Development Of The Limbs Flashcards
Around what time in development do you see the limb buds?
End of 4th week (lower limbs lag two days behind)
What is a limb bud?
A mesenchymal core surrounded by ectoderm
How does a limb bud elongate?
Proliferation of mesenchymal core
What is found at the apex of the limb bud? What is it for? At what point does it regress?
Thickened ectoderm (apical ectodermal ridge - aer)
Has signalling properties- orchestrates limb development (proximal to distal)
After the appearance of paddles on limbs
What are the controllers of axial specification found on the;
- anterior to posterior axis
- proximal to distal axis
- dorsal to central axis
A to p - zone of polarising activity (ZPA)
p to d - apical ectordermal ridge (AER)
D to v - ectoderm (that which covers the bud)
How does proximal - distal axis generation work in the growth of limbs?
Controlled by apical ectodermal ridge
This causes the immediately underlying mesenchymal to remain undifferentiated and just divide to allow the bud to extend.
The proximal mesenchymal then has an opportunity to different into constituent tissues.
Finally the AER will induce the development of the digits within the hand/foot plates.
What does the ZPA stand for and what role does it have in the development of limbs?
Zone of polarising activity (controls anterior to posterior axis development)
There is a signalling centre located at the posterior base of the limb bud
Controls both patterning and maintains the AER
Creates difference between thumb and little finger
What are digital rays and where are they found?
Mesenchymal condensations within plates because aer is regressing
They’re cartilaginous models of digital bones
Found in the hands and feet during foetal development
How are fingers formed?
- Digital rays form
- AER breaks up and is maintained only over the tops of the digital rays
- Interdigital spaces are progressively sculpted by programmed cell cell death (apoptosis)
What is the fusion of digits called?
Syndactyly
May involve just connective tissues or bones may be fused (ie rose bud hands)
What is it called when there are extra digits?
Polydactyly
Genetic recessive trait
What is the complete absence of limbs called?
Amelia
What is the partial absence of one or more limb structures called?
Meromelia
What is phocomelia?
A sub type of meromelia when the hand/feet are far closer to the trunk than normal, limb development is stunted but hands no feet develop as normal
What are the underlying problems in morphogenesis?
Malformation- intrinsic error in coordination of morphogenesis
Deformation- constriction bands
Disruption- external agent (eg thalidomide- disturbs aer which stops the elongation of limb bones/ infections)
