Limb differentiation
 

 
Limb development video 

Limbs grow from limb buds which are seen shortly before the disappearance of somites. They are formed from somatopleure and are placed ventral to cervical and upper sacral somites. An overlying ectodermal ridge is formed over the limb buds as they grow. They  are initially rounded and later flattened dorsoventrally. Initially the flexor aspect of the limb bud is ventral while the extensor is dorsal so that preaxial border is ventral and postaxial is caudal.

Muscle first forms in the limb buds as condensed mesenchyme. Flexor and adductor muscles develop in the ventral mass of condensed mesenchyme while the abductor and extensor muscles are in the dorsal . As the muscles are developing the mesenchyme forming the cartilaginous basis of bones begin to form.

The arteries go along with the structures which they supply as they migrate during development

Initially the muscles are innervated segmentally. As the muscle mass begin to migrate they carry along their nerve supply, hence the formation of plexuses -such as brachial plexus and lumbosacral plexus. The final nerves of individual muscles show functional components and spinal cord levels which are multisegmental

 

 

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Limb at 12 weeks gestation

 

 

 

 

 

 

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