Organogenesis-formation Of The Vertebrate Limb Flashcards
Describe the pentadactyl limb
. Conserved structure in the tetrapods (very simple)
. Modified for different function e.g. in the avian wing yet built to the same plan
Give the characteristic sequence of bones in the forelimb
Humerus, radius and ulna, carpals, metacarpals, digits
Give the characteristic sequence of bones in the hindlimb
Femur, tibia and fibula, tarsals, metatarsals, digits
The limb has 3 developmental axes. What are these?
. Proximal-distal (e.g. shoulder to finger)
. Anterior-posterior (e.g. thumb to little finger)
. Dorsal-ventral (e.g. knuckles and palm)
Give the 3 main limb regions in a chick wing
. Stylopod- humerus
. Zeugopod- ulna and radius
. Autopod- metacarpals and digits
Give the pattern of limb development in a chick/ a mouse (Similar)
. Formation of the limb bud
. Outgrowth (initial bulging of cells)
. Sequential formation of limb bones from proximal (laid down first) to distal- humerus first, digits last
How are limb buds formed?
Formed from mesodermal (mesenchyme) cells which accumulate under ectoderm- capacity to form new limbs
How are multi-legged frogs and salamanders formed in US?
At first formation, half a limb bud can generate a new limb- parasites split these so cause the animal to be multi-legged
How many limb buds are there per embryo in land vertebrates (tetrapods)?
4 limb buds per embryo, always opposite
See top of page 104
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What happens if you replace a limb bud that has started developing with another different limb bud?
It can change what limb is produced- could be a leg rather than a wing
What is apical ectodermal ridge (AER) necessary for?
Outgrowth of the limb
What does implantation if an extra AER do?
Duplicates the limb axis
Implantation of non limb bud mesoderm causes what?
Regression of limb
See par 104 (interactions between ectoderm and mesoderm)
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What happens if the AER is removed?
The limb increases to grow outward (if removal is delayed , there is a progressive appearance of limb bonds in units along the proximal-distal axis)
What does a cells positional information depend on?
Depends on time spent in progress zone (the one that spends the least time will produce the most proximal part and the longest will form the digits)
What are the effects of grafting a young limb bud tip into an older limb bud stump? Why does this happen?
There is a duplication of proximal-distal sequence of bones,
- humerus
- tibia/ fibula
- humerus
- tibia fibula
- digits
Cells in the grafted limb bud too have spent less time in the progress zone so develop into proximal rather than distal bones
What do the four Hox gene complexes in mammals result from?
From chromosome duplications (the Hox genes in vertebrates are on different chromosomes instead of just one as in drosophila). Independent gene duplications and deletions have occurred since split from Drosophila, so relationship with HOM-C complex not 1:1.
Equivalent gene in each complex (e.g. Hoxa1, Hoxb1, and Hoxd1 are paralogues (due to gene duplications, so mammals have at least 4 variances for each Hox gene)
See bottom page 106
Hox gene specification of the limb
A signalling centre is located in the posterior (the little finer side of hand) region of the limb bud. This organising centre is termed the what?
Zone of polarising activity
Implantation of cells from the posterior margin of the limb into the zone of polarising activity causes what?
A reversal of the posterior to anterior organisation
In an experiment they put another group of cells from the posterior margin on a side they would not usually develop and what happened?
It formed a symmetrical arrangement of the same limb bone because there are 2 gradients- one of each side. So, creates two sets of wings when there should be one
Developmental zones along the proximal-distal axis are specified by what?
A Hox gene code
Fibroblast growth factors (proteins) are essential for what?
Induction, maintenance and function of the apical ectodermal ridge
Where is Fgf10 expressed and what does it do?
Is expressed in the limb bud mesoderm and stimulate limb bud growth
See page 107
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Fgf10 from mesoderm induces what?
Cells in the AER to express Fgf8
What does Fgf8 stimulate? What does this enable?
Stimulates mitosis in underlying mesoderm, enabling continues Fgf10 production (positive feedback- which causes the limb to gradually elongate)
Where is the localisation of Fgf8?
In the AER of chick limb buds
Sonic hedgehog (protein), shh (vertebrate orthotic of drosophila hedgehog gene) is expressed specifically where?
In the ZPA (zone of polarising activity cells)
What does hedgehog do? Why is it important in division into segments?
Tells cells whether they are in the front or the back of a segment- important in division into segments. If not their then all the anterior segments clump together and form a bristly looking larvae in drosophila (-sonic hedgehog is the equivalent in humans)
Experimental implant of cells (on the other side of the limb bud) secreting Shh protein mimics ZPA (zone of polarising activity cells). So what happens?
So Shh sets up a gradient that tells cells what to turn into along this posterior-anterior axis
What is programmed cell death called?
Apoptosis
Apoptosis in interdigital necrotic zones controls what?
Amount of webbing between digits
How do the radius and ulna become separated by?
By interior necrotic zones
How is the end of limb shaped?
By anterior and posterior necrotic zones
What is the signal for apoptosis provided by?
Bone Morphogenetic (BMP) proteins
BMP proteins are inhibited by?
Gremlin protein
In what animals in BMP greatly reduced?
Animals with webbed feet
How can duck-like webbing be produced in chicks?
By applying gremlin- soaked beads between digits to inhibit BMP, if you stick gremlin between chick digits at the stage when they have webbing present then you will prevent the webbing cells from dying- so chick ends up with webbed feet like a duck
How is chick limbless mutation caused?
Failure of AER development, limbs do not form
How is polydactyl (additional digit) mutation caused?
Several possible mutations e.g. altered expression of sonic hedgehog
Certain toxins can disrupt limb development. Give an example.
Thalidomide
What was Thalidomide used for?
As a cure for morning sickness in pregnant women in 1950s
What did the use of Thalidomide lead to?
Gave rise to a generation of children with birth defects
How did Thalidomide lead to mutations?
Causes oxidative stress and increased BMP signalling, leading to increased cell death
Where is sonic hedgehog expresses in chick limb bud?
The posterior region
The drosophila wing disc expresses the orthologous gene hedgehog where?
In the posterior compartment
The genetic analysis leads to the conclusion that the development of ‘analogous structures’ frequently involves what?
Similar developmental genes in ‘homologous developmental pathways’
