BG14

Question 1

what did saunders show 1960

Answer 1

that the ap axis of the limb is determined by a small block of mesodermal tissue near the posteriror junction of the limb bud and body wall
* zone of polarising activity

Question 2

what happens if you transplant the zpa to the anterior side of another limb

Answer 2

double the no. of digits, extra digits are a mirror image of the normal ones.

Question 3

French flag model

Answer 3

1969 wolpert - french flag acocunts for directional pattern
cells acquire positional info from morphogens secreted from a point source.
morphogens diffuse becoming lower in concentration away from the source
cells read the gradient and respond directly to it, differentiating into different cells accordingly.

Question 4

examples of morphogens

Answer 4

bicoid fgf, proteins, wnt, bmp, caudin etc.

Question 5

AP limb axis morphogen in zpa

Answer 5

in situ hybridisation shows that Shh is expressed in zpa.

and bmp

Question 6

ectopic addition of shh

Answer 6

via beads to anteiror limb bud produces mirror image digit duplications like those induced by zpa transplants

Question 7

mirror image polydactyly

Answer 7

extreme form
causal mutation maps to 7q36 close to the shh gene.
coding region of gene not interupted evidence suggests cis acting reg element limits shh expression to posterior part is deleted.

Question 8

polydactyly

Answer 8

1/3000 europeans and 1/300 africans ahve an extra digit.
most common is postaxial polydactly in africans
both in europeans.

Question 9

post axiel polydactly cause

Answer 9

loss of function mutation in Gli3 a tf, which neg regs shh in anterior part of developing limb,

Question 10

inhibition of shh in the limb

Answer 10

inhibited by cyclopamine
doesnt just cause loss of patterning in digits - causes loss of digits too
- posterior digits lost first
conditional timed shh ko has same effect

Question 11

signal involved in digit fate

Answer 11

bmp
- although shh clearly specifies ap axis
cells of given mesenchymal condensation arent irreversibly committed to that fate.

Question 12

bmp

Answer 12

produced in inter-digital webbing area
each inter-digital area produces a particular amount of bmp appears to specifiy digit identifty
manipulating inter digital bmp via removal, inhibition and mutants alters digit identity

Question 13

shh roles

Answer 13

1. required for aer maintainance
2. required for ap axis
3. null mice have no nipples, hair, head, collapse into a trunk, limbs amputated phenotype and no paw’
4. required for limb outgrowth
   * * shh null mice probs not a direct effect of shh signalling but rather an interaction with the aer.

Question 14

Aleijandinho

Answer 14

genetic loss of limbs due to inbreeding - recessive mutation
maps to 7q36 and deletes shh reg region
- shh appears to have a large complex cis acting reg region

Question 15

model of limb bud formation

Answer 15

1. fgf10 secreted by lateral plate mesoderm induces fgf8 expression in the competant ectoderm at the dorsal ventral boundary.
2. fgf8 secretion by ectoderm induces the proliferation of the mesenchymal cells and induces shh expression in the posterior region of the limb bud.
3. shh in turn induces fgf4 expression in the posterior portion of the limb bud ectoderm.
4. fgf2 is also made by the aer ectoderm although its not clear yet whether its induced by fgf10