Development Of The Limbs Flashcards
Describe simply, concepts such as compartmentalisation of structures and their positional changes during development
Myogenic precursors migrate into the limbs from somites and coalesce into two common muscle masses around the newly formed skeletal elements.
Ventral - flexor, dorsal - extensor. Individual muscles then split from the common masses. Upper and lower limbs have flexor and extensor compartments on opposite sides.
The upper limb rotates laterally so the future elbow points backward and the lower limb rotates medially so the future knees face forward.
Spinal nerves enter the limb bud early in development otherwise development stalls.
Nerves grow into common muscle masses - anterior divisions for the ventral compartment regroup to form medial and lateral cords that supply flexors. Posterior divisions for the dorsal compartment regroup to form the posterior cord and branches supply extensors.
Give a summary of prenatal limb development
Limb buds appear in 4th week of development, with the lower limbs lagging about 2 days behind the upper limbs. The buds consist of a mass of mesenchyme covered by a layer of ectoderm. Limb buds appear on the ventro-lateral wall and extend ventrally first. The elongation of the limb bud is through proliferation of the mesenchyme core.
Apical Ectodermal Ridge (AER) promotes growth (proximal to distal) and development of the limbs due to signalling molecules and marks the boundary between dorsal and ventral. It induces development of the digits within the plates then regresses.
Zone of Polarising Activity (ZPA) at the posterior base of the limb bud is responsible for generation of asymmetry in the limbs. It controls patterning and maintains the AER.
Anterior –> Posterior (ZPA)
Proximal –> Distal (AER)
Dorsal –> Ventral (ectoderm)
Explain simply, the developmental basis of some common abnormalities of the limbs
Limb defects are rare and usually hereditary but teratogen induced defects have been described.
Amelia - complete absence of a limb
Meromelia - partial absence of one or more limb structures (phocomelia due to thalidomide)
Syndactyly - fusion of one of more digits (CT or bone)
Polydactyly - one or more extra digits
Triradiate cartilage - fusion of three hip bones, underdevelopment of acetabulum and head of femur
