamorphosis
•  Aging
§ 
§ 
Cell movements occur
in distinct ways in
different genera
Invertebrates
§  invagination of cells on
the future ventral side
§ 
Vertebrates
§  fish & amphibians
§  spherical embryos
§  involution of
mesoderm on future
dorsal side at
blastopore lip
2
Developmental Stages
Ø Early Development
•  Fertilization
•  Cleavage
•  Gastrulation
•  Neurulation
Ø Later Development
•  Organogenesis
•  Larval molts
•  Metamorphosis
•  Aging
Ø Vertebrates
•  Reptiles, birds and some
mammals (humans)
– disc embryos
– de-lamination of
ectoderm to form
endoderm &
mesoderm
– delaminating cells
involute at Henson s
node, primitive streak
and groove form &
progresses along
dorsal midline
3
Developmental Stages
Ø Early Development Ø Vertebrates
•  mammals (mouse)
•  Fertilization
– cup embryos
•  Cleavage
– ingression at the dorsal
•  Gastrulation
midline (primitive
streak) as in chick and
•  Neurulation
human
Ø Later Development
•  Organogenesis
•  Larval molts
•  Metamorphosis
•  Aging
4
Questions
How does a single celled zygote become a
gastrula?
How are layers of tissue established?
How do cells get to their proper positions for
development?
•  In part, this is accomplished by various types of
Morphogenetic Movements.
•  Induction
Differentiation
•  Organogenesis is the formation of the organs
(Organo = organs, genesis = creation)
•  Arises from the layering of cells that occurs
during gastrula stage
•  The layers are germ layers; they have specific
fates in the developing embryo:
Differentiation
–  Endoderm
• The innermost layer
• Goes on to form the gut
–  Mesoderm
• In the middle
• Goes on to form the muscles, circulatory
system, blood and many different organs
–  Ectoderm
• The outermost
• Goes on to form the skin and nervous
system
Differentiation of Primary Germ
Layers (from the gastrula)
Ectoderm
Mesoderm
Endoderm
Nervous
system
Epidermis of
skin
Skeleton
Digestive tract
Muscles
Respiratory
system
Liver, pancreas
Circulatory
system
Gonads
Bladder
Morphogenetic Movements
Epiboly
Morphogenetic
Movements Invagination, Evagination
Morphogenetic Movements
Involution
Epiboly over a fold
Involution!
Morphogenetic
Movements
Delamination
Morphogenetic Movements
Ingression
Morphogenetic Movements
Cell Migration
Cell Migration
Morphogenetic
Movements
“Intercalation: rows of cells move between one another,
creating an array of cells that is longer (in one or more
dimensions) but thinner.”
“Convergent Extension: rows of cells intercalate, but the
intercalation is highly directional.” - involved in formation of
the primitive streak in avian and mammalian embryos.
Morphogenetic Movements
Differential Growth
.. ..
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time
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... .. ..
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Fate Maps
Mother cell
Cell division
Cell type A
Cell type B
Cell division places
daughter cells in
different
environments,
which can lead to
different cell fate
choices
Fate Maps
Induction: information from neighbors influences
cell fate
inducer
responder
Induction
Competence: ability to respond to a certain
inductive signal
responder
Cell not competent to respond
inducer
Induction
Succesive inductions: can generate many cell
types from just a few interactions
inducer
Cell not competent to respond
responder
Types of Signals
Inducer
Responder
Types of Signals
Signals can
act globally
throughout
the body
Induction
An initial difference can be
amplified into many cell types
How is cell fate determined
-- the dorsal-ventral axis in newts
How is cell fate determined
-- the dorsal-ventral axis in newts
Spemann & Mangold: the organizer can
influence neighboring cells to form a
secondary body axis:
Figure 7.17
Induction
Induction of the
organizer
can influence
neighboring cells
to form a
secondary
body axis:
Induction
If one transplants
a second inducer
of the organizer
the embryo forms
two body axes
It is not birth, marriage or death, but
gastrulation, which is truly the most
important time in your life.
- Lewis Wolpert (1986)
Gastrulation
Cell Movements Relevant to
Gastrulation
Getting cells inside
Spreading tissues out
Moving cells around
Making tissues longer
Convergence/extension
Gastrulation
•  Definition = migration and division of cells to
set up the 3 primary germ layers.!
•  What positions do presumptive germ layers
occupy in the blastula, before gastrulation? !
–  Experimental answers to this question
provide our basis for understanding cell
movements that occur during
gastrulation.!
•  FATE MAPS = diagram of blastula/blastodisc
showing the fate of each part. !
Gastrulation
•  Fate mapping technique developed by Vogt
in 1920s. Involves …!
–  Marking surface of blastula with vital dyes!
–  Dyes retained by cells for prolonged
periods, but don t interfere with normal
cellular processes!
–  Follow movements of marked cells during
gastrulation to ultimate locations in later
embryos!
Sample Fate Map of Frogs