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Holoprosencephaly is a type of cephalic disorder. This is a disorder characterized by the failure of the prosencephalon (the forebrain of the embryo) to develop. During normal development the forebrain is formed and the face begins to develop in the fifth and sixth weeks of human pregnancy. (The condition also occurs in other species, as with Cy, the Cyclops kitten.) Holoprosencephaly is caused by a failure of the embryo's forebrain to divide to form bilateral cerebral hemispheres (the left and right halves of the brain), causing defects in the development of the face and in brain structure and function.
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Symptoms of holoprosencephaly range from mild (anosmia, or a single central incisor) to moderate (cleft lip or palate) to severe (synophthalmia proboscis or cyclopia).
There are three classifications of holoprosencephaly.
Holoprosencephaly, once called arhinencephaly, consists of a spectrum of defects or malformations of the brain and face. At the most severe end of this spectrum are cases involving serious malformations of the brain, malformations so severe that they often cause miscarriage or stillbirth. At the other end of the spectrum are individuals with facial defects - which may affect the eyes, nose, and upper lip - and normal or near-normal brain development. Seizures and mental retardation may occur.
The most severe of the facial defects (or anomalies) is cyclopia, an abnormality characterized by the development of a single eye, located in the area normally occupied by the root of the nose, and a missing nose or a nose in the form of a proboscis (a tubular appendage) located above the eye. The condition is also referred to as cyclocephaly or synophthalmia and is very rare.
Armand Marie Leroi  describes the cause of cyclopia as a genetic malfunctioning during the process by which the embryonic brain is divided into two. Only later does the visual cortex take recognizable form, and at this point an individual with a single forebrain region will be likely to have a single, possibly rather large, eye (at such a time, individuals with separate cerebral hemispheres would form two eyes).
Increases in expression of such genes as Pax-2, as well as inhibition of Pax-6, from the notochord have been implicated in normal differentiation of cephalic midline structures. Inappropriate expression of any of these genes may result in mild to severe forms of holoprosencephaly.
Leroi goes on to state that:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Holoprosencephaly". A list of authors is available in Wikipedia.|