Gastrulation rearranges a blastula into a triploblastic gastrula with three germ layers - ectoderm, mesoderm and endoderm. These germ layers then differentiate into various tissues and organs through the process of organogenesis. Patterning and morphogenesis in animal development involves changes in cell shape, position and adhesion regulated by the cytoskeleton, extracellular matrix, cell adhesion molecules and growth factors.

2.  Gastrulation rearranges the embryo into a
triploblastic gastrula.
Gastrulation rearranges the
blastula to form a three-layered
embryo with a primitive gut
Primary Germ Layers
Ectoderm
Endoderm
Mesoderm

3. The
Formation of
Primary
Germ Layers

4. Fates of the Primary Germ Layers
• Ectoderm
– hair, nails, epidermis, brain, nerves
• Mesoderm
– notochord (in chordates), dermis, blood
vessels, heart, bones, cartilage, muscle
• Endoderm
– internal lining of the gut and respiratory
pathways, liver, pancreas

5. Germ Layer Patterns
Diploblastic
gutEndoderm
Ectoderm

6. Phylum Cnidaria
Diploblastic- two germ layers

7. Germ Layer Patterns
Triploblastic- 3
germ layers
acoelomate
gutEndoderm
Ectoderm
Mesoderm

8. Gastrulation
in Sea Urchin
Embryo

9. Frog gastrulation

10. Early
organogenesis in
a frog embryo

11. • The amniote embryo is the solution to
reproduction in a dry environment.
Amniote embryos develop in a fluid-
filled sac within a shell or uterus
chorion
amnion
embryo
allantois
yolk sac
Extraembryonic membrane
Fetal portion of placenta
Maternal portion of placenta
reptile & bird mammal

12. • The four extraembryonic membranes are
the yolk sac, amnion, chorion, and
allantois.
–Cells of the yolk sac digest yolk
providing nutrients to the embryo.
–The amnion encloses the embryo in a
fluid-filled amniotic sac which protects
the embryo from drying out.
–The chorion cushions the embryo
against mechanical shocks.
–The allantois functions as a disposal sac
for uric acid.

13. Mammalian Development.
–Recall:
• The egg and zygote do not exhibit any
obvious polarity.
• Holoblastic cleavage occurs in the zygote.
–Gastrulation and organogenesis follows a
pattern similar to that seen in birds and
reptiles.

14. Organogenesis
Differentiation of primary
germ layers into tissues
and organs.

20. • Changes in
cell
shape usually
involves
reorganization
of the
cytoskeleton.
Morphogenesis in animals involves
specific changes in cell shape, position,
and adhesion

21. • The cytoskeleton is also involved in cell
movement.
– Cell crawling is involved in convergent
extension.
• The movements of convergent extension probably
involves the extracellular matrix (ECM).
• ECM fibers may direct cell movement.
• Some ECM substances, such a
fibronectins, help cells move by
providing anchorage for crawling.
• Other ECM substances may inhibit
movement in certain directions.

22. •Cell adhesion molecules
(CAMs): located on cell surfaces
bind to CAMs on other cells.
–Differences in CAMs regulate
morphogenetic movement and tissue
binding.
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23. •Cadherins are also involved in
cell-to-cell adhesion.
–Require the presence of calcium for
proper function.
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24. • In many animal species (mammals may be a
major exception), the heterogeneous
distribution of cytoplasmic determinants in
the unfertilized egg leads to regional
differences in the early embryo
The developmental fate of cells
depends on cytoplasmic determinants
and cell-cell induction: a review
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25. • Subsequently, in induction,
interactions among the embryonic
cells themselves induce changes in
gene expression.
–These interactions eventually bring about
the differentiation of the many
specialized cell types making up a new
animal.
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26. • Fate maps illustrate the developmental
history of cells.
• “Founder cells” give rise to specific tissues in
older embryos.
• As development proceeds a cell’s
developmental potential becomes
restricted.
Fate mapping can reveal cell
genealogies in chordate embryos
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27. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 47.20

28. • Polarity and the Basic Body Plan.
– In mammals, polarity may be established by the
entry of the sperm into the egg.
– In frogs, the animal and vegetal pole determine
the anterior-posterior body axis.
The eggs of most vertebrates have
cytoplasmic determinants that help
establish the body axes and differences
among cells of the early embryo
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29. • Restriction of Cellular Potency.
– The fate of embryonic
cells is affected by
both the distribution
of cytoplasmic
determinants and
by cleavage pattern.
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Fig. 47.21

30. • Induction: the influence of one set of cells on
a neighboring group of cells.
–Functions by affecting gene expression.
• Results in the differentiation of cells into a
specific type of tissue.
Inductive signals drive differentiation
and pattern formation invertebrates
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31. • The “Organizer” of Spemann and Mangold.
• Grafting the dorsal lip
of one embryo onto
the ventral surface of
another embryo
results in the develop-
ment of a second
notochord and neural
tube at the site
of the graft.
– Spemann referred
to the dorsal lip
as a primary
organizer.
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Fig. 47.22

32.  An example of the molecular basis of
induction:
 Bone morphogenetic protein 4 (BMP-4) is
a growth factor promoting promote the
formation of bone and the skeleton
– In amphibians, organizer cells inactivate BMP-4
on the dorsal side of the embryo.
– In humans it’s a critical signaling molecule
required for the early differentiation of the embryo
and establishing of a dorsal-ventral axis
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33. BMP in neural tube
formation
1. Inhibition of BMP
signaling
2. At end of neurulation the
lateral edges of the
neural plate fuse
3. They segregate from the
non-neural epithelia to
form a neural tube
4. Roof plate of neural tube
now produces BMP. BMP
stimulates neural crest
cell formation

34. • Pattern Formation in the
Vertebrate Limb.
–Induction plays a major role in
pattern formation.
• Positional information, supplied by
molecular cues, tells a cell where it is
relative to the animals body axes.
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35. • Limb development in chicks as a model of
pattern formation.
• Wings and legs begin as limb buds.
– Each component
of the limb is
oriented with
regard to
three axes:
– Proximal-distal
– Anterior-posterior
– Dorsal-ventra.
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Fig. 47.23b

36. Organizer regions.
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37. • Apical ectodermal ridge (AER).
– Secretes fibroblast growth factor (FGF)
proteins.
– Required for limb growth and patterning along
the proximal-distal axis.
– Required for
pattern formation
along the
dorsal-ventral
axis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 47.23a

38. • Zone of polarizing activity (ZPA).
–Secretes Sonic hedgehog, a protein
growth factor.
–Required for pattern formation of the limb
along the anterior-posterior axis.
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39. • Homeobox-containing (Hox) genes
play a role in specifying the identity of
regions of the limb, as well as the body
as a whole.
–In summary, pattern formation is a
chain of events involving cell
signaling and differentiation.
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40. Guess whom the following grew up to be:
C. D.B.A.
E.
F. G.
H.

41. C. cat D. humanB. fishA. dolphin
Phylum Chordata
E. Mouse
F. Elephant G. Snake
H. bat