Limb Development In Vertebrates Flashcards
What model organisms are often used in limb development? Why?
• chick - can a carry out microsurgery while embryo still in egg
• Mouse- 5 digits- blue stain is cartilage- red stain is bone
• Zebrafish - pectoral fins homologous to arm o wings of chick
What are the three limb axes?
The three limb axes
* Anteroposterior (AD)
* Dorsoventral (DV)
* Proximodistal (PD)
What is a stylopod?
humerus (forelimb); femur (hindlimb).
What is Zeugopod?
radius and ulna (forelimb) ; tibial and fibula (hindlimb)
What is a autopod?
wrist and finger (forelimb) ankle and toes (hindlimb)
What is the AER?
Tightly packed columns cells thickening the distal end of the limb bud essetntial for outgrowth and patterning along the PD axis. ZPA or polarising region at the posteiro margin is required to pattern AP axis
What is the stages of the outgrowth of the limb bud?
37-42 days make out elbow and where digits will begin to form
44-48 make out individual bones of digits of fingers-
53-54- digits now distinct - separated from one another
Name the order of stylopod, zeugopod and autopod development?
Stylopod first, then zeugopod and finally the autopod
How do skeletal and connective tissues from?
By the condensation (increased local packing) of the local mesenchyme cells.
How does msucle form?
Cells migrating from the somite
What does the limb skeleton arise from?
Epithelial mesenchymal transition and proliferation of cells from the lateral plate mesoderm
What determines where the limb bud forms?
What determines where limb buds form
* Terrestrial vertebrates have four clusters of hox genes encoding homeobox transcription factors
* Hoz genes are expressed in overlapping domain along the AP axis of the body
* They have sharp anterior expression boundaries and diffuse posterior boundaries
Why do snakes have a loss of forelimb?
The hoxc6 and hoxc8 expression domains are extended, eliminating the forelimb bid
Name the transcription factors and Paracrine signalling factor involved in limb bud initiation?
Transcription factors
• Hox
• Tbx5 (forelimb only)
• Tbx4 hind limb only
• Ptx1- hindlimb
Paracrine signalling factors
• retinoic acid (RA) forelimb onl
• FGF10
• FGF8
• Wnt2b (forelimb only)
• Wnt8c (Hindlimb only)
• Wnt3a
What causes liebenberg syndrome?
Pitx1 mutation - lead to expression of the Pitx1 in the forelimb buds in addition hindlimbs bud
How does the retinoic acid signalling pathway work?
RA changes conformation of RAR/RXR heterodimers bound to RA- response elements releasing co-repressor, recruiting co-activators and changing chromatin nucelosomal structure to facilitate target gene transcription
Describe the model for formation of forelimb bud
RA- from somites and diffuses straight through intermediate mesoderm and straight into lateral plate mesoderm- also produced by lateral plate mesoderm itself
One of gf10 targets is TBX5 to maintain loops-
Tbx5 goes to from Wnt2b, which goes to form fgf10
FGf10 gives rise to wnt3a in AER then gves rise fgf8 which maintains formation of fgf10
What happens in transgenic mice which retinaldheyde-2-dehydrogenase has been inactivated?
Do not form forelimbs
Do not express Tbx5 at the site where forelimbs buds should form
What happen when Tbx5 has been inactivated?
- do not form forelimbs
Do not express fgf10 at the sites where forelimb buds should form.
WHAT HAppens if fgf10 has been inactivated?
Do not form limbs
What factors does the AER produce?
Fibroblast growth factors
What does ZPA produce?
Sonic hedgehog
What happen when AER removes?
AER removal causes a zone of cell death of approx 200-300 um regardless of length of lumb bud. I,e death of the progress zone cells adjacent to AEE. Outgrowth ceases and incomplete limb forms. The later the AER is removed, the more complete the resulting wing.
How does AER inhibit cell death and differentiation in the progress zone.
AER expresses FGF8
What happens if removal of apical ectodermal ridge to the wing?
Become shortened
What happens to a wing if AER is removed but source of FGF is given.
Get Normal wings
Describe the progress zone model
The progress zone model
* Cells become specified as they exit the progress zone
* Specification depends on the length of time spent in the progress zone, cells that exit early from proximal structures, this that exit later form progressively more distal structures
* PZ cells have an intrinsic clock
What evidence back up the progress zone model?
The AER removal experiment can be interpreted as supporting this model
* According to the model,the AER produces signals that maintain proliferation of PZ cells and prevent them from differentiating AER removal stops limb outgrowth and only structures that form are thos derived from cells that had already been specified whe they exited the PZ
Describe the two single model
TWO SIGNAL MODEL
* Retinoic acid diffusing from the flank causes cells to be specified as stylopod
* FGFs diffusing from the AER cause specification as autopod
* Cells outside the range of either signal become zeugopod
What happen if rdh enzymes are knocked out
Can produce normal hindlimbs
What happens to mice lacking hoxa11 and hoxd11?
Form forelimbs lacking the zeugopod
