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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