Chapter 19 Part 2: Proximal and Distal Axis of the Limb Flashcards
the distal AER formation is induced by mesodermal ____, which then induces a positive feedback loop
induces fgf10
the _____ is a multipurpose signalling center that influences patterning along all axes of limb development
AER
3 broad functions of AER
1) maintains the mesenchyme beneath it to stay in a plastic and proliferative state for proximal-distal growth
2) maintains the expression of molecules that generate the anterior-posterior axis
3) interacts with proteins that specify the anterior-posterior and dorsal-ventral axis so that each cell is given instructions on how to differentiate.
The PZ (progress zone) is in the ___ ____, and it is responsible for proliferating to extend the limb bud
is in the distal mesenchyme
the PZ proliferation is required for outgrowth of the limb bud, but the AER maintains the proliferation and outgrowth. How does it maintain this?
AER is responsible for the sustained outgrowth and development of the limb because it’s activation allows for positive feedback loop of fgf10, which is in the mesenchyme below repsonsible for mitotic proliferation.
____ is the major active factor of AER. What provides a redundancy function?
fgf8 is the major active factor of AER. Fgf4,9,and 17 provides redundancy for fgf8 function; loss of one fgf does not significantly impact development, but removal of all fgfs will result in complications.
mesoderm specification of the proximal-distal axis: the ___ ____ of mesenchyme specifies the skeletal identity along the prox-distal axis. How?
the PROXIMAL ZONE (proximal mesenchyme) of mesenchyme specifies the skeletal identity along the prox-distal axis. Age of mesenchymal cells determines types of bones formed. Young mesenchyme forms stylopod, whereas older mesenchyme forms autopod.
What are the two opposing gradients involved in proximal-distal axis formation
1) fgf and wnt gradient from distal AER (induced by fgf10) 2) retinoic acid from the proximal flank. proximal stylopod bones are formed if mesenchyme is treated with RA (Still in the presence of Wnt and FGF) bone becomes more sital if mesenchyme is treated with FGF and Wnt only. if FGF is inhibited, bones are more proximal. If RA synthesis is inhibited, bones become more distal.
How does Fgf8 and RA antagonize each other?
RA binds directly to fgf8 and represses it, but Fgf8 can upregulate Cyp26, which degrades RA.
Describe the Gradient model of post-distal limb patterning
operates using the fgf8(AER) and RA (flank) antagonism. Different concentrations of fgf8 and RA influence results in activation of different genes
. IN chicks:
- RA promotes the expression of Meis1/2 in the proximal area, whereas Fgf8 inhibits Meis1/2. Meis codes for the stylopod, and inhibits RA degradation by repressing Cyp26 transcription.
Fgf8 promotes expression of distal Hoxa13, whereas RA inhibits Hoxa13.. Hoxa13 expression is importnat for forming the autopod.
Intermediate levels of RA and Fgf8 results in the expression of Hoxa11, which codes for the zuegopod.
IN chicks: - RA promotes the expression of ____ in the proximal area, whereas Fgf8 inhibits ____. This gene codes for the ___, and inhibits RA degradation by repressing ___ transcription. Fgf8 promotes expression of distal __, whereas RA inhibits ___.. This gene expression is importnat for forming the ___. Intermediate levels of RA and Fgf8 results in the expression of ____, which codes for the ____.
IN chicks: - RA promotes the expression of Meis1/2 in the proximal area, whereas Fgf8 inhibits Meis1/2. Meis codes for the stylopod, and inhibits RA degradation by repressing Cyp26 transcription. Fgf8 promotes expression of distal Hoxa13, whereas RA inhibits Hoxa13.. Hoxa13 expression is importnat for forming the autopod. Intermediate levels of RA and Fgf8 results in the expression of Hoxa11, which codes for the zuegopod.
What mechanism in Hoxa13 (involved in autopod specification) ensures that the zeugopod will be adequately formed?
Hoxa13 is under epigenetic control in addition to being controlled by the RA/Fgf8 antagonism gradients. Hoxa13 remains tightly coiled until the cells get further extended, allowing for the middle part of the limb to form.
As the limb bud formation progresses, how do cells know to start forming cartilage, and when do they activate and lay down the foundation of skeletal systems rather than countinuing to create cells and jelly mass?
The reaction diffusion mehcanism! As you move out of zones with different factors, the behaviour of the cell cahnces. Ex/ Fgf8 an Wnt in AER at the top will keep cells immediately underneath in the mesoderm dividing and proliferating since fgf10 (which fgf8 upregulates) induces mitosis. By having these factors in the AER, the cells underneath them will be pushed into the proliferative state. As the AER grows and moves more distally, they are not affected by the AER, and they form cartilage.
The reaction/diffusion mechanism operates on the premise that the AER divides the mesenchyme into 2 domains, whith a fozen zone being more proximal from the domains and is the site where chondrogenesis will take place. What are the two domains?
1) inhibitory mesenchyme domain (most distal), which expresses fgfr1. The AER expresses fgf8 which induces FGFR1 in the inhibitory domain of the mesenchyme. Cartilage condensation is repressed in this zone.
2) Active zone mesenchyme, which expresses FgfR2. This is where cartilage condensation starts to appear because Fgfr2 expression allows for the expresion of TGFbeta, activin, BMPS and galectin. This reults in aggregations of precartilage mesenchyme. These same cells, however, also synthesize inhibitors of aggregation, such as Noggin and inhibitory galectins, so in the active zone, true cartilage is not yet formed. As a result, what were once cartilage-forming aggregates inhibit the areas surrounding them from forming more such aggregates. These inhibitors thus act as lateral inhibition.
This secretion of both cartilage formers and inhibitors in the active zone result in precartilage condensations in some parts, and none in others. At different zies of the limb, different numbers of precartilaginous condensations can form. Ex/ two condensations can form in the zeugopod.
How do multiple bones form in some areas of the limb but only one bone forms in other areas?
the number of cartilagenous condensations that begin to form in the active zone of the mesechyme depends on the geometry of the active zone and the cell’s strenght of lateral inhibition.
Recall:
the active zone mesenchyme, expresses FgfR2. This is where cartilage condensation starts to appear because Fgfr2 expression allows for the expresion of TGFbeta, activin, BMPS and galectin. This reults in aggregations of precartilage mesenchyme. These same cells, however, also synthesize inhibitors of aggregation, such as Noggin and inhibitory galectins, so in the active zone, true cartilage is not yet formed. As a result, cartilage-forming aggregates inhibit the areas surrounding them from forming more such aggregates. These inhibitors thus act as lateral inhibition.
This secretion of both cartilage formers and inhibitors in the active zone result in precartilage condensations in some parts, and none in others. At different SIZES of the limb, different numbers of precartilaginous condensations can form. Ex/ two condensations can form in the zeugopod.
