The neuroectoderm placode develops ridges (folds) laterally and begins to approximate in the region of the future medulla at 22 days of gestation before most women are aware they are pregnant. This closure, a process known as neurulation, results in a tube that continues to extend by the process of approximation of the neural folds in multiple locations rostrally and caudally until a complete neural tube is formed at 28 days of gestation, with the latter period marking the end of caudal neural tube closure (future spinal cord). The rostral neural tube, which closes at approximately 24 days of gestation, serves as the foundation for further brain development; the caudal end of the tube forms the spinal cord.

Although the specific molecules and genes involved in the processes of neural tube closure in humans are largely unknown, clearly a combination of genetic and environmental factors leads to disorders of these processes. Studies in mice suggest that a codeletion of Paxl, a gene for a transcriptional factor that mediates notochord signaling, and Pdgfra, the platelet-derived growth factor alpha gene, may lead to a spina bifida-like phenotype.

Certain ethnic groups experience a higher incidence of neural tube closure defects than do others, and genetic disorders with an apparent autosomal recessive inheritance that include neural tube defects have been described. In addition, teratogens are involved in neural tube pathology. The most notable teratogens for the neurologist are the anticonvulsants valproate and carbamazepine, each of which imposes a risk of 1% to 6% for a neural tube defect in offspring exposed in utero to these common drugs. Whereas folate supplementation appears to prevent neural tube defects in large population studies, this vitamin may or may not be protective when these disorders are associated with the aforementioned anticonvulsants. Given that short-term treatment with folate is probably benign, it seems prudent to offer this vitamin (400 to 4,000 µg daily, depending on risk factors) to women of childbearing age who are on anticonvulsants.

In humans, multiple genes and factors probably are involved in the pathogenesis of disorders of neural tube formation and closure. The human disorders of neural tube closure include craniorachischisis (a complete failure of neural tube closure along the entire neuraxis), anencephaly (a failure of anterior neural tube closure), myeloschisis (a failure of posterior neural tube closure), spina bifida (myelomeningocele, a failure of closure of a portion of posterior neural tube), and encephalocele (a partial defect in anterior neural tube closure). Fetal recognition of myelomeningocele with correction in utero prior to 26 weeks’ gestation can dramatically alter the need for subsequent ventriculoperitoneal shunting (reduced by 42%) and reduces the incidence of intellectual disability. The risk is to the mother.

Regional specification of the developing nervous system is an important step in human CNS development and is likely under the control of a number of genes that encode transcriptional factors and of a number of molecules that influence these genes. These genes were first described in Drosophila and are involved in the regional specification of the fly embryo. Not surprisingly, the function of the proteins encoded by the comparable genes in mammals differs considerably from that in fruit flies, but the general role of these proteins appears to be that of regional specification of clones of cells destined to form structures in the fully developed nervous system.

Transcriptional factors are proteins with distinct sequences that participate in DNA binding and thus influence DNA. In Drosophila, transcriptional factors encoding homeobox genes, empty spiracles, ems have been shown to specify head structures. For example, the rudimentary structures of mutant flies that are devoid of ems seem to be a result of failure of regional specification of a clone of cells that are destined to form those structures. In humans, pathogenic variants in the comparable gene EMX2 may result in schizencephaly, cleft in the cortical mantle. The clefts in this disorder extend from the pia to the ventricle and are lined with a polymicrogyric gray matter (see the discussion in “Polymicrogyria”). The pia and ependyma are usually in apposition, especially in severe cases. The defect is termed open lipped if the cleft walls are separated by CSF and closed lipped if the walls appose. These clefts may be unilateral or bilateral and the prognosis appears to be dependent on location, bilateral occurrence, or extent of the lesion. Bilateral schizencephaly is associated with mental retardation and spastic cerebral palsy; affected patients often are microcephalic. Seizures almost always accompany severe lesions, especially the openlipped and bilateral schizencephalic clefts. The exact frequency of seizures in patients with the less severe lesions is uncertain, nor is the incidence of asymptomatic schizencephaly. Most patients in whom schizencephaly is diagnosed undergo neuroimaging because of seizure. Therefore, a bias in favor of a universal occurrence of seizure in this disorder is noted. Hence, patients with schizencephaly who do not have epilepsy might exist, but the malformation remains undetected because no imaging is done. With fetal imaging, asymptomatic schizencephaly has been noted (Fig. 133.2).

Seizure type and onset may vary in this disorder. Patients may experience focal or generalized seizures. Some will present with infantile spasms. The onset varies from infancy to the early adult years. Seizures may be easily controlled or may be recalcitrant to standard anticonvulsant therapy.

