Fundamental Disorders of Prosencephalic Formation
1. Aprosencephaly
- Complete absence of prosencephalic structures:
- No telencephalon or diencephalon.
2. Atelencephaly
- Absent telencephalon; preserved diencephalon (thalami present).
- Rare and invariably lethal anomaly.
Disorders of Prosencephalic Cleavage
Holoprosencephaly (HPE)
Definition
- Spectrum disorder from impaired or incomplete forebrain cleavage into cerebral hemispheres.
- Fundamental issue: disturbed dorsoventral patterning during ventral induction phase.
Pathogenesis and Molecular Basis
- Sonic Hedgehog (SHH) signaling:
- Key ventralising factor in forebrain development.
- SHH disruption leads to failure or incomplete cleavage.
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