Chapter 16- Tetrapod limb development

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Chapter 16- Tetrapod limb development
It is one thing to differentiate the
chondrocytes and osteocytes that synthesize the
cartilage and bone matrices it is another thing
to produce those cells in a temporal-spacial
orientation that generates a functional bone.
How is it that the fingers form at one end of
the limb and nowhere else?
How is it that the little finger forms at one
edge of the limb and nowhere else?
We will use limb development to examine the
larger question of morphogenesis
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Human development
5 weeks
11 weeks
Hands
7 weeks
11weeks
5 weeks
Feet
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Morphogenic rules

• These rules appear to be the same in all
  tetrapods

Example- Grafted mammalian limb bud will direct
formation of chick limbs

• These rules must follow a _______________
  coordinate system

Dimension
Proteins families involved
1. ________________ (hip-toe)
_______________
2. _________________ (thumb-pinky)
_______________
_________
3. _________________ (knuckle-palm)
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Recall from Chapter 11- Hox genes dictate
_______________ axis development
Fig. 11.36- Hox genes are organized in a
_______________ that concurs with
_________________ structures
This is referred to as the ____________
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Also recall that ____________ has a profound
effect on development by activating _____gene
expression
Recall amphibian development (Ch. 10)
Fig. 10.41
Structure of retinoic acid (not in textbook)
RA
RA
RA receptor
Retinoic acid activates mammalian ____genes
Hox gene
Retinoic acid bind a _______, then the complex
binds promoter of a ________
Lacks all distal _______
Wild-type mouse embryo
RA-treated mouse embryo
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Another unusual observation-
Experiment- Amputate tadpole tail, then soak the
stump in ___
Result- Several _____ develop
How does a _____develop?
Limb development initiates from __________ of
_____________ of _____________ mesoderm and from
___________
Limb bud
_______
Somatic layer of ___________ mesoderm
Fig. 14.1- mesoderm lineages
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What dictates forelimb from hindlimb?

• Transcription factor ____ is expressed in mouse
  ________

• Transcription factor ____ is expressed in
  mouse _________

• Loss of one copy of _____ gene in humans results
  in __________________

Observations-
Normal
Fig. 16.5

• If Place FGF-soaked bead near forelimb bud,
  generate a _____ ______

• If Place FGF-soaked bead near hindlimb bud,
  generate a __________________

FGF bead inserted

• If Place FGF-soaked bead between hindlimb and
  forelimb buds, generate a ___________

Thus, FGF _______ to make a limb, but tbx5 and
tbx4 instruct ______ of limb
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Fig. 16.5F -Chimera limb from FGF bead placed
between limb buds-
Fig. 16.5E Extra wing from FGF bead placed near
forelimb bud
The apical ectodermal ridge (AER) is a critical
structure
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Generating the ______________ axis
Forelimb AER experiments
Limb devel. ceases
The _______ ____________ (AER) is a critical
structure
AER removed
AER duplicated
Duplicate wing
AER replaced with leg mesoderm
chimera

• AER is required for
• ______________,
• _______________ axis formation,
• ______ axes specification

AER replaced with other mesoderm
Limb devel. ceases
AER replaced by FGF bead
normal
Fig. 16.8
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Generating the _____________________ axis
Hox genes specify two things-
1. Where the limb _______ (recall chapter 11)
2. Whether a cell will become a ________
(humerus), ________ (Ulna and radius) or an
__________ (metacarpels and digits)
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10
11
12
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Fig. 16.1
Fig. 16.14
Ulna, Radius
Scalpula
Digits
Humerous
Metacarpels
Example- If KO Hoxa-11 and Hoxd11 in mouse, no
_________________
Example- Humans with defective HOX-13 display
________________
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Specifying the anterior-posterior limb axis the
proximal-distal axis
What dictates thumb-pinky orientation??
1. A group of cell located near ____________
region of limb bud- called _______________________
___ (ZPA) directs polarity
2. These cells in the ZPA express
__________________ (Shh)
Evidence
then observe second set of __________
Extra set
If place Shh bead in anterior limb bud
Normal
3. Sonic hedgehog (Shh) activates __________ genes
Hoxd genes specify _____________ orientation
Fig. 6.15
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Specifying the _____________ limb axis
The surrounding ectoderm signal dorsal-ventral
axis

• _____ activates ______expression in mesenchyme

• _____ is required to specify ______cell fate

• Lmx1 mutant humans have loss of _________,
  ________________
• May also have ____________

All three axes are ________________
Ectoderm secretes ____
..which maintains ____ expression in ___..
..which activates ___ genes in limb bud..
..which specifies _____________ limb axis.