Central Nervous System Malformations and Embryonic Development

Congenital Malformations of the Central Nervous System

Neural Tube Formation Defects

These defects arise from incomplete closure of the neural tube during neurulation:

• Anencephaly: Malformation of the rostral neural tube resulting in no development of the cerebral hemispheres. Differentiation occurs up to the metencephalon caudally. Characterized by the lack of the cranial vault (calvaria).
• Amyelia: Spinal Cord (SC) defect characterized by the lack of formation of the neural arch of vertebrae (also known as rachischisis).
• Myeloschisis: A severe defect in neural tube closure affecting all or part of the spinal cord. Often associated with anencephaly.
• Spina Bifida: Lack of formation of the neural arch of vertebrae, most frequently occurring in the lumbar region.

Prosencephalon Cleavage Defects

Malformations resulting from abnormal division of the forebrain vesicle:

• Holoprosencephaly (HPE): A defect in prosencephalon cleavage, resulting in the failure to form rostral telencephalic vesicles. Characterized by a single central vesicle. Associated with cyclopia (due to lack of optic vesicle fusion).
• Arhinencephaly: Defect characterized by missing cleavage of the olfactory bulb. Associated with cebocephaly (often linked to chromosomal trisomy).
• Microphthalmia/Anophthalmia: Defects related to the size or complete absence of the optic vesicle.
• Cerebellar Agenesis: Absence of the cerebellum, related to hindbrain cleavage or developmental issues.

Neuronal Migration Defects

Abnormalities in the movement of neurons during cortical development:

• Lissencephaly: Characterized by a smooth cerebral cortex (agyria), lacking normal gyri and sulci.
• Pachygyria: Abnormally thick gyri; only large sulci may be visible.
• Microgyria: The cortex is formed by numerous, small, and abnormal gyri.
• Status Verrucosus Cortex (Cobblestone Lissencephaly): The cortex shows nodular excrescences.

Other CNS Malformations

• Congenital Aqueductal Stenosis: Occlusion of the cerebral aqueduct, causing dilation (hydrocephalus) of the lateral and third ventricles.
• Dandy-Walker Syndrome: Characterized by cystic dilation of the fourth ventricle.
• Hydranencephaly: Severe enlargement of the ventricles, often occurring late in development, typically due to defective irrigation of the internal carotid arteries (ICA).

Early Embryonic Development and Neurulation

Germ Layers and Early Structures

• Ectoderm: Epithelium located below the amniotic cavity.
• Endoderm: Epithelium lining the common vitelline cavity.
• Mesoderm: Mesenchyme that extends extraembryonically.
• Neural Plate: Derived from the ectoderm.
• Neural Crest: Forms at the junction between the neural plate and the surface ectoderm via proliferation.

Neurulation Process

Neurulation is the process by which the dorsal neural groove closes to form a hollow tube (the neural tube). The edges fuse and separate to form the neural crest.

Neurulation Timeline

• Day 22: Neurulation begins centrally. The cranial (head) and caudal (tail) ends of the neural plate remain open.
• Day 25: Closure of the cranial neuropore occurs.
• Day 27: Closure of the caudal neuropore occurs.

Neural Tube Differentiation

• Neural Tube: Develops into the spinal cord (SC) and the encephalon (brain).
• Spinal Cord (SC): Formed in the caudal region of the neural tube (starting around the level of the 4th somite).
• Encephalon (Brain): Formed from the cranial neural tube (rostral to the 4th somite level).

Primary Neural Tube Vesicles (3)

1. Prosencephalon (Forebrain): Origin of the optic vesicles.
2. Mesencephalon (Midbrain).
3. Rhombencephalon (Hindbrain).

Note on Third Week Development: During the third week, the embryonic disc separates from the amniotic sac and cavity.