Improvements in neuroimaging have enhanced the recognition of these disorders and have broadened the spectrum of the radiographic appearance of schizencephalic lesions. The lesions may occur in isolation or may be associated with other anomalies of brain development. An especially common association is made between septo-optic dysplasia and schizencephaly because as many as 50% of patients with septo-optic dysplasia also have schizencephaly.

Other genes for transcriptional factors have been described in the mouse. Pax3, PaxS, Pax6, Dlxl, DIx2, Dbx, and the Hox genes are found in specific brain regions. In general, the Pax genes tend to be expressed in midbrain and the Dlx genes tend to be expressed in the ventral forebrain, ventral telencephalon, and dorsal telencephalon; Dlx2 is important for the genesis of GABAergic interneurons from the median ganglionic eminence. The Hox genes seem to specify the neuromeres and rhombomeres of the hindbrain.

Furthermore, in an interesting link to the ventral inductive events described later, when applied to proliferating cells at critical times in development, the protein sonic hedgehog can alter the expression of the homeobox genes. This ties the inductive proteins to the expression of homeobox genes and gives a hint as to the mechanisms involved in inductive processes. Further, a gradient of retinoic acid in a rostral (less) to caudal (more) fashion seems to be important for Hox and other patterning gene expression patterns in the hindbrain, neck, and other structures of the head. Retinoic acid activity also decreases the sonic hedgehog signaling (see “Ventral Induction [Prosencephalon Cleavage]”). It is for this reason that retinoids are highly regulated and should not be given to women who may become pregnant.

Septo-optic dysplasia (de Morsier syndrome) is a disorder characterized by absence of the septum pellucidum, optic nerve hypoplasia, agenesis of the corpus callosum, and hypothalamic dysfunction. This disorder should be considered in any patient who exhibits at least two of these, and all such patients should have hypothalamic function screening. Fifty percent of patients with septo-optic dysplasia have schizencephaly. Although this is a rare disorder, genetic syndromes with this phenotype and with some risk of recurrence have been described. For instance, pathogenic variants in homeobox gene expressed in ES cells, HESX1, have caused a dominantly inherited disorder in siblings. This disorder can be suspected in utero but magnetic resonance imaging (MRI) of the fetus may be necessary to assure a proper diagnosis. The prognosis for development is highly variable, with learning, intellectual, and motor impairments often described. Asymptomatic patients probably only come to medical attention if there are needs for neuroimaging. Pathogenic variants in COL11A2 and PAX6 have also been described. Because most of the genes related to this phenotype are undiscovered and no genetic panels exist, it is recommended that clinicians consider exome sequencing in order to delineate the recurrence risk in a family.

The telencephalon is formed by medial division of a rostral single tube-like structure (prosencephalon); the two vesicles (telencephalon) formed in this division become the two cerebral hemispheres. The ventral and anterior portion of this division is induced by midline facial structures and the notochord via soluble factors. Abnormalities of this induction and division lead to midline abnormalities of the brain such as holoprosencephaly. These disruptions of normal development occur before 42 days of gestation.

Sonic hedgehog, which was described first in Drosophila as a soluble factor (protein) that influences dorsoventral patterning of the developing embryo, is probably the most important of the soluble factors influencing ventral induction. Sonic hedgehog is expressed in the notochord (also in ventral forebrain and floor plate— future facial structures), interacts through a well-defined signaling pathway that includes PTCH, a human homolog of patched, and alters the expression of transcriptional factors (homeobox and other related gene products).

Other molecules of interest in this inductive process are the retinoids (discussed previously in segmentation), which are lipids capable of crossing membranes and have been shown to exist in gradients across embryos. Retinoic acid can alter the pattern of transcriptional factors in neuroepithelial cells, and it can also downregulate sonic hedgehog, perhaps explaining some of the midfacial defects and holoprosencephaly seen in retinoid embryopathy. Also, cholesterol and cholesterol-derived lipids serve as a cofactor for sonic hedgehog, thus perhaps explaining telencephalic cleavage issues (holoprosencephaly) in Smith-Lemli-Opitz syndrome (7-dehydrocholesterol reductase deficiency).

