Embryology of the Central Nervous System (CNS)

Introduction

• CNS embryology focuses on organogenesis of the CNS.
• Involves the ectoderm, mesoderm, and endoderm germ layers.
  • Ectoderm: Initiates CNS embryogenesis, differentiates into surface and neural ectoderm (forms brain, spinal cord, peripheral nerves).
  • Endoderm: Forms lining of gastrointestinal and respiratory systems, and organs like liver and pancreas.
  • Mesoderm: Differentiates into paraxial, intermediate, and lateral plate mesoderm affecting skeletal and muscular development.

Embryological Transformations

• Weeks 1-3: Zygote formation, blastocyst, gastrulation.
• Mid-fourth week: Notochord formation.
• Late-fourth to eighth week: Differential growth, limb bud and organ system formation.
• Weeks 9-38: Body growth, limb development, organ maturation.

Development

Embryogenesis (Weeks 2-8)

• Germ disc development, primitive streak formation, and notochord induction.
• Neurulation: Neural plate folds into the neural tube (forms CNS).
• Neural Crest Cells: Form dorsal root ganglia and connective tissue.
• Notochord: Defines the axis, part of intervertebral discs.

Spinal Cord

• Develops from neural plate.
• Contains ventricular, mantle, and marginal layers.
• CNS myelination by oligodendrocytes; PNS myelination by Schwann cells.

Brain

• Formation of three primary brain vesicles into five secondary vesicles.
• Prosencephalon: Develops into forebrain structures (consciousness, sensorimotor transformation).
• Mesencephalon: Becomes midbrain with minimal reorganization.
• Rhombencephalon: Divides into segments affecting cerebellum and medulla functions.
• Pituitary Gland: Arises from hypophysis with dual origins (anterior from ectoderm, posterior from hypothalamus).

Testing

• Non-invasive tests (10 weeks gestation).
• Amniocentesis (14-20 weeks) for fetal anomalies.
• First-trimester testing for trisomies and neural defects.
• Anatomy sonogram (16-22 weeks) for fetal measurements.

Pathophysiology

• Complications include teratogenesis and dysraphism (like spina bifida).
• Spina Bifida Types: Occulta, aperta, cystica.
• Cranial dysraphism leads to issues like encephalocele, anencephaly, holoprosencephaly, and craniorachischisis totalis.

Clinical Significance

• Rare, but potential for serious craniofacial and spinal defects.
• Emphasis on avoiding teratogens during embryogenesis.
• Importance of prenatal vitamins (folic acid) for neurodevelopment.

References

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