Embryological and Fetal Development of the Nervous System

The best way to understand congenital malformations of the nervous system is in the context of embryology, which encompasses not only classical descriptive morphogenesis but also the molecular genetic programming of development. Neuroimaging and electrocerebral maturation, as determined by electroencephalogram (EEG) in preterm infants, contribute other aspects of ontogenesis of normal and abnormal brain formation that are particularly relevant to clinical neurology. Maturation refers to both growth, a measure of physical characteristics over time, and development, the acquisition of metabolic functions, reflexes, sensory awareness, motor skills, language, and intellect. Molecular development, by contrast with molecular biology, refers to the maturation of cellular function by changes in molecular structures such as the phosphorylation of neurofilaments. In neurons, it also includes the development of an energy production system that actively maintains a resting membrane potential, the synthesis of secretory molecules as neurotransmitters, and the formation of membrane receptors. Membrane receptors respond to various transmitters at synapses, to a variety of trophic and adhesion molecules, and during development to substances that attract or repel growing axons in their intermediate and final trajectories. Molecular biology is the basis of linking a DNA sequence to a specific gene and a particular locus on a specific chromosome, and ultimately a correlation with normal function and a particular disease.

Table 60.1 shows known genetic loci and mutations in human central nervous system (CNS) malformations. In most cases, mutations affect the genetic programming of the spatial and temporal sequences of developmental processes. These range from early processes that establish the axes of the neural tube and gradients of genetic expression, to late processes that establish the identity of specific types of neurons, the type of neurotransmitter they synthesize, and the synaptic connections they make. The role of homeobox genes in the differentiation of neural structures is an aspect of development recognized relatively recently. Molecular genetic data are rapidly becoming available because of intense interest in this key to understanding neuroembryology in general and neural induction in particular (Sarnat and Menkes, 2000). Other aspects of current investigative interest include the roles of neurotropic factors, hormones, ion channels, and neurotransmitter systems in fetal brain development. Genetic manipulation in animals has created many genetic models of human cerebral malformations. These contribute greatly to our understanding of human dysgeneses and provide insights into the pathogenesis of epilepsy and other functional results of dysgeneses (Chevassus-au-Louis et al., 1999).

Maturation progresses in a predictable sequence with precise timing. Insults that adversely affect maturation influence events occurring at a particular time. Some are brief (e.g., a single exposure to a toxin), whereas others act over many weeks or throughout gestation (e.g., some congenital infections, diabetes mellitus, and genetic or chromosomal defects). Even brief insults may have profound influences on later development by interfering with processes essential to initiate the next stage of development. Often this makes the timing of an adverse event difficult.

The anatomical and physiological correlates of neurological maturation reflect the growth and development of the individual neuron and its synaptic relations with other neurons. The mature neuron is a secretory cell with an electrically polarized membrane. Though endocrine and exocrine cells are secretory and muscle cells possess excitable membranes, only neurons embrace both functions. The precursors of neurons are neither secretory nor excitable. The cytological maturation of neurons is an aspect of ontogenesis that is as important as is their spatial relations with other cells, both for future function and for the pathogenesis of some functional neurological disorders of infancy such as neonatal seizures. Neuroblasts are postmitotic neuroepithelial cells committed to neuronal lineage. These cells have not yet achieved all functions of mature neurons such as membrane polarity, secretion, and synaptic relations with other neurons, and often they are still migratory. Use of the term blast is different for neural development than for hematopoieses, in which blast cells are still in the mitotic cycle or may even be neoplastic. The events of neural maturation after initial induction and formation of the neural tube are each predictive of specific types of malformation of the brain and of later abnormal neurological function. These are (1) neurulation or formation of the neural tube, (2) mitotic proliferation of neuroblasts, (3) programmed death of excess neuroblasts, (4) neuroblast migration, (5) growth of axons and dendrites, (6) electrical polarity of the cell membrane, (7) synaptogenesis, (8) biosynthesis of neurotransmitters, and (9) myelination of axons.

Malformations of the nervous system are unique. No two individual cases are identical, even when categorized as the same anatomical malformation, such as alobar holoprosencephaly, syndromic or isolated agenesis of the corpus callosum, and types 1 and 2 lissencephaly. Functional expression of anatomically similar cases also may vary widely. For example, two cases of holoprosencephaly with nearly identical imaging findings and similar histological patterns of cortical architecture and subcortical heterotopia at autopsy may differ in that one infant may have epilepsy refractory to pharmacological control, whereas the other may have no clinical seizures at all. The difference may be at the level of synaptic organization and the relative maturation of afferent input and neuronal maturation (Sarnat and Born, 1999; Sarnat et al., 2010a). A discussion of the critical sequence of events in neural maturation follows.

Neurulation

Neurulation refers to the formation and closure of the neural tube. The formation of the neural tube from the neural placode starts with the establishment of the axis in the neural plate. The three early axes—longitudinal, horizontal, and vertical—persist during life and correspond to the basic body plan of all vertebrates (Sarnat and Flores-Sarnat, 2001b). At neurulation, grooving of the neural placode occurs in the anteroposterior axis. Subsequently, closure of the lateral margins of the folding neural placode ensues in the dorsal midline to form the neural tube. To accomplish closure, intercellular filaments interdigit cells of the two sides to form a veil at midline closure points and the neuropores. At this time, the neural crest separates bilaterally at the two fusing lips of the closing neural tube, and its cells migrate along predetermined pathways to form the peripheral nervous system, chromaffin tissue, melanocytes, and various other cells. Neural crest cells terminally differentiate only after reaching their final destination. The inhibitory function of versican, a chondroitin sulfate proteoglycan, is an important factor of the extracellular matrix for neural crest cell migration (Dutt et al., 2006).

The process just described is primary neurulation. Another process, secondary neurulation, occurs in the most caudal regions of the spinal cord and is limited to the lower sacral region, the part of the incipient spinal cord that formed caudal to the posterior neuropore, which is not at the extreme posterior end of the neural placode. During secondary neurulation, rather than the ependyma forming from the dorsal surface of the placode, which then becomes folded, a central canal grows rostrally from the posterior end of the solid cylinder of neural tissue within its core. It may or may not reach the central canal of primary neurulation more rostrally, and often in the midgestational or earlier fetus in particular, a transverse section through the lower sacral spinal cord reveals two ependymal-lined central canals, both in the vertical axis and one above the other. This is a normal condition.

