lecture four limb axes

Question 1

what is the apical ectodermal ridge

Answer 1

a morpholoically distinct actodermal thickening

Question 2

where is the aer

Answer 2

at the most distal part of limb bud

between dorsal and ventral

Question 3

what does removal of aer at different time points show

Answer 3

causes truncation along proximal distal axis
earlier remove the less distal strcutres that develop shows that aer is needed for the outgrowth and positioning of the limb

Question 4

what is expressed in the aer

Answer 4

fgf8 and fgf4

Question 5

what is expressed in the mesenchyme in the lpm underlying the aer

Answer 5

fgf10

Question 6

describe experiment which showed fgf8 to be the functional agent of aer

Answer 6

remove aer
fgf8 soaked beads
implant
partial rescue

Question 7

what is the role of fgf10 in lateral pate mesoderm (mesenchyme underlying the aer)

Answer 7

induces the formation of aer

Question 8

what is the role of the aer and

Answer 8

signalling centre for limb dev

maintains the mesenchyme underneath proliferating

Question 9

what did the cut and paste experiments with the aer and underlying emsenchyme show

Answer 9

no aer = no limb dev
double aer = duplicate limb
leg mesenchyme = leg
non limb mesenchyme = limb dev cease
no aer/fgf bead = norm dev

shows that identity from the underlying mesenchyme
is a feedback loop
communicating between mesenchyme and ectoderm

Question 10

what are the two main theories for the how mesenchyme provides the proximal distal pattern

Answer 10

progress zone

space

Question 11

role of the underlying mesenchyme

Answer 11

maintains aer
maintains limb identity
maintains proximal distal growth of the limb (NOT BY THE AER)

Question 12

what is the progress zone

Answer 12

progress zone is the mostl distal part of the mesenchyme of the limb bud
aer expresses fgf4 and fgf8 which maintains the proliferation of the mesenchyme
limb bud grows
when cells far from the aer = no less fgf4/8
exit cell cycle
early exit from cell cycle become more proximal strucutres
exit late = distal structures

Question 13

what experimental finding doesn’t support the progress zone model

Answer 13

when ko fgf4/8 after the initiation of the limb bud there is early exit from teh cell cycle yet formation of distal structures

Question 14

what is the space hypothesis

Answer 14

zones are specified early in dev
an all these zones expand during distal progression of outgrowth
when aer removed apoptosis upto 200microm away
this model suggest aer release antio apop signals
when aer removed early loss of intermideiate and distal elemnts as within the 200microm
remove later = loss of only distal elements as expansion has happened only the distal specfied areas is within the 200microm

Question 15

what are other possible mechanisms for the how the mesenchyme and aer give proximal distal identitiy

Answer 15

two signal gradient (fgf at distal and ra at proximal) cells then express diff Tf in te different areas
differentiation front

Question 16

what genes give p-d idenitity

Answer 16

hoxa and hoxd

Question 17

what is the expression pattern of the hox genes p-d axis

Answer 17

hoxa9 is expr most proximal
hoa9-11 middle
hoxa9-13 in most distal

hoxd9- anterior prox
hoxd9-11
hox9-13 posterior and distal

Question 18

what do ko of hox genes show

Answer 18

mutant limb formation
the colinearity of hox expression 
hox9 ko effect the scapula 
hox13 ko effect digits
proximal to distal

Question 19

what do hox9 and hox10 define and what do ko show

Answer 19

stylopods

Question 20

what do hox11 define

Answer 20

zeugopods

Question 21

what do hox 12 and hox 13 define

Answer 21

autopods

Question 22

does the aer give p-d identitiy

Answer 22

no the mesenchyme underneath does

Question 23

what is the organising centre for the anterior posterior axis

Answer 23

zone of polarising activity at the posterior
(zpa)
not a morpho structure (the aer is )

Question 24

what is the signal from the zpa and what doe sit control

Answer 24

shh (shown by graft/overexpr/ko and replace)

hox expr

Question 25

what experiment proved identity of the zpa

Answer 25

transplant into mirror image and get mirror image limb

Question 26

what do the experiments with variable number of zpa cells transplated hshow

Answer 26

number of cells effect the axis
large number = dup. of axis
small nu,ber = not all duplicated

Question 27

what is the role of shh

Answer 27

required for digit specifcation

not formation

Question 28

what is the role of bmp in digits

Answer 28

expr in tissue etweeen digits
remove the interdigit tissue transforms the digit into idenitty more anterior (eg 2 to 1)
inhibition of bmp also does this
shows importan in interdigit space in defining digit identity

Question 29

what is the dorsal side of hand categorised by

Answer 29

hair
knuckles
nails

Question 30

describbe the distinct gene expression patterns along the d/v of the limb

Answer 30

wnt 7a expr most dorsal then lmx1 (homeobox pr)

engrailed homeobox pr expr most ventral

Question 31

what is the ficntion of wnt7a and engrailed

Answer 31

to dorsalise and ventralise the wing respectively
if ko wnt7a the dorsal is ventralised
vice versa if ko engrailed

Question 32

what is the human conditon with lmx1 mutation

Answer 32

human nail patella syndrome

incomplete dorrsalisation of the hand

Question 33

during proliefartion what are the levels of fgf and bmp in the limb bud

Answer 33

low bmp
high fgf
as shh inh bmp by grem. no inh of fgf by bmp

Question 34

during termination of proliferaation what are the levels of fgf and bmp

Answer 34

high bmp
low fgf
as distance increases less inh of shh on bmp = ncr bmp