BG13 Flashcards
Limb axis
AP
DV
PD
limb zones
stylopod -> humerus and femur
Zeugopod: radius and ulna
autopid: hand
limb first formation
when limbs first form they are a sack of ectoderm filled with undifferentiated mesoderm.
human development
appear around day 26 after conception
day 50 fingers vsiible
day 56 fetus can touch nose
what do limbs form from
4 limb frield
what did limbs develop from in different vertebrates
different somite levels
how are homologous limbs defined
by the same hox gene expression along the AP axis
give two examples of hox gene specification in limb development
- chicks have more cervial neck vertebrate than humans but the boundary is encoded by the same combo of hox genes.
- in fish/amphibians/birds and mammals, the forelimb buds are found at the most anterior expression region of hoxc-6 the position of the first throacic vertebrate
what does RA do in limbs
specified limb fields via hox activity - produced by the organiser
RA exposure to amputated tadpole tail
if exposed to ra in the first days of regeneration, several legs formed from tail as tail stump. as ra acitvates hox genes to form limbs
what does the signal for limb formation come from
the lateral plate mesoderm that will become the prospective limb messenchyme
- cells secrete fgf10 paracrine factor
ectopic addition of fgf10
added ectopically beneath flank ectoderm, extra limbs emerge
mutation in fgf3
causes achrondroplasia dwarfism
what did john saunders note
limbs were covered in ridge of unusual ectoderm
- cells tightly packaed and columnar rather than pancake shaped
- called apical ectodermal ridge
what did saunders hypothesize
that it had something to do with proximal distal outgrowth and patterning of the limb
what were saunders experiments
surgically removed Aer
- removal stopped limb outgrowth,
- removal interupted patterning of the limb
removed in young limb bud nearly all structures were lost. older limb only distal structures were lost
Meromelia
partial failure of PD growth in one or more limbs in humans
causes of meromelia
suggests due to failure of aer
however interdependence of aer with other signalling systems in limb means interpretation of these syndromes is not straightfoward
AER and FGF
aer secretes fgf
specially fgf8
* remove aer but add fgf8 ectopic bead = still get more or less properly developed limb.
fgfs in humans
around 20 in humans and other mammals, many expressed in the limb, several in aer
clock model who proposed
summerbell
* model to explain how aer works and makes limb
limb bud elongation
limb bud elongates by proliferation of mesenchymal cells under aer (progress zone) which extend 200nm from aer.
fgfs from aer keep progress zone mesenchymal cells dividing
clock model
longer cells remain in the progress zone the more distal they become
stem cells maintained in the progress zone of proliferation and cells behind them dont proloferate as much and become specialised proximally.
problem with the clock model
removal of aer only causes a reduction in proliferation of progress zone
*basis of model
** cell death also increases which is unexpected, aer is doing more complicated things.
basis for pre-specification model
lineage tracing shows that even in the earliest limb bud - before growth has started, the cells were commmitted to becoming different parts along the PD axis; stylo/zeugo/autopod.
suggests pre-dermtinated pattern
pre-specification model
proposed there is an initial population of cells that are pre-specificed in the ectoderm and that the aer merely maintained and expanded these populations
aer removal explained by pre-specification model
proliferation occurs in a particular order, proximal proliferate first, then more distal - so removing aer kills a constant depth of cells 200m at all times,
- done early affects cells of all future parts of the limb
- done late however only affects most distal (autopod structures)
recombinant limb technology
2011
rLs are limb buds made by dissociating mesenchymal cells from a limb bud; re-aggregating them and putting them in a jacket of limb bud ectoderm
recombinant limb buds can then be grafted back onto a new chick where it will grow.
counter gradient model basis
if stage hh18 cells are formed into a rL and grafted back, they can form a proper three part limb.
cultured with wnt3a (another aer sig) and fgf8 they become increasingly restricted to form only distal structures.
culture with ra too form the full limb
counter gradient model
suggests there is no clock but that the pd fate of the mesenchymal cells depends on balance between exposure to RA vs exposure to wnt3a and fgf8
- experiments also show some patterning occurs later in development, so morphogen gradients form an extra later of pd patterning which is thought to control progresszone
