Limb Patterning: Proximal-Distal Axis Gilbert - Chapter 16 Do you want to hear a talk from an ASTRONAUT 4/17??
Jan 18, 2016
Limb Patterning: Proximal-Distal 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)