5. MSK - Embryology of the limbs Flashcards
When does prenatal limb development start and what is the first stage?
- Fourth week of development
- Small projections on lateral body (which are called LIMB BUDS)
Would you say limb development is early or late in the development process? Why?
- Relatively late as just started making them in the fourth week (no where near complete) - Embryo doesn’t need its limbs until after birth and hence makes the CNS, CVS etc first
What is the limb bud? It has a structure at the end of it, what is this called? It has a structure within it too, what is this called?
- Mass of mesenchymal cells covered by a layer of ectoderm
- ectodermal cells divide at the tip = apical ectodermal ridge (AER)
- Zone of polarising activity (ZPA)
Do both sets of limbs develop at the same time?
- No, the lower limb lags by 2 days relative to the upper limb
- This is significant as an insult in development of the lower limb could occur within these two days leading to damage/non-existence of the lower limb
How does elongation of the limb buds occur?
- proliferation of the mesenchyme core
How many spatial axes are there in limb development? Why?
- 3
- we want asymmetry of the limbs!
1. proximal to distal axis (shoulder to finger tips)
2. dorsal to ventral axis (palms of hands different to back)
3. anterior to posterior axis (thumb to pinky)
What controls the axial specification for the 3 axes?
- Proximal to distal (shoulder to finger tips) - AER = has influence on mesenchyme below and encourages it to differentiate into constituent tissues (shoulder, humerus, ulnar, fingers) - AER then regresses and dies
- Dorsal to ventral (palm, back of hand) - ectoderm controls mesenchymal cells
- Anterior to posterior (thumb, pinky) - zone of polarising activity (ZPA) = signalling centre located in posterior base of limb bud - controls patterning - maintains AER
We have now formed paddles, what happens next?
- The AER regresses in some places and remains in others (i.e. remains on tips of fingers)
- Mesenchymal condensations occur where the AER has regressed = cartilaginous model of bones of hand
- Apoptosis gets rid of tissues between the digits = helping to form our fingers
Describe the following limb defects: 1. Syndactyly 2. Polydactyly
- syndactyly = fusion of digits (CT/bone too)
- polydactyly = extra digits
Describe the following limb defects: 1. amelia 2. meromelia 3. phocomelia
- amelia = complete absence of limb
- meromelia = partial absence e.g. humerus, ulna
- phocomelia = hand/foot directly attached to trunk
How do limb defects occur?
- Malformation - intrinsic error in coordination of morphogenesis e.g. genetic
- Deformation - starts normal then stop e.g. amniotic band syndrome
- Disruption - external agents e.g. thalidomide
What is amniotic hand syndrome?
Amniotic fluid wraps around limb = constriction sac = amputation naturally
The entire limb skeleton has formed and is cartilaginous, what happens next?
- endochondrial ossification for bones
Where does the upper limb bud and lower limb bud appear respectively? What is its relevance?
- upper limb bud appears opposite the caudal cervical spinal segments (C5-T1)
- lower limb bud opposite lumbar and sacral spinal segments
- spinal nerves enter the limb bud early in its development
How does muscle enter the limbs?
- Myogenic precursors migrate into limbs from somites (body segments)
- Coalesce into 2 common muscle masses around the newly formed skeletal elements
- Ventral = flexor
- Dorsal = extensor
- Individual masses then split from common masses
Describe rotation of limbs and its relevance.
- upper limbs rotate laterally
- lower limbs rotate medially
- rotation of limbs means innervated nerve pattern not so simple