Mitotic Proliferation of Neuroblasts (Neuronogenesis)

After formation of the neural tube, proliferation of neuroepithelial cells in the ventricular zone associated with mitoses at the ventricular surface generates neurons and glial cells. The rate of division is greatest during the early first trimester in the spinal cord and brainstem and during the late first and early second trimester in the forebrain. Within the ventricular zone of the human fetal telencephalon, 33 mitotic cycles provide the total number of neurons required for the mature cerebral cortex. Most mitotic activity in the neuroepithelium occurs at the ventricular surface, and the orientation of the mitotic spindle determines the subsequent immediate fate of the daughter cells. If the cleavage plane is perpendicular to the ventricular surface, the two daughter cells become equal neuroepithelial cells preparing for further mitosis. If, however, the cleavage is parallel to the ventricular surface, the two daughter cells are unequal (asymmetrical cleavage). In that case, the one at the ventricular surface becomes another neuroepithelial cell, whereas the one away from the ventricular surface separates from its ventricular attachment and becomes a postmitotic neuroblast ready to migrate to the cortical plate. Furthermore, the products of two genes that determine cell fate, called numb and notch, are on different sides of the neuroepithelial cell. Therefore, with symmetrical cleavages, both daughter cells receive the same amount of each, but with asymmetrical cleavage, the cells receive unequal ratios of each, which also influences their subsequent development (Mione et al., 1997). The orientation of the mitotic spindle requires centractin.

Active mitoses cease well before the time of birth in most parts of the human nervous system, but a few sites retain a potential for postnatal mitoses of neuroblasts. One recognized site is the periventricular region of the cerebral hemispheres (Kendler and Golden, 1996). Another is the external granular layer of the cerebellar cortex, where occasional mitoses persist until 1 year of age. Postnatal regeneration of these neurons after destruction of most by irradiation or cytotoxic drugs occurs in animals and may occur in humans as well. Primary olfactory receptor neurons also retain a potential for regeneration. In fact, if a constant turnover of these neurons did not occur throughout life, the individual would become anosmic after a few upper respiratory infections, which transiently denude the intranasal epithelium. A population of “stem cells” with mitotic potential in the subventricular zone and hippocampal dentate gyrus is reported (Johansson et al., 1999). These have generated considerable interest because of a potential for regeneration of the damaged adult brain and because they may be induced to mature as neurons (Schuldiner et al., 2001).

Neuronogenesis also involves the biosynthesis of cell-specific proteins. Many of these are detectable in the germinal matrix as evidence of early commitment of cells not only to a neuronal lineage but also to a fate as a specific type of neuron. The previously held concept that germinal matrix cells were uniformly undifferentiated postmitotic neuroepithelial cells was incorrect.

Disorders of Neuronogenesis

Destructive processes may destroy so many neuroblasts that regeneration of the full complement of cells is impossible. This happens when the insult persists for a long time or is repetitive, destroying each subsequent generation of dividing cells. Inadequate mitotic proliferation of neuroblasts results in hypoplasia of the brain (Fig. 60.1). Such brains are small and grossly malformed, either because of a direct effect on neuroblast migration or by destruction of the glial cells with radial processes that guide migrating nerve cells. The entire brain may be affected, or portions may be selectively involved. Cerebellar hypoplasia often is a selective interference with proliferation of the external granular layer. In some cases, cerebral hypoplasia and microcephaly are the result of precocious development of the ependyma before all mitotic cycles of the neuroepithelium are complete, because ependymal differentiation arrests mitotic activity at the ventricular surface. The mutation of a gene that programs neuronogenesis may be another explanation for generating insufficient neuroepithelial cells, although this pathogenesis remains hypothetical in humans.

Fig. 60.1 Severe cerebral hypoplasia. The brain of this full-term neonate weighed only 12.6 g (normal mean is 350 g), although the cranium was closed and mainly filled with fluid. The dysplastic architecture of the telencephalon, including dysplastic cerebellar tissue, extended into a frontal encephalocele (e) and was not that of a neural tube defect or fetal infarction. The spinal cord (sp) is well formed except for the absence of descending tracts. The cerebellum (c) is small but normally laminated. This brain probably represents lack of neuronal proliferation. Note the well-formed fossae at the base of the skull, despite the absence of cerebral development.

(Reproduced with permission from Sarnat, H.B., de Mello, D.E., Blair, J.D., et al., 1982. Heterotopic growth of dysplastic cerebellum in frontal encephalocele in an infant of a diabetic mother. Can J Neurol Sci. 9, 31-35.)

Programmed Cell Death (Apoptosis)

Normal mitotic proliferation produces excessive neuroblasts in every part of the nervous system. Reduction of this abundance by 30% to 50% is by a programmed process of cell death, or apoptosis, until achieving the definitive number of immature neurons. The factors that arrest the process of apoptosis in the fetus are multiple and are in part genetically determined. Cells that do not match with targets are more vulnerable to degeneration than those that achieve synaptic contact with other cells. Endocrine hormones and neuropeptides modulate apoptosis. Some homeotic genes such as c-fos are important in the regulation of apoptosis in the nervous system, and other suppressor genes stop the expression of apoptotic genes.

Two phases of apoptosis are distinguished. One involves as-yet undifferentiated neuroepithelial cells or neuroblasts with incomplete differentiation; the other phase involves fully differentiated neurons of the fetal brain. The first phase begins during embryonic life and may extend to midgestation in some parts of the brain (e.g., periventricular telencephalic neuroepithelium) until ependyma differentiates at the ventricular surface. The second phase may be ongoing throughout life, as occurs in primary olfactory neurons and probably in the hippocampus, closely associated with a reservoir of stem cell progenitors.

Neuroblast Migration

No neurons of the mature human brain occupy their site of generation from the neuroepithelium. They migrate to their mature site to establish the proper synaptic connections with appropriate neighboring neurons and send their axons in short or long trajectories to targets. The subependymal germinal matrix (Fig. 60.2) is the subventricular zone of the embryonic concentric layers and consists of postmitotic premigratory neuroblasts and glioblasts. In general, the movement of maturing nerve cells is centrifugal, radiating toward the surface of the brain. The cerebellar cortex is exceptional in that external granule cells first spread over the surface of the cerebellum and then migrate into the folia. Migration of neuroblasts begins at about 6 weeks’ gestation in the human cerebrum and is not completed until at least 34 weeks of fetal life, although the majority of germinal matrix cells after midgestation are glioblasts. Glioblasts continue to migrate until early in the postnatal period. Within the brainstem, neuroblast migration is complete by 2 months’ gestation. Cerebellar external granule cells continue migrating throughout the first year of life.

