Limb Patterning: ProximalDistal Axis
Gilbert - Chapter 16
Do you want to hear a talk from an
ASTRONAUT 4/17??
Today’s Goals
• Become familiar with several aspects of limb
formation in the tetrapod
– Limb initiation
• Forelimb vs. hindlimb
• Where to make a limb?
– Limb patterning
• Dorsal/ventral
• Anterior/Posterior
• Proximal/distal
Generating Limb Axes
• Classical embryonic manipulations from
the dating from the 1940’s
– John Saunders
– Series of surgical rearrangements that later
became the basis for what we know about
molecular signals in limb formation
– He identified specialized areas in the limb
that were necessary for development of the
different axes
John Saunders
Proximal Distal Axis
• Growth along P-D axis made possible by
interaction between 2 tissues
• Apical ectodermal ridge (AER)
– Thickening of ectoderm at distal tip of limb bud
– Very important for several aspects of limb
formation
• Progress zone (PZ)
– Mesenchyme directly underneath AER
– Proliferates to lengthen the limb bud
PZ
Experimental Manipulations
• Remove AER at any time
– Distal limb development ceases
• Graft extra AER to existing limb
– Extra distal structures form
• CONCLUDE:
– AER is necessary and sufficient for distal
limb formation
Removal of AER at different
times: loss of various structures
More Experimental
Manipulations
• Remove PZ from wing, replace it with leg PZ
– Toes form at distal tip of wing!
• Remove PZ from wing, replace with
mesenchyme from non-limb
– Limb development ceases, AER disappears
• CONCLUDE:
– PZ involved in specifying limb type
– AER dependent on PZ to continue developing
Overall conclusions: AER, PZ
• AER required for distal limb
development
• AER keeps PZ in a proliferative state,
and PZ sustains AER (feedback loop)
• What molecules are involved in this
interaction??
FGFs and the AER
• FGFs, including FGF10, FGF8
– Important in formation, sustaining the AER
• FGF10 expression in the mesenchyme of the
forming limb bud induces formation of AER in
the overlying ectoderm
– This occurs at the distal tip of the limb
– FGF10 induces expression of FGF8 in AER
– FGF8 is secreted from AER, signals to PZ to keep
dividing
– FGF8 acts in feedback loop to instruct PZ to
maintain FGF10 expression
FGF8 expression
FGFs and The AER
• If the AER is removed from the
developing limb
– Normally, development of distal limb
ceases
– BUT - if remove AER and put in a bead
coated in FGF
• NORMAL DEVELOPMENT OF LIMB!
How are cells specified with
regard to P-D axis?
• The AER and PZ allow for limb
outgrowth, but how do proximal cells
know to form humerus, distal cells know
to form digits?
• 2 Models
– Progress zone model
– Early allocation and progenitor expansion
model
P-D axis specification: Guess
who?
• Regardless of which model for the P-D
axis wins out . . .
• Hox genes appear to be involved!
– These genes are expressed in a nested
pattern that changes during limb
development
– When mutations occur, changes can occur
in the P-D axis
Specific Hox Paralog groups are
expressed in specific regions of the limb
A= Wild Type Mouse; B = Hox a-11, Hox d11 double mutant
mouse; C = Human Polysyndactyly (HomozygousHoxD13
mutation)