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This is holoprosencephaly, often accompanied by the failure of fetal facial midline structures to form properly. Hence, there are usually midline facial defects (cleft lip, cleft palate, cyclopia, etc) accompanying this condition. The "alobar" form of holoprosencephaly is shown here in which there a single large ventricle, because there is no attempt to form separate cerebral hemispheres.

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The skull is opened here to reveal the "semilobar" form of holoprosencephaly, because there is a small cleft representing an attempt to separate the hemispheres. There is no gyral pattern here because this stillborn fetus was under 20 weeks gestation. Holoprosencephaly is a grave condition with little or no brain function. Holoprosencephaly can be associated with chromosomal anomalies (such as trisomy 13), with maternal diabetes mellitus, and can be seen sporadically.

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Here is a cross section though a brain with holoprosencephaly, revealing a single ventricle. Toward the base of the brain are the fused thalami, typical of this process.

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