Limb development and embryology

Question 1

Development of the upper limb 1

Answer 1

• Upper limb develops from the upper limb bud, which receive cells from 5 somites (from mesoderm, each innervated by a spinal cord segment: C5-T1)
• The somites form the muscles of the limbs (dermomyotome), and they form the spinal cord (sclerotome)
• The developing limb bud is separated into an anterior compartment (medial-facing) and posterior compartment (lateral-facing)
• These compartments are on either side of the humerus bone

Question 2

Development of the upper limb 2

Answer 2

• The upper limb then rotates laterally, so that the anterior compartment eventually faces anteriorly (before was medially) and the posterior compartment faces posteriorly (was laterally)
• Anterior compartment will be responsible for arm flexion (biceps)
• Posterior compartment will be responsible for arm flexion (triceps)

Question 3

Myogenesis

Answer 3

• The myoblasts in each compartment differentiate into muscle fibers via myogenesis
• 3 steps in myogenesis: myoblasts proliferate, differentiate into myotubes (fusion of myoblasts), then mature into muscle fibers (multinucleate) by producing myofibrils
• The muscle fibers are incapable of replicating, but some myoblasts do not fuse and remain as a reserve for muscle regeneration (satellite cells)

Question 4

Dermatomes of upper limbs

Answer 4

• As the upper limb bud grows it pulls the ventral rami of the regions C5-T1 with it
• All of these dermatomes innervate parts of the upper limbs, with much overlap
• A few regions do not have overlap and can reliably test certain dermatomes
• C6 is tested on the thumb, C7 is tested on the middle finger, and C8 is tested on the pinky

Question 5

Lower limb development

Answer 5

• Begins the same as the upper limbs: anterior compartment is medial-facing and posterior compartment is lateral facing
• However the lower limbs rotate opposite to the upper limbs
• Lower limbs rotate in the medial direction, meaning the anterior compartment (hamstrings, flexors) ends up on the posterior side and the posterior compartment (quads, extensors) ends up on the anterior side
• Thus, before rotation (for both upper and lower) the anterior compartment contains flexors and the posterior compartment contains extensors
• These just reach opposite sides of the upper and lower limbs because the two rotate in opposite directions

Question 6

Lower limb dermatomes

Answer 6

• Develop in a process similar to upper limbs, the limb bud pulls the ventral rami as it grows
• However in the lower limbs there is more rotation of the dermatomes, which creates the spiral-effect of the dermatomes
• The reliable places to test lower limb dermatomes: knee cap and medial side of big toe for L4, dorsum of foot and middle toe for L5, and little toe and lateral side of foot for L1