lecture four limb axes Flashcards
what is the apical ectodermal ridge
a morpholoically distinct actodermal thickening
where is the aer
at the most distal part of limb bud
between dorsal and ventral
what does removal of aer at different time points show
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
what is expressed in the aer
fgf8 and fgf4
what is expressed in the mesenchyme in the lpm underlying the aer
fgf10
describe experiment which showed fgf8 to be the functional agent of aer
remove aer
fgf8 soaked beads
implant
partial rescue
what is the role of fgf10 in lateral pate mesoderm (mesenchyme underlying the aer)
induces the formation of aer
what is the role of the aer and
signalling centre for limb dev
maintains the mesenchyme underneath proliferating
what did the cut and paste experiments with the aer and underlying emsenchyme show
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
what are the two main theories for the how mesenchyme provides the proximal distal pattern
progress zone
space
role of the underlying mesenchyme
maintains aer
maintains limb identity
maintains proximal distal growth of the limb (NOT BY THE AER)
what is the progress zone
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
what experimental finding doesn’t support the progress zone model
when ko fgf4/8 after the initiation of the limb bud there is early exit from teh cell cycle yet formation of distal structures
what is the space hypothesis
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
what are other possible mechanisms for the how the mesenchyme and aer give proximal distal identitiy
two signal gradient (fgf at distal and ra at proximal) cells then express diff Tf in te different areas
differentiation front
what genes give p-d idenitity
hoxa and hoxd
what is the expression pattern of the hox genes p-d axis
hoxa9 is expr most proximal
hoa9-11 middle
hoxa9-13 in most distal
hoxd9- anterior prox
hoxd9-11
hox9-13 posterior and distal
what do ko of hox genes show
mutant limb formation the colinearity of hox expression hox9 ko effect the scapula hox13 ko effect digits proximal to distal
what do hox9 and hox10 define and what do ko show
stylopods
what do hox11 define
zeugopods
what do hox 12 and hox 13 define
autopods
does the aer give p-d identitiy
no the mesenchyme underneath does
what is the organising centre for the anterior posterior axis
zone of polarising activity at the posterior
(zpa)
not a morpho structure (the aer is )
what is the signal from the zpa and what doe sit control
shh (shown by graft/overexpr/ko and replace)
hox expr