Fig. 60.2 Coronal section of forebrain of 16-week normal fetus, showing extensive subependymal germinal matrix (g) of neuroblasts and glial precursors that have not yet migrated. The surface of the brain is just beginning to develop sulci (arrowheads). Migrating neuroblasts (m) are seen in the subcortical white matter. The corpus callosum (cc) is artifactually ruptured, and the two hemispheres should be closely approximated. (Hematoxylin-eosin stain.) cn, Caudate nucleus; ic, anterior limb of internal capsule.

Neuroblast migration permits a three-dimensional spatial relationship to develop between neurons, which facilitates the formation of complex synaptic circuits. The timing and sequence of successive waves of migrating neuroblasts are precise. In the cerebral cortex, immature nerve cells reach the pial surface and then form deeper layers as more recent arrivals replace their position at the surface. Neurons forming the most superficial layers of neocortex are thus the last to have migrated, although in the three-layered hippocampus, the most superficial neurons represent the earliest migratory wave. Three major groups of molecules control neuroblast migration (Gressens, 2006): (1) molecules of the cytoskeleton that determine the initiation (filamin-A and ADP-ribosylation factor GEF2) and ongoing progression (doublecortin and LIS1) of neuroblast movement; (2) signaling molecules involved in lamination, including reelin and other proteins not yet associated with human diseases; and (3) molecules modulating glycosylation that provide stop signals to migrating neuroblasts (e.g., POMT1 [protein O-mannosyl-transferase], involved in Walker-Warburg syndrome; POMGnT1 [protein O-mannose β-1,2-N-acetylglucosaminyltransferase], involved in muscle-eye-brain disease; and fukutin, involved in Fukuyama muscular dystrophy).

The laminated arrangement of the mammalian cerebral cortex requires a large cortical surface area to accommodate increasing numbers of migrating neuroblasts and glioblasts. Initially the cortical plate shows no histological layering, a process beginning at about midgestation, but rather has an immature columnar architecture. The lamination is superimposed upon this columnar pattern, but columnar architecture is still seen postnatally, particularly at the crowns of gyri and the depths of sulci. Even before histological lamination is evident, RNA probes for specific neuronal identities can already detect future organization of the cortical plate (Hevner, 2007). Convolutions provide this large surface area without incurring a concomitant increase in cerebral volume. The formation of gyri and sulci is thus a direct result of migration (Fig. 60.3). Most gyri form in the second half of gestation, which is a period of predominant gliogenesis and glial cell migration. Therefore, the proliferation of glia in the cortex and subcortical white matter may be more important than neuroblast migrations in the formation of convolutions, but the growth of dendrites and synaptogenesis also may influence gyration by contributing mass to the neuropil.

Fig. 60.3 Lateral (A) and ventral (B) views of a normal brain of a 16-week fetus. Primary sulci (e.g., sylvian fissure, calcarine fissure, central sulcus) are forming, but secondary and tertiary sulci and gyri are not yet developed, and the surface of the brain is smooth.

Major Mechanisms of Neuroblast Migration: Radial Glial Fiber Guides and Tangential Migration Along Axons

The majority of neuroblasts arriving at the cortical plate do so by means of radial glial guides from the subventricular zone. A second route, tangential migration, uses axons as the guides for the migratory neuroblasts. The genetically determined programming of neuroblast migration begins when cells are still undifferentiated neuroepithelial cells and even before all their mitotic cycles are complete. Neuroepithelial cells express the gene products of the lissencephaly gene (LIS1), as do ependymal cells and Cajal-Retzius cells of the molecular layer of cerebral cortex. The expression of this gene is defective in type 1 lissencephaly (Miller-Dieker syndrome), a severe disorder of neuroblast migration (Clark et al., 1997). An understanding of its function in migration is incomplete. The guidance of most neurons of the forebrain to their predetermined site from the germinal matrix (embryonic subventricular zone) is by long radiating fibers of specialized fetal astrocytes (Fig. 60.4). The elongated processes of these glial cells span the entire wall of the fetal cerebral hemisphere; their cell bodies are in the periventricular region, and their terminal end-feet are on the limiting pial membrane at the surface of the brain (see Fig. 60.4). Radial glial cells are the first astroglial cells of the human nervous system converted into a mature fibrillary astrocyte of the subcortical white matter; some are still present at birth. Mature astrocytes are present throughout the CNS by 15 weeks’ gestation, and gliogenesis continues throughout fetal and postnatal life. Several types of glial cells are recognizable between 20 and 36 weeks’ gestation.

Fig. 60.4 Radial glial fibers extending from subependymal region (right) toward cerebral cortex (left), guiding migrating neuroblasts in a 16-week fetus. (Glial fibrillary acidic protein reaction. Bar = 10 µm.)

Facilitating the mechanical process of neuroblasts gliding along a radial glial fiber are several specialized proteins at the radial glial fiber surface membrane or extracellular space. An example is astrotactin, secreted by the neuroblast (Zheng et al., 1996). Glial cells and neural cell adhesion molecules also facilitate gliding (Jouet and Kenwrick, 1995). Fetal ependymal cells have radiating processes that resemble those of the radial glial cell but do not extend beyond the germinal matrix and secrete molecules in the extracellular matrix. Some adhesion molecules are present in the extracellular matrix (Thomas et al., 1996). These molecules serve as lubricants, as adhesion molecules between the membranes of the neuroblast and the radial glial fiber, and as nutritive and growth factors. They stimulate cell movement by a mechanism still poorly understood. Deficient molecules lead to defective migration. For example, the abnormality of the L1 adhesion molecule is the defective genetic program in X-linked hydrocephalus accompanied by polymicrogyria and pachygyria.

