Session 1 Lecture - Development of the limbs Flashcards
When do the limb buds appear?
End of 4th week of embryonic development
Which develop first - the upper or lower limbs?
How many days difference between their development?
The upper limbs - the embryo develops in a head to tail sequence
The lower limbs develop 2 days later than the upper limbs
What are the limb buds composed of?
Core of proliferating (multiplying) mesenchymal cells with an ectoderm covering
What are mesenchymal cells?
Undifferentiated cells
How does the limb bud elongate?
Through proliferation of the mesenchymal core
The thickened ectoderm at the apex of the limb bud is known as what?
The AER - the apical ectodermal ridge
What are the 3 spatial axis in limb development?
- shoulder to fingertips (proximal to distal)
- palmar surface to dorsal surface (dorsal to ventral)
- side to side (anterior to posterior)
What controls the anterior to posterior axis?
The ZPA - zone of polarising activity
What controls the proximal to distal axis?
The AER - Apical ectodermal ridge
What controls the dorsal to ventral axis?
the ectoderm (covering the limb bud)
What are the final stages of the AER development?
Appearance of paddles (the limb bud flares) and then the AER regresses (cell apoptosis)
What effect does the AER have on underlying mesenchyme core?
It keeps the mesenchyme undifferentiated
As the mesenchyme proliferates the proximal mesenchyme begins to differentiate into tissues
Where is the ZPA located?
The zone of polarising activity is a signalling centre located at the posterior base of the limb bud
What does the ZPA maintain?
The zone of polarising activity maintains the AER - the apical ectodermal ridge
What happens if you implant a second ZPA?
A second zone of polarising activity would result in duplication of the digits
What is happening to the mesenchymal cells as AER regresses?
As the apical ectodermal ridge regresses the mesenchymal cells start to differentiate, condensing and developing into cartilaginous models of bones. They will create digital rays (continuous digit of hand or foot)
In the hand and foot plates, where does AER remain and where does it go through apoptosis?
Apical ectodermal ridges remains at the tips of the digital rays (continuous digit of hand or foot) and goes through apoptosis (programmes cells death) in the interdigital spaces
What is the term used to describe fusion of digits?
Syndactyly
What is the term used to describe extra digits?
Polydactyly
This is a genetic recessive trait
What is the term used to describe a complete absence of limb?
Amelia
What is the term used to describe partial absence of one of more limb structures?
Meromelia
What is the term used to describe missing arms, forearms, thighs and legs (where hands and feet are attached directly to trunk)?
Phocomelia
Give three ways in which a problem may occur in morphogenesis (embryo developing its shape)?
- Malformation - error in co-ordination of morphogenesis (usually a signalling problem)
- Deformation - due to construction bands (cutting off circulation to part of the embryo and causing the limb to fall off)
- Disruption - e.g. through infections such as rubella or through cancer treatment (such as thalidomide)
Define caudal
Near the tail or posterior of the body
The upper limb bud appears opposite which spinal segments?
Caudal cervical spinal segments
The lower limb bud appears opposite which spinal segments?
The lumbar and sacral spinal segments
Where do myogenic precursors migrate into the limbs from?
Somites (body segments)
What do myogenic precursors coalesce into? (come together as)
2 common muscle masses - individual muscles split from common masses
In muscle development, where are the flexors located and where are the extensors located?
(one is ventral and one is dorsal)
Ventral (directed towards front) - flexor
Dorsal (directly towards back) - extensor
In which direction do limbs first extend?
Ventrally (towards the front). Palms of hands and soles of feet are facing one another
As limbs elongate they rotate. Which direction do the upper and lower limbs rotate?
Upper limbs - laterally (away from midline
Lower limbs - medially (towards midline)
