Foundations of the Developing Nervous System

Understanding the timing of developmental events is crucial; a teratogenic insult or genetic mutation at a specific gestational week yields a predictable structural anomaly.
A. Neurulation (Weeks 3–4)

• Process: Formation of the neural plate → neural groove → neural tube.
• Key Clinical Correlates:
  • Failure of anterior neuropore closure: Anencephaly (incompatible with life).
  • Failure of posterior neuropore closure: Myelomeningocele (open spina bifida). Critical association with folate deficiency.
  • Defects in secondary neurulation (canalization): Tethered cord (often presents later in childhood with bladder dysfunction or orthopedic deformities).

B. Prosencephalic Development (Weeks 5–10)

• Process: Cleavage and midline development of the forebrain.
• Key Clinical Correlate:
  • Holoprosencephaly: Failure of the hemispheres to separate. Severity ranges from alobar (single ventricle, cyclopia) to semilobar/lobar (often presents with midline facial defects such as a single central incisor, hypotelorism, or cleft lip/palate, plus developmental delay and seizures).

C. Neuronal Proliferation & Migration (Weeks 8–24)

• Process: Neurons are born in the periventricular germinal matrix and migrate radially along glial fibers to the cortex.
• Key Clinical Correlates:
  • Periventricular Hemorrhagic Infarction (PVHI): The germinal matrix is highly vascular and fragile. In preterm infants (especially <32 weeks), hemodynamic instability leads to hemorrhage (Grade I-IV IVH).
  • Migration Disorders:
    • Lissencephaly (smooth brain): Failure of migration (e.g., LIS1 or DCX mutations). Presents with severe developmental delay, early-onset epilepsy (infantile spasms), and hypotonia evolving to spasticity.
    • Periventricular Heterotopia: Clusters of neurons lining the ventricles; classic presentation is a female with normal intellect but refractory epilepsy.

D. Myelination (Prenatal to Adulthood)

• Process: Insulation of axons occurs in a predictable caudal-to-rostral and central-to-peripheral pattern.
• Key Clinical Correlate:
  • Myelination patterns dictate the normal primitive reflexes (e.g., Moro disappears as corticospinal tracts myelinate).
  • Leukodystrophies: Inborn errors of metabolism (e.g., Krabbe, Pelizaeus-Merzbacher) present with regression (loss of milestones) as previously myelinated white matter degenerates.