Limb Development
Major questions:
Site of Limb formation
Limb identity
Establishment of 3 axes
Development of various structures in correct positions
Formation of the Limb Field
Morphogenetic field—set of cells whose position and fate are specified with respect to
the same intrinsic reference points
-can transplant, rotate etc and the organ forms at the new site and new orientation
the position of the limb field is specified by segment identity (Hox genes)
eg. fish, amphibians, birds and mammals—the anterior limb buds form at the
anteriormost HoxC-6 expressing region.
RA is also required
Limb Bud Formation
Proliferation of mesodermal mesenchyme
-lateral plate mesenchyme produces skeleton
-somite produces muscle
Induced by FGF10 from the lateral plate mesoderm (limb mesenchyme precursors)
-exogenous FGF10 can induce ectopic limb formation
-requires competent ectoderm—only at the lateral midline (boundary) between Dorsal
and ventral
Apical ectodermal ridge (AER) —
-mesoderm induces overlying ectoderm (FGF10)
-runs along the distal margin of the limb bud at the D-V boundary
-will become a major signaling center
AER + subjacent mesodermal mesenchyme = limb bud
Specification of Hindlimb vs. Forelimb
Determined by expression of 2 transcription factors
-Tbx5 ! forelimb
-Tbx4 ! hindlimb
controlled by the mesodermal mesenchyme
Proximal-distal axis formation
Involves interactions between AER and limb bud mesenchyme
AER:
-if remove, growth stops and cells differentiate
-graft on an extra AER, get supernumary limb structures
∴ AER promotes proliferation and inhibits differentiation of subjacent mesenchyme
limb mesenchyme:
-if replace with non-limb mesenchyme, development stops
-if replace forelimb mesenchyme with hindlimb mesenchyme, a hindlimb forms
∴ limb mesenchyme induces and sustains the AER, and specifies limb identity
Progress zone—mesenchyme directly beneath AER (within ~ 200µm)
-cells proliferate
-as cells exit the progress zone, they differentiate in a region-specific manner (ie. the
first cells to leave the progress zone form proximal structures. If stop development by
removing AER at different times, get the progressive production of structures along the
p-d axis)
The progress zone controls p-d identity. If an early (proximal) progress zone is
transplanted to a late limb, get duplicated limbs (p—d-p—d). If a late (distal) progress
zone is transplanted to an early limb, get deleted intermediate structures.
If only the AER is transplanted, the resultant limb looks normal.
AER maintains the progress zone—signals with FGF8