• limb buds appear at about 4 weeks gestation (embryo 4 mm)
  • arise as evaginations of ectoderm and mesoderm
  • mesoderm forms bones, joint cartilage, blood vessels
  • ectoderm forms epidermis
  • dermis and muscle formed from migrating dermomyotome of somite
  • polarity in 3 directions:
    • proximo-distal : controlled by time in progress zone (under apical ectodermal ridge)
    • pre-/post axial: controlled by zone of polarizing activity
    • posterior/anterior : somites divide into doral and ventral muscle mass
  • digits formed by mesenchyme/cartilaginous modelling of skeletal elements and programmed cell death to seperate
  • limbs rotate to reach final position. Lower limb more than upper.

 

7.1 EMBRYOLOGY OF THE LIMBS

Concept of pattern formation in the developing limb along the three axes:

Proximo-distal polarity, pre-/ postaxial polarity, extensor / flexor polarity.

Origin of segmental nerve supply to skin and muscles
Congenital anomalies: polydactyly, syndactyly