The process of transformation of radial glial cells into astrocytes and ependymal cells begins during the first half of gestation and completes postnatally. During midgestation when neuronal migration is at a peak, many radial glial cells remain attached to the ventricular and pial surfaces, increasing in length and curving with the expansion and convolution of the cerebral wall. From 28 weeks’ gestation to 6 years of age, astrocytes of the frontal lobe shift from the periventricular to the subcortical region. The centrifugal movement of this band of normal gliosis marks the end of neuronal migration in the cerebral mantle. Ependyma does not completely line the lateral ventricles until 22 weeks’ gestation.

Radial glial cells also act as resident “stem cells” in the fetal brain. In the presence of fetal brain injury, such as a cortical microinfarct, radial glia are capable of differentiating as neurons to replace neurons that were lost. Radial glia express nestin and other primitive proteins found only in cells of multipotential lineage or that participate in early developmental processes, such as floor-plate ependymal cells.

In addition to the radial migration to the cerebral cortex, tangential migration also occurs, but the number of neuroblasts is far smaller (Rakic, 1995; Takano et al., 2004). These migrations perpendicular to the radial fibers probably use axons rather than glial processes as guides for migratory neuroblasts. This explains why not all cells in a given region of cortex are from the same clone or vertical column. Most of the tangentially migrating neuroblasts in the cerebral cortical plate are generated in the fetal ganglionic eminence, a deep telencephalic structure of the germinal matrix that gives origin to basal ganglionic neurons and to the γ-aminobutyric acid (GABA)ergic inhibitory interneurons of the cerebral cortex. These neurons in the cortex from tangential migration have some unique metabolic features such as calretinin synthesis (Takano et al., 2004; Ulfig, 2002). Calretinin-reactive inhibitory interneurons in the cerebral cortex comprise about 12% of total neurons and are a subset of total neurons arriving at the cortical plate by tangential migration, which represent about 20% of total cortical neurons.

Tangential migrations occur in the brainstem and olfactory bulb as well as in the cerebrum. The subpial region is another site of neuroblast migration that does not use radial glial cells. Calretinin-reactive neurons are in the cerebellum as well as the cerebral cortex (Yew et al., 1997), particularly Purkinje cells, basket cells, and neurons of the dentate and inferior olivary nuclei of the cerebellar system, but not those of the pontine nuclei, which similarly originated in the rhombic lip of His.

Disorders of Neuroblast Migration

Nearly all malformations of the brain are a direct result of faulty neuroblast migration or at least involve a secondary impairment of migration. Imperfect cortical lamination, abnormal gyral development, subcortical heterotopia, and other focal dysplasias relate to some factor that interferes with neuronal migration, whether vascular, traumatic, metabolic, or infectious. The most severe migratory defects occur in early gestation (8 to 15 weeks), often associated with even earlier events in the gross formation of the neural tube and cerebral vesicles. Heterotopia of brainstem nuclei also occurs. Later defects of migration are expressed as disorders of cortical lamination or gyration such as lissencephaly, pachygyria, and cerebellar dysplasias. Insults during the third trimester cause subtle or focal abnormalities of cerebral architecture that may express in infancy or childhood as epilepsy.

Most disturbances of neuroblast migration involve arrested migration before the journey is complete. These disorders are divisible into three anatomical phases, depending on where the migratory arrest occurred. An example of neuroblasts never having begun migration from the periventricular region is periventricular nodular heterotopia, an X-linked genetic disorder due to defective expression of the gene, filamin-A (FLNA). Subcortical laminar heterotopia results when neuroblasts begin migration but arrest in the subcortical white matter before reaching the cortical plate. This is another X-linked recessive trait but is due to a different gene called doublecortin (DCX). The term double cortex is sometimes used, but this name is incorrect because unlike a true cortex, the subcortical heterotopia lacks lamination. If the neuroblasts reach the cortical plate but lack correct lamination, accompanying this abnormal architecture of the cortical plate are abnormalities of gyration such as lissencephaly or pachygyria. Several different genes, including LIS1 and reelin (RLN), are important in cortical plate organization (Curran and D’Arcangelo, 1998) and mutated in malformations of the terminal phase of neuroblast migration.

Lissencephaly is a condition of a smooth cerebral cortex without convolutions. Normally at midgestation, the brain is essentially smooth; the interhemispheric, sylvian, and calcarine fissures are the only ones formed. Gyri and sulci develop between 20 and 36 weeks’ gestation, and the mature pattern of gyration is evident at term, although some parts of the cerebral cortex (e.g., frontal lobes) are still relatively small. In lissencephaly type 1 (Miller-Dieker syndrome), the cerebral cortex remains smooth. Lesser degrees of this gross morphological defect exist, with a few excessively wide gyri (pachygyria) or multiple excessively small gyri (polymicrogyria). The histopathological pattern is that of a 4-layer cortex in which the outermost layer (1) is the molecular layer, as in normal 6-layered neocortex. Layer 2 corresponds to layers 2 through 6 of normal neocortex, layer 3 is cell-sparse as a persistent fetal subplate zone, and layer 4 consists of incompletely migrated neurons in the subcortical intermediate zone. In lissencephaly type 2 (Walker-Warburg syndrome), poorly laminated cortex with disorganized and disoriented neurons is seen histologically, and the gross appearance of the cerebrum is one of a smooth brain or a few poorly formed sulci (Fig. 60.5). The term cobblestone refers to the aspect of the surface with multiple shallow furrows not corresponding to normal sulci. The cerebral mantle may be thin, suggesting a disturbance of cell proliferation as well as of neuroblast migration. Malformations of the brainstem and cerebellum often are present as well (see Fig. 60.5). Lissencephaly type 1 and type 2 (Walker-Warburg syndrome, Fukuyama muscular dystrophy, muscle-eye-brain disease of Santavuori) are genetic diseases. Lissencephaly also results from nongenetic disturbances of neuroepithelial proliferation or neuroblast migration, including destructive encephaloclastic processes such as congenital infections during fetal life. More recently it has been recognized that the lissencephalies, including those resulting from mutations in LIS1, DCX, and ARX genes, are disturbances not only in radial migration, but also involve tangentially migrating neuroblasts (Marcorelles et al., 2010).

