Meroencephaly• failure of the rostral neuropore to close during the fourth week• forebrain primordium is abnormal and development of the Most of the embryos brain is exposed or extruding from the cranium- exencephaly.•.
• abnormal structure and vascularization of the embryonic exencephalic brain-nervous tissue degenerates.• Remaining brain appear as a spongy, vascular mass; mostly hindbrain structures
• often called anencephaly(Gr. an, without, + enkephalos, brain);a rudimentary brainstem and functioning neural tissue are always present in living infants.• meroencephaly (Gr.meros, part) is the better term
• common lethal anomaly, occurring at least once in every 1000 births.• most common serious anomaly seen in stillborn fetuses• Female preponderance- 2-4 times>males• always associated with acrania (absence of the calvaria) and may be associated
• suspected in utero when there is an elevated level of AFP in the amniotic fluid• easily diagnosed by ultrasonography and MRI fetoscopy, and radiography because extensive parts of the brain and calvaria are absent.• Associated polyhydramniosis- common
Holoprosencephaly• severe and relatively common• Maternal diabetes and teratogens, such as high doses of alcohol, can destroy embryonic cells in the median plane of the embryonic disc during the third week→ defective formation of the forebrain.• infants have a small forebrain, merged lateral ventricles