Neural Tube Formation and Development

Overview

• Neural tube formation is critical for the development of the spinal cord and brain.
• Complications in this process can lead to neural tube defects.
• Neural development begins in the third week of gestation.

Primary Neurulation

• Definition: The process forming the functional central nervous system.
• Begins with the development of the embryonic nervous system by day 18.

Germ Layers

• Ectodermal Germ Layer: Shaped like a disc with cranial and caudal ends by day 18.
• Three Germ Layers:
  • Ectoderm
  • Mesoderm
  • Endoderm

Formation of the Neural Plate

• Central nodal cord induces ectoderm to thicken and form the neural plate.
• Folding of the neural plate leads to the neural tube.

Formation of the Neural Tube

• By the end of week three:
  • Neural plate edges form neural folds.
  • Neural groove forms in the mid-region.
  • Neural folds fuse at the midline to form a hollow neural tube.

Neural Crest Cells

• Separate from neural fold tips.
• Migrate to form various cell types:
  • Schwann cells
  • Meninges
  • Endocardial cushions
  • Parafollicular cells
  • Adrenal medulla

Closure of the Neural Tube

• Cranial and caudal ends communicate with the amniotic cavity.
• Cranial neural pore closes on day 25.
• Caudal neural pore closes on day 27.

Development of Brain Vesicles

• Cranial end development:
  • Forms three primary vesicles:
    • Forebrain (Prosencephalon)
    • Midbrain (Mesencephalon)
    • Hindbrain (Rhombencephalon)
  • Develops into five secondary vesicles:
    • Forebrain: Telencephalon, Diencephalon
    • Midbrain: Mesencephalon
    • Hindbrain: Metencephalon, Myelencephalon

Neural Tube Defects

• Occur if the neural tube fails to close properly.
• Can be caused by:
  • Genetic syndromes
  • Chromosomal disorders
  • Environmental factors (e.g., folic acid deficiency)

Types of Neural Tube Defects

1. Anencephaly

   • Cranial end fails to close; leads to absence of brain development.
   • Associated with high alpha-fetoprotein and polyhydramnios.

2. Spina Bifida

   • Caudal end fails to close.
   • Vertebral arch remains open.
   • Types include:
     • Spina Bifida Occulta: Asymptomatic, possible tuft of hair, no alpha-fetoprotein increase.
     • Spina Bifida with Meningocele: Meninges protrude, no spinal cord protrusion.
     • Spina Bifida with Myelomeningocele: Meninges and spinal cord protrude, associated with Chiari II malformation and hydrocephalus, increased alpha-fetoprotein.
     • Spina Bifida with Myeloschisis: Most severe, skin does not cover defect, increased alpha-fetoprotein.

Conclusion

• Understanding neural tube formation and defects is crucial for early diagnosis and intervention of congenital abnormalities.