Abstract
Within the context of embryogenesis, and particularly with regard to prosencephalic cleavage, we describe the anatomical characteristics of holoprosencephaly, which results from a disorder of ventral induction. To interpret morphogenesis in this anomaly, it is important to know the difference between the hemispheric telencephalon (hemispheric brain) and the holospheric telencephalon (holospheric brain). While the hemispheric brain represents normal development of the paired neocortical primordium (telencephalon 'totopar'), the holospheric brain, which is an essential feature of holoprosencephaly, results from failed development of the neocortex, which may originate from a single or unpaired primordium. Thus, holoprosencephaly shows an undivided telencephalon 'totopar' (supralimbic lobe), where the neocortex shows midline continuity in the more rostral part of the telencephalon. In addition, while the dorsal cyst is usually absent in lobar holoprosencephaly, the dorsal sac, which represents the unfolded diencephalic roof plate, covers the dorso-caudal aspect of the prosencephalic ventricle. These anatomical features, such as midline continuity of the cerebral neocortex and an unfolded diencephalic roof plate, are common to the holospheric brain. On the other hand, callosal defect, which results from maldevelopment of the telencephalon 'impar' (medium) after complete hemispheric cleavage, may show an elevated roof of the third ventricle. Marked dilatation of the third ventricle with absent or hypoplastic falx cerebri is called primary interhemispheric cyst, and its configuration mimics holoprosencephaly with a dorsal cyst. Since a primary interhemispheric cyst consists of an elevated but folded diencephalic roof plate (roof of the third ventricle) in the hemispheric brain, it should be differentiated from holoprosencephaly.