Fig. 60.5 Sagittal T1-weighted magnetic resonance image of a 10-month-old girl with lissencephaly type 2 and Dandy-Walker malformation. The cerebral mantle is thin, and the lateral ventricles are greatly enlarged. A few abnormal shallow fissures at the cerebral surface may indicate abortive gyration or pachygyria. The cerebellum is severely hypoplastic (arrow indicates anterior vermis), and the posterior fossa contains a large fluid-filled cyst. The brainstem also is hypoplastic, and the basis pontis is nearly absent. A differential diagnosis of this image is pontocerebellar hypoplasia, but the high position of the torcula indicates a Dandy-Walker malformation.

Other abnormal patterns of gross gyration of the cerebral cortex occurs secondary to neuroblast migratory disorders. Pachygyria signifies abnormally large, poorly formed gyri and may be present in some regions of cerebral cortex, with lissencephaly in other regions. Polymicrogyria refers to excessively numerous and abnormally small gyri that similarly may coexist with pachygyria; it does not necessarily denote a primary migratory disorder of genetic origin. Small, poorly formed gyri may occur in zones of fetal ischemia, and they regularly surround porencephalic cysts due to middle cerebral artery occlusion in fetal life.

In the cerebral hemisphere, most germinal matrix cells become neurons during the first half of gestation, and most form glia during the second half of gestation. Nonetheless, a small number of germinal matrix cells are neuronal precursors, migrating into the cerebral cortex in late gestation. Because the migration of the external granular layer in the cerebellar cortex is incomplete until 1 year of age, a potential for acquired insults to interfere with late migrations persists throughout the perinatal period. Anatomical lesions such as periventricular leukomalacia, intracerebral hemorrhages and abscesses, hydrocephalus, and traumatic injuries may disrupt the delicate radial glial guide fibers and prevent normal migration even though the migrating cell itself may escape the focal destructive lesion. Damaged radial glial cells tend to retract their processes from the pial surface. The migrating neuron travels only as far as its retracted glial fibers carry it. If this fiber retracts into the subcortical white matter, the neuroblast stops there and matures, becoming an isolated heterotopic nodule composed of several nerve cells that were migrating at the same time in the same place. In these nodules, neurons of various cortical types differentiate without laminar organization and with haphazard orientations of their processes, but a few extrinsic axons may prevent total synaptic isolation of the nodule. Interference with the glial guide fibers in the cerebral cortex itself results in neurons either not reaching the pial surface or not being able to reverse direction and then descending to a deeper layer. The consequence is imperfect cortical lamination, which interferes with the development of synaptic circuits. These disturbances of late neuroblast migration do not produce the gross malformations of early gestation and may be undetectable by imaging techniques. They may account for many neurological sequelae after the perinatal period, including seizures, perceptual disorders, impairment of gross or fine motor function, learning disabilities, and mental retardation.

In sum, either defective genetic programming or acquired lesions in the fetal brain that destroy or interrupt radial glial fibers may cause disorders of neuroblast migration. Cells may not migrate at all and become mature neurons in the periventricular region, as occurs in X-linked periventricular nodular heterotopia (Eksioglu et al., 1996) and in some cases of congenital cytomegalovirus infection. Cells may become arrested along their course as heterotopic neurons in deep subcortical white matter, as occurs in many genetic syndromes of lissencephaly-pachygyria and in many metabolic diseases including cerebrohepatorenal (Zellweger) syndrome and many aminoacidurias and organic acidurias. The same aberration may occur in acquired insults to the radial glial cell during ontogenesis. Cells may overmigrate beyond the limits of the pial membrane into the meninges as ectopic neurons, either singly or in clusters known as marginal glioneuronal heterotopia, or brain warts. Rarely, herniation of the germinal matrix into the lateral ventricle may occur through gaps in the ependyma; those cells mature as neurons, forming a non-neoplastic intraventricular mass that may or may not obstruct cerebrospinal fluid (CSF) flow. Whether disoriented radial glial fibers actually guide neuroblasts to an intraventricular site or neuroblasts are physically pushed in a direction of less resistance is uncertain.

Fissures and Sulci of Cortical Structures

Fissures and sulci are grooves that form in laminated cortices, principally cerebral and cerebellar. Such folding accomplishes a need for an enlarging surface area without a concomitant increase in tissue volume as development proceeds. Without gyration of the cerebral cortex and foliation of the cerebellar cortex, the brain would be so large and voluminous at birth that neither the neonate nor the mother would survive delivery. Fissures and sulci both result from mechanical forces during fetal growth, but they differ in that fissures form from external forces and sulci form from internal forces imposed by the increased volume of neuronal cytoplasm and the formation of neuropil, the processes of neurons and glial cells (Sarnat and Flores-Sarnat 2010a). The ventricular system acts as another external force, surrounded by but outside of the brain parenchyma. Whereas fissures generally form earlier and often are deeper than sulci, these are not the most important differences. Box 60.1 lists the various fissures of the brain, and Fig. 60.6 is a drawing of the development of the human telencephalic flexure, which becomes, after closure of the operculum, the Sylvian fissure. It should be noted that the ventral bending of the primitive oval-shaped telencephalic hemisphere results in the original posterior pole becoming the temporal—not the occipital—lobe, and that the lateral ventricle bends with the brain. The occipital horn of the lateral ventricle is a more recent diverticulum of the original simple ventricle and as such remains the most variable part of the ventricular system, symmetrical in only 25% of normal individuals. Cerebellar folia are the equivalent of cerebral cortical gyri. A temporally and spatially precise sequence of the development of fissures, sulci, and cerebellar folia is genetically programmed and enables the neuroradiologist and neuropathologist to also assess maturational delay of this aspect of ontogenesis. The gestational age of a premature infant may be determined to within a 2-week period or less from the convolutional pattern of the brain.

Fig. 60.6 The telencephalic flexure that forms the Sylvian fissure.

Growth of Axons and Dendrites

During the course of neuroblast migration, neurons remain largely undifferentiated cells, and the embryonic cerebral cortex at midgestation consists of vertical columns of tightly packed cells between radial blood vessels and extensive extracellular spaces. Cytodifferentiation begins with a proliferation of organelles, mainly endoplasmic reticulum and mitochondria in the cytoplasm, and clumping of condensed nuclear chromatin at the inner margin of the nuclear membrane. Rough endoplasmic reticulum becomes swollen, and ribosomes proliferate.