The holoprosencephaly syndromes are heterogeneous disorders of ventral induction and telencephalic cleavage that result from a failure of the prosencephalic vesicle to medially cleave normally. At least three forms of this disorder have been described: alobar, semilobar, and lobar. In the alobar form, the telencephalic vesicle completely fails to divide, producing a single horseshoeshaped ventricle, sometimes with a dorsal cyst, fused thalami, and a malformed cortex (Fig. 133.3). In the semilobar form, the interhemispheric fissure is present posteriorly, but the frontal and, sometimes, parietal lobes continue across the midline. In the lobar form, only minor changes may be seen: The anterior falx is absent, the frontal lobes and horns are hypoplastic, there may be partial fusion of the thalami, and the genu of the corpus callosum may be abnormal. Holoprosencephaly is a common human malformation leading to spontaneous abortion.

Because sonic hedgehog and patched are expressed in the developing face, it is not surprising that holoprosencephaly is associated with a spectrum of midline facial defects. These include cyclopia, in which there is a single central eye and a supraorbital proboscis; ethmocephali, in which the nose is replaced by a proboscis located above the hypoteloric eyes; cebocephaly, in which hypotelorism and a nose with a single nostril are seen; and premaxillary agenesis, with hypotelorism, a flat nose, a single frontal incisor, and a midline cleft lip.

Only children who have the semilobar and lobar forms are known to survive for more than a few months. An infant affected with the severe form is microcephalic (unless aqueduct stenosis and hydrocephalus are present), hypotonic, and visually inattentive. In infants with the less severe forms of holoprosencephaly, myoclonic seizures frequently develop and, if the infant survives, autonomic dysfunction, failure to thrive, psychomotor retardation, and atonic or spastic cerebral palsy often are present. Some infants with the lobar form may be only mildly affected. Pituitary defects may be associated with these malformations and may result in neuroendocrine dysfunction.

Holoprosencephaly has been reported to be associated with maternal diabetes, retinoic acid exposure, cytomegalovirus, and rubella. Chromosomal abnormalities associated with this disorder include trisomies 13 and 18; duplications in 3p, 13q, and 18q; and deletions in 2p, 7q, 13q, and 18q. Autosomal dominant forms exist in which the pathogenic variant is in sonic hedgehog or in ZIC2. In this form, the clinical features vary. In their mildest form, presence of a single central incisor, attention deficit disorder, or a choroid fissure coloboma may be the only clinical manifestation of an autosomal dominant disorder.

Lining the interior of the newly developed telencephalic vesicles is a proliferative, primitive neuroepithelium. Neuroepithelial processes extend from the ventricular surface to the pial surface, and the nuclei of the primitive neuroepithelial cells move from the cortical surface in a premitotic phase to a mitotic phase near the ventricle. Cells divide at the most ventricular aspects of the developing telencephalon and, after division, move back toward the pial surface. The pial processes of neuroepithelial cells near the ventricle often will detach from the cortical surface before a new cycle begins.

Neuroepithelial cells divide in so-called proliferative units such that each unit will undergo a specific number of divisions resulting in the appropriate number of cells for the future cortex. Abnormalities in the number of proliferative units or in the total number of divisions can lead to disorders of brain manifested by abnormal brain size and, therefore, an unusually small (microcephaly) or large head (macrocephaly). Disorders in which too many cells are generated in the proliferative phase result in megalencephaly (large brain) or, if proliferative events go awry on only one side of the developing cortex, hemimegalencephaly. Once the appropriate complement of cells is generated in the proliferative phase, the cells that will become the neurons of the cerebral cortex become postmitotic and are referred to as neuroblasts. Others undergo a programmed cell death—apoptosis. The genetics of apoptosis are best characterized in the simple nematode Caenorhabditis elegans. Approximately 10% of the cells generated during development of this worm will undergo apoptotic or programmed cell death. Pathogenic variants leading to smaller or larger numbers of surviving cells result in smaller or larger nematodes, respectively. The molecular characterization of these pathogenic variants has led to identification of a number of “death” genes and of genes that prevent apoptosis. The mammalian counterparts of the nematode death genes encode enzymes that are cysteine aspartate-specific proteases, also known as caspases. At least one function of such enzymes is that of an interleukin-converting enzyme common in the inflammatory response of the body. These enzymes have been shown to promote neuronal death; in addition, when an animal is produced without caspase 3, a larger than normal brain results.

