Nervous System Embryology

Introduction

• Embryology of the nervous system involves the transformation of a blastula into three germ layers.
• Focus is on the ectoderm, which becomes the entire nervous system.

Formation of Germ Layers

• Blastula folds to form three germ layers:
  • Ectoderm (focus for nervous system)
  • Mesoderm
  • Endoderm (not relevant here)

Development Process

• Notochord develops between mesoderm and secretes growth factors.
• Growth factors act on the ectoderm to form a neural plate.
• The ectoderm moves laterally as the neural plate forms medially, termed neuroectoderm.

Neural Plate Folding

• Neural plate folds over, pushing ectoderm up and out.
• Folding results in the formation of:
  • Neural tube (from neural plate)
  • Neural crest (from tissue between neural folds and neural plate)

Neuralation

• Process described above is called Neuralation.
• Important for exams as it relates to embryological folding and movement.

Pathology

• Spina Bifida: Failure of the neural tube to close caudally.

  • Types:
    1. Spina Bifida Occulta
       • No protrusion of spinal contents.
       • Normal Alpha Fetoprotein (AFP) levels.
       • Possible features: tuft of hair or sacral dimple.
    2. Spina Bifida with Meningocele
       • Protrusion of meninges, not spinal cord.
       • Increased AFP levels.
    3. Spina Bifida with Myelomeningocele
       • Protrusion of meninges and spinal cord.
       • Increased AFP levels.
       • Associated with Chiari Type 2 malformations.

• Anencephaly: Failure of neural tube closure at the cephalic end leading to partial absence of the brain.

  • Incompatible with life.
  • Increased AFP and polyhydramnios.

Primary Vesicle Formation

• Formation of three primary vesicles from the top of the neural tube:
  • Prosencephalon (forebrain)
  • Mesencephalon (midbrain)
  • Rhombencephalon (hindbrain)

Secondary Vesicle Derivatives

• Prosencephalon:
  • Telencephalon → Cerebral hemispheres, Globus pallidus
  • Diencephalon → Thalamus, Hypothalamus
• Mesencephalon:
  • Mesencephalon → Midbrain
• Rhombencephalon:
  • Metencephalon → Pons, Cerebellum
  • Myelencephalon → Medulla

Ventricular System

• Telencephalon → Lateral ventricle
• Diencephalon → Third ventricle
• Mesencephalon → Cerebral aqueduct
• Metencephalon → Upper part of the fourth ventricle
• Myelencephalon → Lower part of the fourth ventricle and central canal

Conclusion

• Top part of the neural tube forms primary and secondary vesicles, leading to neural structures and ventricular cavities.
• Bottom part of neural tube becomes the spinal cord.

Study Tips

• Understand neuralation and its potential pathologies.
• Memorize derivatives of primary and secondary vesicles using neuroanatomy knowledge.
• Re-watch material for complex topics and use mnemonics where helpful.