The outgrowth of the axon always precedes the development of dendrites, and the axon forms connections before the differentiation of dendrites begins. Ramón y Cajal first noted the projection of the axon toward its destination and named this growing process the cone d’accroissement (growth cone). The tropic factors that guide the growth cone to its specific terminal synapse, whether chemical, endocrine, or electrotaxic, have been a focus of controversy for many years. However, we now know that diffusible molecules secreted along their pathway by the processes of fetal ependymal cells and perhaps some glial cells guide growth cones during their long trajectories. Some molecules (e.g., brain-derived neurotropic growth factor, netrin, S-100β protein) attract growing axons, whereas others (e.g., the glycosaminoglycan, keratan sulfate—not to be confused with the protein, keratin) strongly repel them and thus prevent aberrant decussations and other deviations. Matrix proteins such as laminin and fibronectin also provide a substrate for axonal guidance. Cell-to-cell attractions operate as the axon approaches its final target. Despite the long delay between the migration of an immature nerve cell and the beginning of dendritic growth, the branching of dendrites eventually accounts for more than 90% of the synaptic surface of the mature neuron. The pattern of dendritic ramification is specific for each type of neuron. Spines form on the dendrites as short protrusions with expanded tips, providing sites of synaptic membrane differentiation. The Golgi method of impregnation of neurons and their processes with heavy metals such as silver or mercury, used for more than a century, continues to be one of the most useful methods for demonstrating dendritic arborizations. Among the many contributions of this technique to the study of the nervous system, beginning with the elegant pioneering work of Ramón y Cajal, none has surpassed its demonstration of the sequence of normal dendritic branching in the human fetus. Newer immunocytochemical techniques for demonstrating dendrites also are now available, such as microtubule-associated protein 2. These techniques are applicable to human tissue resected surgically, as in the surgical treatment of epilepsy, and to the tissue secured at autopsy.

Electrical Polarity of the Cell Membrane

The development of membrane excitability is one of the important markers of neuronal maturation, but knowledge is incomplete about the exact timing and duration of this development. Membrane polarity establishes before synaptogenesis and before the synthesis of neurotransmitters begins. Because the maintenance of a resting membrane potential requires considerable energy expenditure to fuel the sodium-potassium pump, the undifferentiated neuroblast would be incapable of maintaining such a dynamic condition as a resting membrane potential. The development of ion channels within the neural membrane is another important factor in the maturation of excitable membranes and the maintenance of resting membrane potentials.

Synaptogenesis

Synapse formation follows the development of dendritic spines and polarization of the cell membrane. The relation of synaptogenesis to neuroblast migration differs in different parts of the nervous system. In the cerebral cortex, synaptogenesis always follows neuroblast migration. In the cerebellar cortex, however, the external granule cells develop axonal processes that become the long parallel fibers of the molecular layer and make synaptic contact with Purkinje cell dendrites before migrating through the molecular and Purkinje cell layer to their mature internal position within the folium. Synaptophysin immunoreactivity with thermal intensification has proved to be a useful marker for studying normal and abnormal synaptogenesis in the fetus and newborn (Sarnat et al., 2010).

Afferent nerve fibers reach the neocortex early, before lamination occurs in the cortical plate. The first synapses are axodendritic and occur both external to and beneath the cortical plate in the future layers I and VI, which contain the first neurons that have migrated.

An excessive number of synapses form on each neuron, with subsequent elimination of those that are not required. Outside the CNS, muscle fibers also begin their relation with the nervous system by receiving multiple sources of innervation from multiple motor neurons, later retaining only one. Transitory synapses also form at sites on neurons where they no longer exist in the mature condition. The spinal motor neurons of newborn kittens display prominent synapses on their initial axonal segment, where they never occur in adult cats. Somatic spines are an important synaptic site on the embryonic Purkinje cell, but these spines and their synapses disappear as the dendritic tree develops. A structure/function correlation is possible in the developing visual cortex. In preterm infants of 24 to 25 weeks’ gestation, the visual evoked potentials (VEPs) recorded at the occiput exhibit initial long-latency negativity, but by 28 weeks’ gestation, a small positive wave precedes this negativity. The change in this initial component of the VEP corresponds to dendritic arborization and the formation of dendritic spines that occurs at that time.

The EEG of the premature infant follows a predictable and time-linked progression in maturation. The EEG reflects synaptogenesis more closely than any other feature of cerebral maturation and thereby provides a noninvasive and clinically useful measure of neurological maturation in the preterm infant. Fetal EEG may even detect neurological disease and seizures in utero.

Biosynthesis of Neurotransmitters

The basis for synthesis of neurotransmitters and neuromodulating chemicals is the secretory character of the neuron, without which, synaptic transmission is impossible. Several types of substances serve as transmitters: (1) acetylcholine (ACh); (2) monoamines including dopamine, norepinephrine, epinephrine, and serotonin; (3) neuropeptides including substance P, somatostatin, and opioid-containing peptide chains such as the enkephalins; and (4) simple amino acids including glutamic acid, aspartic acid, GABA, and glycine. Some transmitters are characteristically inhibitory (e.g., glycine, GABA, and ACh in the CNS). Each neuronal type produces a characteristic transmitter—motor neurons produce ACh, cerebellar Purkinje cells produce GABA, and granule cells produce glutamic acid in the adult. Neuropeptides may coexist with other types of transmitters in some neurons.

In some parts of the brain, transitory fetal transmitters may appear during development and then disappear. Substance P and somatostatin are present in the fetal cerebellum at midgestation, but these neuropeptides are never in the mature cerebellum. In the cerebral cortex of the frontal lobe, the pattern of laminar distribution of cholinergic muscarinic receptors in the mature brain is the inverse of that in the fetus. The functions of these transitory transmitter systems are unknown. Some serve as tropic molecules rather than transmitters in early development. Even amino acid transmitters such as GABA may serve mainly a tropic function at an early stage in development. In situ hybridization and immunocytochemical techniques demonstrate neurotransmitters in neurons of the developing brain of experimental animals and may be applicable to human tissue under some circumstances (Dupuy and Houser, 1997). The ontogeny of neurotransmitter systems depends not only on the mechanisms of synthesis of chemical transmitters but also on the development of highly specific receptors of these chemical signals and their ability to modify excitability of neuronal membranes and trigger action potentials after the recognition of specific molecules (Rho and Storey, 2001; Simeone et al., 2003).

