MSK- Development Of The Limbs Flashcards
What are ‘limb buds’?
- Core of proliferating mesenchymal cells with an ectoderm covering
- Thickened ectoderm at apex of limb buds (apical ectodermal ridge AER)
Give a brief overview of the development of the limbs
- Limb buds appear in ventro-lateral body wall, lower limb bud appears after upper limb bud
- Limb buds extend ventrally at first
- Primordium of hands and feet develop
- Limbs adopt their adult orientation
When do limb buds start to appear and how far behind the upper limb bud is the lower limb bud?
- Appear at end of 4th week
- ~2 days behind upper limb
What do the following embryonic orientations become in an adult?
- Anterior
- Posterior
- Ventral
- Dorsal
- Anterior -> Superior
- Posterior -> Inferior
- Ventral -> Anterior
- Dorsal -> Posterior
What is meant by the antero-posterior and proximo-distal axis?
- Antero-posterior axis: 1st to 5th digit
- Proximo-distal axis: Base of limb to tips of finger
What does limb development stimulate?
- Activation of mesenchymal within lateral mesoderm
- Derived from somatic layer of lateral plate mesoderm
What is a ‘slipped disk’
- Nucleus pulposus (remnant of notochord) can herniate.
What does the mesoderm contribute to the limbs?
- Lateral plate mesoderm: limb skeleton
- Somites: musculature
What is meant by the P-D, D-V and A-P axis and why do we have the D-V axis?
- P-D: top and bottom (shoulder to fingertips)
- D-V: front and back -> no hairy palms
- A-P: side to side (L&R hands are mirror images)
What is the role of the AER?
- Apical ectodermal ridge
- Critical for limb bud outgrowth
- Orchestral limb development - proximal to distal
- Final stage’s appearance of paddles (flattening)
- Then AER regresses
How is the proximal-distal axis generated?
- AER exerts on inductive influence in the immediately underlying mesenchyme: remains undifferentiated
- Proximal mesenchymal: begins to differentiate
- AER induces development of digits
- AER marks dorsal/ventral boundary
- Ectoderm exerts dorsalising and ventralising influences over mesenchymal core
What does ZPA stand for and what’s its role?
- Zone of polarising activity
- Generation if a symmetry of limbs: anterior-posterior axis determination
- A signalling centre located at posterior base of limb bud
- Controls patterning and maintains AER
How do hand and foot plates form?
- Digital rays: mesenchyme condensations within plates
cartilaginous models of digital bones - Apoptosis of tissues between digits
- AER breaks up and only maintains over tips of digital rays
- Interdigital spaces are progressively sculpted by programmed cell death
What is syndactyly?
- Fusion of digits - connive tissue of bones fused
What is polydactyly?
- Extra digits, genetic recessive trait
