Fourth to Eight Weeks of Human Development

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The immediate fate of paraxial mesoderm?
- The immediate fate of paraxial mesoderm :
  - The paraxial mesoderm develops into somites.
  - At week 4, it develops into segmented round cell clusters (somites) along the neural tube
  - At the beginning of week 5,42-44 somite pairs are formed in a cranocaudal direction. Several pairs degenerate leaving only 35-37 pairs.
  - Somites determine body segmentation.
Developmental derivatives of somites
- Dermomyotome
  - Dermatome: Migrate towards the surface ectoderm. Forms the dermis of the neck and trunk.
  - Myotome: Subdivides into the dorsal epaxial and ventral hypaxial skeletal muscles in the lateral and anterior trunk.
    - Hypaxial muscles – thoracic and anterior abdominal muscles
    - Epaxial muscles – extensor muscles of the neck and back
- Sclerotome: Each pair migrates medially toward the notochord and fuse. Forms the vertebrae, rib cartilage, and part of the occipital bone
10 derivatives of neural crest cells
- Nervous tissue: Neurons and glia of the PNS, Dorsal root ganglion, Autonomic ganglion cells, Schwann cells, Satellite glial cells, Enteric Nervous System
- Cranial and head structures: Dermis and Subcutaneous tissue, Bones, Cartilage, Connective tissue, Arachnid mater, Pia mater, Odontoblasts (Dentin), Dental cement
- Chromaffin cells of the adrenal medulla
- Melanocytes
- Carotid body
- Cardiac septum in the outflow tract
- Cardiac tissue: Endocardial cushions, Aorticopulmonary septum
Derivatives of neural crest found in the head and neck region
- Cranial nerves
- Bones
- Cartilage
- Dermis and Subcutaneous tissue
- Odontoblasts
- Dental cement
Derivatives of the neural crest cells in the: Nervous system, Skin, and Oral cavity
- Nervous system: Neurons and glia of the PNS, Dorsal root ganglion, Autonomic ganglion cells, Schwann cells, Satellite glial cells, Enteric Nervous System
- Skin: Melanocytes
- Oral cavity: Odontoblasts, Dental cement

Fourth to Eight Weeks of Human Development

Jeffrey Kalei

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