Msk Embryology

Question 1

Describe the early formation and developmental role of neural crest cells.

Answer 1

• immediately after grastrulation
• ectoderm differentiates into the midline neural ectoderm and non-neural ectoderm
• neural ectoderm folds over on self making neural groove and ultimately neural tube (by day 21)
• neural tube is covered by non-neural ectoderm and is surrounded by paraxial mesoderm.
• between neural and non-neural area is dubbed the neural fold
• neural fold cells delaminate and are now neural crest cells that can actively migrate throughout the embryo and differentiate (* unique in humans, ectoderm becomes mesoderm - changes layers)

Question 2

detail the developmental fate of the paraxial mesoderm

Answer 2

• paraxial mesoderm differentiates into segmental blocks called somites
  
  • less distinct in head region and dubbed somitomeres
• both somites and somitomeres give rise to mesenchyme (embryonic connective tissue)
• first somite is formed in occipital region and development progresses caudally, somite number used as proxy for developmental stages

Question 3

Describe formation of the vertebrae

Answer 3

• somites differentiate into sclerotomes (mesenchyme that will form the vertebral column through endochondral ossification)
• somite first differentiates into discrete layers (medial to lateral)
  
  • sclerotome - cartilage/bone
  • syndetome - tendons
  • myotome - muscle
  • dermatome - dermis of skin
• each sclerotome differentiates
  
  • cranial low density
  • caudal high density
  • they migrate toward eachother and merge with successive sclerotomes
  • cells remaining from caudal half become outer portion of IV disc (annulus fibrosis)
• numbering scheme
  
  • initially 8 cervical somites, cranial half of first sclerotome migrates up to fuse with occipital bone
  • so 7 complete cervical sclerotomes
  • spinal nerves come out between sclerotomes
• after reorganizing, mesenchyme moves medially and surrounds notochord and neural tube (model of centrum and neural arch)
• mesenchyme differentiates to chondroblasts for cartilage vertebral models
• 3 primary ossification centers emerge, one in the notochord, eliminating it, with its remnants becoming the center of the IV disc - nucleus pulposus