Dermomyotome

• Dorsolateral wall of somite
• Myotome region - muscles
• Dermotome region - dermis of skin

Muscle development

• Myotomes cell masses
• Cell of myotome leave myotome to become elongated and spindle shaped - myoblasts
• Myoblasts differentiate and fuse with other myoblast - myotubes
• Fusion of myotubes form muscle fibres

Muscles and innervation

• Myotomes develop from one of the two areas of the somite
• Small dorsal division - epaxial
• Large ventral division - hypaxial
• Each division of a myotome forms a specific group of muscles with elongation - groups split into flexor and extensor components
• Further splitting - forms individual muscles that may have originated in more than one somite segment
• Epimeres (epaxial division) form the extensors of the back and vertebral column and extensors of the lumbar
• Hypomeres (hypaxial division) form the lateral and ventral flexor musculature of the trunk
• Branchiomeres - myoblasts from the branchial arches, which develop into the face

Limb and facial bone

• Lateral plate mesoderm forms bones of the shoulder, pelvic girdle, limbs and sternum
• Neural crest cells in the head form the craniofacial bones of the skull
• Most of limb musculature develops in-situ from the mesenchyme surrounding the developing bones

• Muscle masses separate into dorsal (extensor) and ventral (flexor)

Innervation of skeletal muscles

• Each myotome receives its innervation from spinal nerves from the same somite segment as the muscle cell
• Each spinal nerve divides and sends a branch to each division at its somite root level
• Dorsal primary ramus - to the epaxial
• Ventral primary ramus - to the hypaxial

Limb development

• Limb bud begin development begins in fourth week of gestation
• Initially consists of mesenchymal core from the parietal (somatic) layer of the lateral plate mesoderm covered by an outer layer of ectoderm
• Upper limb develops 1-2 days before lower limb

Apical ectodermal ridge

• Ectoderm at the distal border of the limb thickens to form apical ectodermal ridge (AER)
• Exerts an inductive influence of the adjacent mesenchyme causing it to remain undifferentiated, rapidly proliferating cells - progression zone
• Cells further from AER differentiate into cartilage and muscles

Digit formation

• Distal end on the limb bud a flatten hand or foot plate forms
• Cell death in the AER creates 5 separate AER regions for finger growth
• Cell death in the interdigital spaces separates the fingers

Innervation of limbs

• Ventral primary rami of these spinal nerve root levels penetrate the mesochyme with dorsal and ventral branches
• These ramus branches of the respective divisions unite to form large ventral and dorsal nerves - development of the plexus
• Radial nerve is formed by a combination of dorsal segment branches (posterior cord)
• Ulnar and median nerves are formed by a combination of ventral branches (medial and lateral cords)

Dermotome development

• As limb bud grows the dermatome is dragged with it
• Limb grows from the centre so the dermatome pattern