BG13

Question 1

Limb axis

Answer 1

AP
DV
PD

Question 2

limb zones

Answer 2

stylopod -> humerus and femur
Zeugopod: radius and ulna
autopid: hand

Question 3

limb first formation

Answer 3

when limbs first form they are a sack of ectoderm filled with undifferentiated mesoderm.

Question 4

human development

Answer 4

appear around day 26 after conception
day 50 fingers vsiible
day 56 fetus can touch nose

Question 5

what do limbs form from

Answer 5

4 limb frield

Question 6

what did limbs develop from in different vertebrates

Answer 6

different somite levels

Question 7

how are homologous limbs defined

Answer 7

by the same hox gene expression along the AP axis

Question 8

give two examples of hox gene specification in limb development

Answer 8

1. chicks have more cervial neck vertebrate than humans but the boundary is encoded by the same combo of hox genes.
2. 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

Question 9

what does RA do in limbs

Answer 9

specified limb fields via hox activity - produced by the organiser

Question 10

RA exposure to amputated tadpole tail

Answer 10

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

Question 11

what does the signal for limb formation come from

Answer 11

the lateral plate mesoderm that will become the prospective limb messenchyme
- cells secrete fgf10 paracrine factor

Question 12

ectopic addition of fgf10

Answer 12

added ectopically beneath flank ectoderm, extra limbs emerge

Question 13

mutation in fgf3

Answer 13

causes achrondroplasia dwarfism

Question 14

what did john saunders note

Answer 14

limbs were covered in ridge of unusual ectoderm

• cells tightly packaed and columnar rather than pancake shaped
• called apical ectodermal ridge

Question 15

what did saunders hypothesize

Answer 15

that it had something to do with proximal distal outgrowth and patterning of the limb

Question 16

what were saunders experiments

Answer 16

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

Question 17

Meromelia

Answer 17

partial failure of PD growth in one or more limbs in humans

Question 18

causes of meromelia

Answer 18

suggests due to failure of aer
however interdependence of aer with other signalling systems in limb means interpretation of these syndromes is not straightfoward

Question 19

AER and FGF

Answer 19

aer secretes fgf
specially fgf8
* remove aer but add fgf8 ectopic bead = still get more or less properly developed limb.

Question 20

fgfs in humans

Answer 20

around 20 in humans and other mammals, many expressed in the limb, several in aer

Question 21

clock model who proposed

Answer 21

summerbell

* model to explain how aer works and makes limb

Question 22

limb bud elongation

Answer 22

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

Question 23

clock model

Answer 23

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.

Question 24

problem with the clock model

Answer 24

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.

Question 25

basis for pre-specification model

Answer 25

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

Question 26

pre-specification model

Answer 26

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

Question 27

aer removal explained by pre-specification model

Answer 27

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)

Question 28

recombinant limb technology

Answer 28

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.

Question 29

counter gradient model basis

Answer 29

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

Question 30

counter gradient model

Answer 30

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