Session 1-Development Of The Limbs Flashcards
When do the limb buds appear?
Towards the end of the 4th week
Why does lower limb development lag by ~2 days behind upper limb?
The embryo develops in a head to tail order
What is a limb bud?
Core of proliferating mesenchymal cells with an ectoderm covering
True or false: the limb bud has a thickened mesoderm at the apex of the limb bud
FALSE - ectoderm, not mesoderm
What are the three spatial axes in limb development?
1) Shoulder to fingertips (proximal-distal axis)
2) Palmar to dorsal surface (dorsal-ventral axis)
3) Side-to-side (anterior-posterior axis)
What is the control of anterior-posterior axial specification?
Zone of polarising activity (ZPA)
What is the controller of proximal-distal axial specification?
Apical ectodermal ridge (AER)
What is the controller of dorsal-ventral axial specification?
Ectoderm
What does the limb bud consist of?
Mesenchyme core (flexible mesoderm) with thickened ectoderm at the apex (apical ectodermal ridge)
Complete the sentence:
The elongation of the limb bud is through the ______________ of the mesenchyme core.
Proliferation
True or false: AER keeps the mesenchyme that is immediately underlying it undifferentiated
TRUE
What is the final stage of limb bud development by AER?
Appearance of paddles
Where is the ZPA located?
Posterior base of the limb bud
What is the ZPA responsible for?
Generation of asymmetry in the limbs
What does the ZPA control? (2)
Patterning and maintains the AER
What progressively sculpts interdigital spaces?
Apoptosis
When the AER breaks up, where is the only place where it is maintained?
Over the tips of the digital rays
What is syndactyly?
Fusion of digits (may involve just CT or bones may be fused)
What is polydactyly?
Extra digits, genetic recessive trait
What is amelia?
Complete absence of limb
What is meromelia?
Partial absence of one or more limb structures
What is phocomelia?
Hands or feet are attached directly to the trunk
What are the three underlying problems in morphogenesis?
1) malformation
2) deformation - constriction bands eg. Amniotic band syndrome (limb is truncated)
3) disruption by external agent eg. Thalidomide
How does thalidomide cause an issue in morphogenesis?
Disrupts AER and stops elongation of limb bones
What are the five stages of bone formation?
1) signals from AER to remain undifferentiated stop
2) lateral plate mesoderm condenses and differentiates
3) cartilage model forms
4) endochondral ossification
5) primary and secondary ossification centres
Complete the sentences:
Limbs extend __________ at first but, as they elongate, they rotate. The upper limb rotates __________ and the lower limb rotates ____________.
Ventrally
Laterally
Medially
