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In the normal human embryo, the neural plate arises approximately 18 days after fertilization (fig 2 and 3).
During the fourth week of development, the neural plate invaginates along the embryonic midline to form the neural groove.
The neural tube is formed as closure of the neural groove proceeds from the middle of the groove and progresses toward the ends in both directions, with completion between day 24 for the cranial end and day 26 for the caudal end (fig. 4).
Disruptions of the normal closure process give rise to NTDs.
Anencephaly results from failure of neural tube closure at the cranial end of the developing embryo. Absence of the brain and calvaria may be partial or complete.
neural tube formed neural folds in apposition deepening neural groove flatplate stage
MRI images of cerebral cortex in a normal human being (Normal),
a patient with LIS1 mutation (LIS1),
a female patient with DCX mutation (DCX-Female) and
a male patient with DCX mutation (DCX-Male).
Notice that patients with mutations in LIS1 and male DCX patients show strikingly similar lissencephalies, whereas female patients with DCX mutations present with 'double cortex' syndrome, in which a band of grey matter is embedded within the white matter beneath the normal cortex.
Agyria-pachygyria-band spectrum, grade 2 Sagittal T1-weighted image ( A ) shows hypoplastic corpus callosum ( arrows ). The splenium is not bulbous; the genu and rostrum are not well seen. The telencephalic brain appears smooth and poorly sulcated, although this can be difficult to discern on midline sagittal images. Axial T1-weighted image ( B ) confirms the smooth lissencephalic brain; note the very primitive sulcation limited to the frontal region ( arrows ). A more cephalad axial T2-weighted image ( C ) shows primitive, enlarged ventricles, figure-8 shaped lissencephalic brain, and hyperintense cell-sparse layer A B C
Agyria-pachygyria-band spectrum, grade 3 Sagittal T1-weighted MPRage image ( D ) shows smooth, broad gyri typical of pachygyria. Axial T1-weighted image ( E ) also shows pachygyria which is more severe over the posterior than anterior regions. Also note the mildly enlarged ventricles with rounded margins. The abnormal hypointense foci along the periatrial margins are atypical, and their significance is not clear. Axial T2-weighted image ( F ) shows similar changes. D E F
Agyria-pachygyria-band spectrum, grade 5 Coronal T1-weighted MPRage image ( G ) shows bilateral undulating bands of abnormal gray matter ( arrows ) in the subcortical region. Note the normal olfactory sulci. Coronal proton density-weighted image at the same location ( H ) also shows bands of heterotopic gray matter ( arrows ). Further posteriorly, coronal MPRage image ( I ) again shows bands. However, the bands terminate superiorly in frank pachygyria (short arrows). Also note the subtle additional heterotopia (long arrows) deep to the band along the ventricles which are occasionally seen in subcortical band heterotopia. G H I
Agyria-pachygyria-band spectrum, grade 6 Axial T2-weighted image ( J ) shows typical subcortical band heterotopia deep to the overlying cortex. The signal intensity parallels gray matter. The ventricles are slightly enlarged and rounded. Slightly more cephalad, axial proton density-weighted ( K ) and T2-weighted ( L ) images show similar changes. The bands have the signal intensity of gray matter ( K, arrows ). The inner margin of the bands are usually smooth, as in this case. The outer margins may be smooth or may undulate with fingers of subcortical white matter, as in this case. The overlying cortex may be radiographically normal or pachygyric J L K
Polymicrogyria the surface of the cerebral hemispheres shows multiple small bumps, suggesting an excessive number of gyri. The cortex is thick and consists of the molecular and one other broad neuronal layer (in some cases there are three poorly defined neuronal layers). These layers are irregularly overfolded and fused, eliminating the sulci. Patients with polymicrogyria have severe psychomotor retardation and seizures.
A focally thickened cortex with a disordered cytoarchitecture, large abnormally oriented neurons and hypertrophic astrocytes.
Such lesions are often seen in specimens resected for epilepsy.
The lesion is thought to represent a focal abnormality of neuronal migration and differentiation.
It resembles the cortical lesions of tuberous sclerosis.
Q04.8 Other specified congenital malformations of brain
Q04.82 Agenesis of septum pellucidum
Q04.84 Diorders of neuronal migration
Q04.840Cortical lamination abnormality
Band heterotopia (double cortex) Band heterotopia, a. MR IR TSE coronal image, a continuous, bilateral band of grey matter interposed between the ventricles and the cortex. The laminar-shaped heterotopic grey matter parallels the signal of the normal cortex in all imaging sequences and seems a "double cortex".
Band heterotopia (double cortex) a, b. MR, T1- and T2-weighted images. A very thick cortex, with two layers separated by a very thin strand of white matter (double cortex) is seen bilaterally. c, d. Coronal images show the same pattern, and absence of digitations of the white matter leading towards the cortical convolutions. A B C D