Myelination

Myelin insulates individual axons and provides greatly increased speed of conduction. It is not essential in all nerves, and many autonomic fibers of the peripheral nervous system remain unmyelinated throughout life. Conduction velocity in central pathways is important in coordinating time-related impulses from different centers that converge on a distant target and in ensuring that action potentials are not lost by synaptic block. The basis of nervous system functions is the temporal summation of impulses to relay messages across synapses.

Myelination of pathways in the CNS occurs in a predictable spatial and temporal sequence. Some tracts myelinate as early as 14 weeks’ gestation and complete their myelination cycle in a few weeks. Examples include the spinal roots, medial longitudinal fasciculus, dorsal columns of the spinal cord, and most cranial nerves. Between 22 and 24 weeks’ gestation, myelination progresses in the olivary and cerebellar connections, the ansa lenticularis of the globus pallidus, the sensory trigeminal nerve, the auditory pathways, and the acoustic nerve, as well as the trapezoid body, lateral lemniscus, and brachium of the inferior colliculus. By contrast, the optic nerve and the geniculocalcarine tract (i.e., optic radiations) do not begin to acquire myelin until near term. Some pathways are late in myelinating and have myelination cycles measured in years. The corpus callosum begins myelinating at 4 months postnatally and is not complete until mid-adolescence. Some ipsilateral association fibers connecting the frontal with the temporal and parietal lobes do not achieve full myelination until about 32 years of age.

Myelination can now be accurately measured in specific central pathways by using T2-weighted MRI sequences, but the time at which myelination can be detected is somewhat later than with traditional myelin stains of brain tissue sections, such as Luxol fast blue. Newer neuropathological methods using gallocyanin and immunoreactivity to myelin basic protein may detect myelination even earlier than the traditional stains. Electron microscopy remains the most sensitive method of demonstrating the earliest myelination in tissue sections.

Cajal-Retzius Neurons of the Fetal Brain

Cajal-Retzius cells are large, mature, stellate neurons in the marginal (outermost) zone of the fetal cerebral cortex. They are the first cells to appear at the surface of the embryonic cerebrum, preceding the first wave of radial migration from the subventricular zone and forming a plexus in the marginal (later the molecular) zone. They migrate to the surface either from the ganglionic eminence or from the midbrain neuromere (Sarnat and Flores-Sarnat, 2002). The first afferent processes to enter the marginal layer are dendrites of pyramidal cells of layer VI; synapses between Cajal-Retzius and pyramidal neurons of layer VI form the first intrinsic cortical circuits (Marín-Padilla, 1998). They eventually have synaptic contacts with cortical neurons in all layers.

Cajal-Retzius cells contain acetylcholinesterase and oxidative enzymes and secrete GABA and probably ACh as neurotransmitters. Their long axons extend parallel to the surface of the brain, plunging short branches into layer II (Fig. 60.7). Cajal-Retzius neurons are sparse by term but persist even in the adult, though their function after maturity is uncertain. They strongly express the transcription product of the LIS1 gene, which is defective in X-linked hydrocephalus associated with polymicrogyria and defective neuroblast migration. Cajal-Retzius neurons also strongly express RLN, another gene essential for radial neuroblast migration (Clark et al., 1997; Sarnat and Flores-Sarnat, 2002). This is the only specific disease involving Cajal-Retzius neurons.

Fig. 60.7 Silver stain of molecular layer of motor cortex in a 20-week fetus. The long fibers (arrowheads) extending parallel to the surface of the brain are axons of Cajal-Retzius neurons. (Bielschowsky stain. Bar = 10 µm.)

The fetal cerebral cortex has a subpial or external granular layer that histologically resembles that of the cerebellum but is of quite a different character. Cells of the cerebral cortex rise in columns from the germinal matrix of the hippocampus to form a thin layer on the surface of the archicortex at 12 weeks’ gestation. They rapidly spread over the neocortex in a predictable sequence to cover the entire convexity by the 16th to 18th week, with the layer reaching the greatest thickness by 22 weeks’ gestation. Subsequent involution of the external granular layer results from migration of these cells into the cerebral cortex, where they can no longer be distinguished. Only remnants of this once prominent layer persist at term, confined to the inferior temporal and orbital surfaces. These surfaces are the last sites from which they finally disappear from the neocortex, although a few may persist over the paleocortex even into adult life. Their fate within the cerebral cortex is unknown, but speculation is that they mature into glial cells, because they lack ultrastructural features of neurons, and they stain immunocytochemically for glial fibrillary acidic protein but not for vimentin. The subpial granular layer of the cerebral hemispheres is partially or totally absent in most cases of holoprosencephaly, even at the gestational period when it is normally most prominent; this absence may contribute to the marginal glioneural heterotopia found in the meningeal spaces and superficial cortical layers. The layer of the subpial granule cells may serve as a barrier to reverse the direction of migration in neuroblasts reaching the surface. In the Fukuyama type of congenital muscular dystrophy associated with cerebral cortical dysplasia, a heterotopic layer of stellate glial cells forms at the surface of the cerebral cortex, into which migrating neurons accumulate as they reach the surface, rather than reversing direction and entering deeper layers of the cortex.

Suprasegmental Influences on Muscle Maturation

The motor unit is capable of developing normally in the absence of suprasegmental modification, such as occurs in infants with severe hypoplasia of the brain (see Fig. 60.1). Malformations of the brainstem and cerebellar hypoplasia in particular are associated with a variety of aberrations in histochemical differentiation. These aberrations include (1) delayed maturation; (2) more than 80% predominance of type 1 or type 2 myofibers, with or without uniform hypoplasia of one or the other type (Fig. 60.8); and (3) classic congenital muscle fiber–type disproportion. Malformations limited to the cerebral cortex do not cause fiber-type predominance. Muscle histology of children with cerebral palsy from birth asphyxia or other perinatal insults shows only nonspecific type 2 muscle fiber atrophy. The normal ratio of fiber types is preserved. This is similar to the changes that follow disuse or immobilization of muscle. The speculation is that many of the small bulbospinal “subcorticospinal” tracts (i.e., vestibulospinal, reticulospinal, olivospinal, tectospinal, and rubrospinal) are more important than the larger corticospinal tract during the stage of histochemical differentiation of muscle between 20 and 28 weeks’ gestation. These small descending pathways are generally well myelinated and functional at that time, whereas the corticospinal tract does not even begin its myelination cycle or proliferation of axonal terminals until after muscle development is complete.

