MSK - Embryology Flashcards
Give a very brief overview of what structures of limbs develop from
Begins with activation of mesenchyme within the somatic layer of the lateral mesoderm
This somatic mesoderm forms the limb skeleton
Somites form the limb structure
When, where and what are limb buds?
Limb buds appear in the 4th week of development, lower limb buds lag two weeks behind the upper limbs
Limb buds appear on the ventro-lateral body wall and extend ventrally first
They consist of a mesenchyme (flexible mesoderm) core with a thickened ectoderm at the apex at the apical ectodermal ridge
How does elongation of a limb bud occur?
Elongation is through proliferation of the mesenchymal core
Describe the apical ectodermal ridge and its role in limb bud development
Critical for limb bud outgrowth
Orchestrates limb development proximally to distally AER keeps the mesenchyme underlying it undifferentiated (this proliferates and elongates the limb)
Proximal mesenchyme begins to differentiate into constituent tissues as it is too far away from the AER to receive signals to stay undifferentiated
Finally, AER induces development of the digits of the hand and foot plates then regresses
What is the zone of polarising activity and how does it influence development?
The ZPA is the signalling centre located at the posterior base of the limb bud It is responsible for the generation of asymmetry in the limbs
Controls both patterning and the maintains the apical ectodermal ridge
What is dorsoventral patterning?
The AER marks the boundary between dorsal and ventral ectoderm
Ectoderm exerts ‘dorsalising’ or ‘ventralising’ influences over the mesenchymal core
What are the important axes in limb development?
Give structures that control developmental changes along each axis
Antero-posterior:
- ZPA
Dorso-ventral:
- Dorsal/Ventral ectoderm
Proximal/Distal:
- AER
Describe the initial development of the hand and foot plates
AER controls development of widened plates at the distal region of the limbs
Mesenchyme condensations within the plates form cartilaginous models of the digital bones
AER then breaks up and is maintained only over the tips of the digital rays
Interdigital spaces sculpted by apoptosis
Describe the formation of bones in the limb buds
Signals from the AER to remain undifferentiated stop as AER moves away from region about to differentiate
Lateral plate mesoderm condenses and differentiates
Cartilage models form
Endochondral ossification occurs (primary and secondary ossification centres)
Describe the differentiation of muscle tissue
Myogneic precursors from the somites coalesce into two common muscle masses around newly formed skeletal elements
Ventral - Flexors
Dorsal - Extensors
Individual muscles then split from the common masses
Describe rotation of the limbs during development
Both upper and lower limbs have flexor and extensor compartments but they are on opposite sides
As the limbs extend ventrally they rotate:
- Upper limb rotates laterally (thumb is lateral)
- Lower limb rotates medially (Big toe is medial)
Compare the positions of the limbs before and after rotation
Before:
- Thumbs up, elbows out
- Soles facing in, knees out
After:
- Thumbs out, elbows down
- Soles down, knees up
Describe limb innervation during development
Upper limb appear opposite the caudal cervical spine segments
Lower limb appears opposite the lumbar and sacral spinal segments
Spinal nerves from these areas enter the limb bud early in development, without this, development stalls
Describe how the brachial plexus relates to limb development
Anterior (ventral) compartment supplied by medial and lateral cords, these innervate flexors
Posterior (dorsal) compartment supplied by the posterior cord, this innervates extensors
Describe how the development of nails relates to their innervation
Nails develop from the nail fields at the tips of fingers then move dorsally, they bring their nerve supply with them (Median and ulnar)
What is meant by:
- Post-axial
- Pre-axial
Post-axial:
- Little finger and toe
Pre-axial:
- Thumb and big toe
Give a brief overview of limb defects
Upper limb affected more
6/10,000 live births
Rare, usually hereditary but teratogen induced has been seen
Give two common limb defects
Amelia:
- Complete absence of limb
Meromelia:
- Partial absence of one or more limbs E.g. Phocomelia (Thalidomide)
Give two digit defects
Syndactole:
- Lack of apoptosis between digits
- Digits fused
- May involve just connective tissues or bones
Polydactyly:
- Too many digits
- Genetic recessive trait
Label this diagram
What is this a diagram of?
Labels (top to bottom):
- Undifferentiated mesenchyme
- AER
- Differentiating mesenchyme
This diagram shows a growing limb bud and the process of mesenchymal differentiation occuring as AER moves away from the mesenchyme in question
What does this diagram show?
Label the boxes
This diagram shows the process of ectoderm overlying the limb bud directing dorsal or ventral differentiation in the underlying mesoderm (Dorsoventral patterning)
Blue:
- Dorsal ectoderm
- Dorsalising
Purple:
- Ventral ectoderm
- Ventralising
Green:
- AER (Dorsoventral boundary)
Label the boxes
What does the second picture (right) show?
Top to bottom, left to right:
- AER
- Loose mesenchyme core ( of the Limb bud)
- ZPA
- Dorsal ectoderm
- Ventral ectoderm
Right image shows a limb bud in cross section
What does this image show?
Label the boxes
This image shows the regression of the AER around the limb plates and the process of apoptosis sculpting the digits
Labels left to right:
- Digital rays
- AER maintained
- AER regresses
