Limb Development Flashcards
At what point of development does limb formation begin in humans?
End of the fourth week
How does limb development begin?
Activation of a group of mesenchymal cells in the lateral plate mesoderm.
Describe a limb bud.
Bulges from the body wall as a mass of mesodermal cells covered by a layer of ectoderm.
What are the two portions of the limb bud?
Apical ectodermal ridge (AER)
Progress zone
What is the apical ectodermal ridge derived from?
Ectoderm
What does the apical ectodermal ridge interact with?
The underlying progress zone
What makes up the progress zone?
Proliferating cells
What is the progress zone derived from?
Two types of mesoderm
From what types of mesoderm is the progress zone derived?
Lateral plate (bones)
Somites (muscles)
How does the embryo decide where the place the limb field?
2
- The somites send a signal to the lateral plate
- The lateral plate mesoderm must be competent to receive this signal.
What makes the lateral plate competent to receive signalling from the somites?
Absence of Fgf8
What happens when different regions of the paraxial mesoderm are tranplanted into the limb field?
Changes in limb bud size.
What determines what part of the body will form the limb bud?
Somites
What determines where the forelimb buds will form?
Expression of Hoxc6 in the somites
What genes pattern the somites?
Hox genes
What directs the lateral mesoderm to form limb buds?
Somites
What is RARE?
A reporter for retinoic acid expression.
Where is RARE expressed?
Between Fgf8 in the heart and Fgf8 in the posterior progenitor zone
What is a result of mutants that do not produce Retinoic acid?
Loss of Tbx5 expression
What is the result of a loss of Tbx5 expression?
Expansion of Fgf8 expression and loss of the limb field.
What two T-box transcription factors does retinoic acid from the paraxial mesoderm initiate the level-specific expression of?
Tbx5
Tbx4
Where is Tbx5 expressed?
In the area of the future forelimb
Where is Tbx4 expressed?
In the area of the future hindlimb.
What do the Tbx genes activate and why is this important for limb development.
Activate Fgf10, which is essential for limb bud initiation.
Outline the molecular initiation of limb buds.
RA expression
Tbx4/5 expression
Fgf10 expression
Limb bud initiation
Where is Fgf10 expressed?
In the lateral plate mesoderm in the positions where limbs form.
What determines if a wing, leg or chimera forms in a chick embryo?
Proximity of an FGF-10 bead to Tbx4 or Tbx5 expression.
Is Fgf10 sufficient to induce an ectopic limb?
Yes
What develops in a chick embryo if the FGF10 bead is closer to Tbx5 expression?
Wing
What develops in a chick embryo if the FGF10 bead is closer to Tbx4 expression?
Leg
What develops if the FGF bead in the chick embryo is equidistant between Fgf4 and 5?
Chimera
What is a chimeric limb?
A limb containing anterior wing structures and posterior leg structures.
What drives forelimb growth?
Feedback loops involving Tbx-5/FGF-10 and FGF-10/FGF-8
Early in limb development, what is the primary bearer of the limb blueprint: The mesoderm or the ectoderm?
Mesoderm
What does Fgf10 in the mesoderm induce?
Expression of Fgf8 in the overlying ectoderm
What induces the apical ectodermal ridge?
Fgf-10 in the mesoderm
What is secreted by the apical ectodermal ridge, and what does this do?
Fgf-8 promotes proliferation of the underlying mesoderm
What happens if the apical ectodermal ridge is removed at any time of development?
Arrested distal outgrowth of the limb
What is the consequence of extra apical ectodermal ridge in the forelimb mesenchyme of the chick embryo?
Wing duplication
What is the consequence of the apical ectodermal ridge being in contact with the leg mesenchyme of the chick embryo?
Development of a wing attached to a leg.
What is the consequence of the apical ectodermal ridge coming into contact with nonlimb mesenchyme in the chick embryo?
The AER regresses and limb development ceases.
What develops when AER in the forelimb mesenchyme of the chick embryo is replaced with an FGF bead?
Normal wing
What causes diplopodia in humans?
Formation of two AERs on the limb bud
What happens when a ZPA is grafted to the anterior limb bud mesoderm?
Duplicated digits emerge as a mirror image of the normal digits.
What is the morphogen secreted by the ZPA?
Shh
What causes polydactyly in humans and cats?
Shh gain of function enhancer mutations
What establishes the anterior-posterior axis of the limb?
Shh
Where are limb buds located?
Bilaterally at the presumptive locations of the forelimbs and hindlimbs
What is the Zone of Polarizing Activity? (ZPA)
Cells in the most posterior region of the progress zone.
What is the maximum amount of limb buds a vertebrate embryo can have?
4
Describe the transplantation that results in a larger forelimb bud.
Transplanting PSM from level of presumptive forelimb to flank
Describe the transplantation that results in a smaller forelimb bud.
Tranplantation of PSM from the flank region to the level of the presumtive forelimb.
Outline the four steps in the initiation of limb bud formation.
- Making mesoderm permissive for limb formation
- Specifying forelimb and hindlimb
- Inducing epithelial-mesenchymal transitions
- Establishing two positive feedback loops for limb bud formation.
Describe how Shh specifies digit identity.
Concentration of Shh in a certain region of the limb bud and then duration of that area’s exposure to Shh specifies the digit it will become.
Describe how Shh helps shape the limb bud.
Shh stimulates proliferation and expansion of the limb bud mesenchyme, helping with its shape.
