Limb development Flashcards
LO
- Organogenesis
- Development of the Vertebrate Limb– or why is your leg different to your arm
What are the main players of limb development?
- Secreted signals
- Transcription factors
- Regions
Name some secreted signals in limb development
- FGFs
- SHH
- BMPs
- Wnts
Name some transcription factors in limb development
- Hox genes
- Tbx
Name some regions in limb development
- Limb field
- AER
- ZPA
- Dorsal limb ectoderm
What are some tools of limb development?
- Chick- manipulation and staining (protein/mRNA)
- Amphibian- manipulation
- Mouse-staining and genetic manipulation
- Null mutants
- Cre lox –stage specific deletion
- Overexpression
- Man-natural mutations
Tell me about the development of the pentadactyl limb
- What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions?
- 3 planes (stylopod, zeugopod and autopod (this changes the most between our species))

With a limb (in this examples a wing) tell me about the segmental structures
Stylopod: The upper part of the arm
Zeugopod: consists of radius (ra) and ulna (ul) in the forelimb, and tibia (ti) and fibula (fi) in the hindlimb
Autopod: comprises wrist and five fingers in the forelimb, and ankle and five toes in the hindlimb.

Tell me Edward Lewis’ role in limb development
- Homeotic mutations explained mechanisms of segment identity.
- Many homeotic genes are clustered in two chromosomal locations in Drosophila.

Whats the Homeobox?
What does it consist of and encode?
- 180bp conserved sequence in developmental regulator genes.
- encodes the homeodomain, 3 helices a DNA binding helix-turn-helix motif (helices 2-3).
- A homologous protein-coding sequence in Drosophila homeotic genes and its conservation in other metazoans. (Garber et al.,1984 Cell 3)
- 2 and 3 are important as they bind into the DNA

What are Hox genes and how do they act?
- Hox genes are a subgroup which regulate the body plan
- They may act as inhibitors (repressors) or enhancers of transcription and therefore regulate gene expression which is important in body plan regulation

Define the following:
Homeobox gene
Homeotic gene
Hox gene
- Homeobox gene- gene with a homeobox sequences (homeodomain) (not all Homeobox genes are Homeotic)- blue
- Homeotic gene- regulates a regions fate (almost all have homeobox motifs)- green
- Hox genes - are a subgroup group of homeotic genes – have homeoboxes (some homeotic genes are not Hox genes)- important in segmentalisation- red
- Some homeobox genes are homeotic genes and some homeotic genes are hox genes

Give an example of a Homeotic gene and what does it regulate
E.g., PAX genes-which are homeotic and regulate eye development bind DNA via a homeodomain (but are not Hox genes)
Tell me about the conserved hox gene cluster
- 4 sets of hox genes in mice where only 1 in drosophila
- Full set= 13 hox genes found in mouse on chromosome??
- Expression starts at 3’ end (towards head) and 5’ towards tail
- Regulated expression moving towards back end
- Expression is coming on in temporal manner (3’-5’)

What do the Hox expression domains set?
Transitions and the anterio-posterior axis in species

Limb development is about decisions, what are some things that need to be established before a limb can develop in a particular area?
The first decision is where the limb can form along the body axis.
The Limb field is where limb development will initiate

What are somites and what do they form?
somites are bilaterally paired blocks of mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somite’s give rise to skeletal muscle, cartilage, tendons, endothelial cells, and dermis.
What is the Lateral plate mesoderm?
Its a type of mesoderm that is found at the periphery of the embryo

Limb fields

How do we define where limb development occurs?
limb development occurs at limb fields
The borders of Hox expression in the lateral plate mesoderm (LPM) decides where the limb field is
In chick and mouse Hoxc6, Hoxc8 and maybe Hoxc9 are expressed in the flank between the forelimb and hindlimb fields
These Hox genes repress limb formation in the flank and drive rib formation

How does the presence of Hoxc6/8/9 repress limb formation?
Where do the limbs start to grow now?
They function by preventing expression of the secreted signalling molecules Fibroblast growth factors, FGF10 and FGF8, in the flank
The presence of the Hox genes in the flank prevent the lateral plate mesoderm forming into the limb bud and therefore forming a limb. Therefore, meaning the limb field can be defined in the forelimb and hindlimb
Extended Hoxc6 and Hoxc8 expression domains in the python LPM inhibits fore limb development

As Hoxc6/8/9 are not expressed in the Fore- and hindlimb what does this mean for limb development. Tell me the next process that occurs/ feedback loops
- Hoxc6/8/9 stop the secretion of FGF10 and FGF8
- In the fore/hindlimb (the limbfields) FGF10 is expressed (FGF8 expression is only transient and soon lost from the LPM but acts to start FGF10)
- In The hindlimb, FGF8 expression –> Wnt8c –> FGF10
- In the forelimb, FGF8 –> Wnt2b –> FGF10
- Wnt has a positive feedback loop with FGF10 to stabilise its expression

What are FGF involved with?
involved in angiogenesis, wound healing, and embryonic development.

