At the most rostral end of the neural tube in the 40-day-old fetus, the first mature neuron arrives at the developing cortical surface. These first neurons are the Cajal-Retzius cells, which are the major neurons of the cortex by day 43. Cajal-Retzius cells, along with corticopetal nerve fibers, form a so-called preplate. These cells will be the major cell type of the most superficial layer of the cerebral cortex, layer I. At the same time that the Cajal-Retzius cells are arriving at the most superficial layer of cortex, other pioneering neurons differentiate and form a so-called subplate. The pioneering neurons of the subplate and the preplate act as the police officers of the developing nervous system and define the limits of the developing cortical plate, which will become the six-layered cortex of the adult brain. Most of the cells of the subplate will die postnatally in a programmed cell death (see earlier discussion of apoptosis).

Near the end of the proliferative phase of neurodevelopment, billions of postmitotic neurons are poised to begin the trip to the cortical surface and to form the cortical plate. This tremendous number of neurons accomplishes this task, for the most part, by attaching to and migrating along radial glia (radially spanning from ventricle to pial surface) in a process known as radial migration (Fig. 133.4). In the process of migration, the deepest layer of the cortical plate forms before the other layers. Therefore, the first neurons to arrive at the future cortical plate are layer VI neurons. More superficial layers of cortex then are formed such that the neurons of layer V migrate and pass the neurons of layer VI; the same process occurs for layers IV, III, and II. The cortex therefore is formed in an inside-out fashion (see Fig. 133.4).

The molecules and the interactions between neurons and glia are extremely important in this process of neuronal migration. Reelin, the protein involved in the migration mouse mutant, reeler, appears to be one such important molecule. Reelin appears to promote an attachment of neurons to glia and, when abnormal, as in the mouse mutant reeler, it leads to an inverted cortex such that the most superficial neurons are the first to arrive and the later-arriving neurons are deeper. This mouse mutant cortex appears to be due to an abnormally adhesive interaction between neurons and glia. Additional molecules that appear to act in an adhesive manner are laminin, astrotactin, L1 antigen, fibronectin, neural cell adhesion molecules (NCAM), and adhesion molecule on glia (AMOG).

Neuroblast movement on radial glia involves an extension of a leading process, a neural outgrowth having an orderly arrangement of microtubules. Microtubules are cytoskeletal elements with a polymerizing (positive) end and a depolymerizing (negative) end. They serve as the major structural element that gives shape to long neural processes. When microtubules depolymerize, or slide, long neural processes are shortened. Shortening of the leading process of migrating neurons has been associated with forward movement of the soma of migrating neurons in vitro. Cytoskeletal changes in leading processes have been purported to be responsible for this shortening and somal movement.

A possible mode of movement in neuronal migration on glia would be the attachment of the neuron to a matrix secreted by either the glia or the neurons. This matrix is likely to consist of the aforementioned adhesion molecules. The attachment of the neuron would be through integrin receptors, cytoskeletal-linking membrane-bound recognition sites for adhesion molecules. That attachment serves as a stronghold for the leading process and soma of the migrating neuron. Shortening of the leading process owing to depolymerization or shifts of microtubules results in movement of the soma relative to the attachment points. This theory of movement of neurons also must include a phase of detachment from the matrix at certain integrin receptors so that the neuroblast can navigate successfully along as much as 6 cm of developing cortex (the maximum estimated distance of radial migration of a neuroblast in humans). Finally, the movement of cells must stop at the appropriate location, the boundary between layer I and the forming cortical plate. Therefore, some stop signal must be given in order for the migrating neuroblast to detach from the radial glia and begin to differentiate into a cortical neuron. This is obviously deficient in such disorders as cobblestone lissencephalies (discussed in the following section).

Other forms of neuronal migration occur in brain development. Some evidence exists for a tangential migration of neurons in the cortex and in the early granule cell migration in the cerebellum. A so-called chain migration of neurons on other neurons occurs in the formation of the olfactory bulbs. In this chain migration, neuroblasts from the subventricular zone of the lateral ventricle
migrate to the olfactory bulb through a sheath of glial cells, but the actual migration occurs on other neurons.

Many clinical entities are associated with neuronal migration disorders. In some, abnormalities are limited to the nervous system but, in others, malformations involving other organs also are present. The responsible genetic mechanism has been identified in some of these disorders, and new genetic mechanisms are being identified regularly. Although the role of the gene product in producing many of these migration disorders may not yet be entirely clear, the disorders provide important clues to mechanisms that are responsible for normal brain development.

Modern neuroimaging techniques, particularly MRI, have allowed the recognition of major migration disorders. Some of these disorders are associated with typical clinical features that might alert the clinician to the presence of such abnormalities even before imaging is obtained. In other disorders, the clinical features are so varied that a strong correlation between imaging and clinical presentation does not exist. High-field MRI and other research techniques may further refine the clinician’s ability to refine diagnoses.