Fig. 60.8 Histochemical type 2 myofiber numerical predominance and relative smallness of type 2 fibers. Type 1 fibers are stained lightly; type 2 fibers are dark. The small type 2 fibers do not show the angular contour characteristic of denervated muscle fibers in the adult or of those atrophic secondary to disuse. This muscle biopsy was taken from a 3-year-old boy with generalized hypotonia and cerebellar hypoplasia. (Myosin ATPase stain preincubated at pH 10.4. Bar = 15 µm.)

Etiology of Central Nervous System Malformations

The causes of cerebral malformations generally fall into one of two categories. The first category is genetic and chromosomal disease in which programming of cerebral development is defective. This genetic category also includes many inborn metabolic diseases in which cerebral dysgenesis may be due to biochemical insults during development, rather than (or in addition to) primary errors in molecular genetic codes for neural programming. The second category includes all induced malformations in which a teratogenic influence acts at a particular time in ontogenesis; the malformation depends on the timing of the insult in relation to brain development at that moment. The timing may be brief, as with a single exposure to a toxic drug, a dose of radiation, or a traumatic injury of the fetal brain. It may be repeated two or more times or may be prolonged and involve the fetus at several stages of development. Examples of the latter include certain congenital infections such as toxoplasmosis and cytomegalovirus infection, which may be active throughout most of gestation, even into the postnatal period. Genetic factors are the most frequent causes of malformations during the first half of gestation. Environmental factors are more important in late gestation and may cause disturbances of late neuroblast migrations, particularly in premature infants. In some cases, no definite inductive factor is identifiable despite intensive clinical investigations during life and meticulous postmortem studies.

Molecular Genetic Classification of Malformations of the Nervous System

Classification is a fundamental human thought process, allowing us to organize data in a systematic manner and understand relations. The traditional basis for classification of CNS malformations is descriptive morphogenesis. New insights into the molecular genetic programming of neural development require the integrations of this information with the anatomical criteria (Sarnat and Flores-Sarnat, 2001b, 2004; Simeone, 2002). For example, lissencephaly and holoprosencephaly are two important malformations, each formerly thought to be distinctive. It is now recognized that many different genetic defects cause each; hence they are end stages of ontogenetic errors with diverse causes (see following discussion). A pure genetic classification to replace anatomical criteria, by contrast, would not be useful to clinicians, radiologists, or pathologists and would be incomplete because many genetic mutations remain unknown. A compromise that addresses the deficiencies of both pure anatomical and pure genetic schemes of classification is one based on patterns of genetic expression in which the precise genetic mutation may or may not be known but is stated while preserving anatomical criteria (Sarnat, 2000; Sarnat and Flores-Sarnat, 2001b). The up-regulation or down-regulation of a dorsalizing or ventralizing gene may be recognizable by its anatomical effect on neural tube development, even if the precise gene is unknown.

The traditional categories of CNS development that allow categories of ontogenetic processes, such as neuronogenesis, neuroblast migration, and synaptogenesis, and their disturbances in malformations may be preserved in the proposed new scheme of classification. They are supplemented by new categories such as “disturbances of cellular lineage” (e.g., tuberous sclerosis; hemimegalencephaly) and disorders of embryonic neuromeric segmentation (e.g., absence of the midbrain and upper pons; absence of the basal ganglia; Chiari malformations). Some genes specify particular types of cellular differentiation and may change the cell type at different stages of development (Marquardt and Pfaff, 2001). One of the most important concepts in the integrated morphological-molecular-genetic scheme is the gradients of genetic expression (Sarnat and Flores-Sarnat, 2001b). The gradients are those of the axes of the neural tube: dorsoventral and ventrodorsal, rostrocaudal and sometimes caudorostral, and mediolateral. Nearly all genes have gradients of expression, with stronger expression in some regions and gradually lesser influence more distally. For example, if the rostrocaudal gradient in holoprosencephaly extends as far as the midbrain, mesencephalic neural crest migration is impaired, and midfacial hypoplasia results regardless of the severity of the forebrain malformation (see following discussion). Some authors attempt to develop schemes of regional classification for malformations (e.g., limited to the cerebral cortex for use in genetic epilepsies). All classifications should consider the entire CNS, however, because the rostrocaudal gradients of genetic expression may cause important subcortical defects, and indeed some seizure disorders may even originate in subcortical structures. The up-regulation and down-regulation of genes also is sometimes easier to understand in the anatomically simpler structures of the brainstem and spinal cord, allowing extrapolation to more complex forebrain structures.

A simple chronological listing of genes in the order from those that are initially expressed in the embryo is not feasible because most genes express at several different stages of development. Genes subserve different functions at each stage, initially as organizer genes for the basic architecture of the neural tube such as axes, cephalization, dorsal and ventral surfaces, and segmentation. These same genes later express as regulator genes for the differentiation and maintenance of particular cellular identities and functions.

Clinical Expression of Selected Malformations of the Nervous System

Table 60.2 summarizes the clinical features of major malformations of the brain.

Disorders of Neurulation (1 to 4 Weeks’ Gestation)

Incomplete or defective formation of the neural tube from the neural placode is the most common type of malformation of the human CNS. Anencephaly has an incidence of 1 per 1000 live births; meningomyelocele is almost as frequent. Geographical and ethnic differences occur among various populations in the world. Nonetheless, it is a medical problem and human tragedy of much greater proportions because the majority of infants affected with defects of the posterior neural tube survive with major neurological handicaps. The causes of these disorders in the first month of gestation are usually not evident, despite intensive epidemiological, genetic, dietary, and toxicological surveys.

Anencephaly (Aprosencephaly with Open Cranium)

Anencephaly is a failure of the closing of the anterior neuropore at 24 days’ gestation. Death in utero occurs in approximately 7% of anencephalic pregnancies, 34% of such babies are premature, and 53% at term. Stillbirth, presumably resulting from intrapartum death, occurs in 20% of these deliveries. In one study of 211 pregnancies, 72% (153) of anencephalic offspring were liveborn; of those, 67% (103) died within 24 hours, but 6 survived 6 or more days (maximum 28 days) (Jaquier et al., 2006). The prenatal diagnosis of anencephaly is by examination of amniotic fluid for elevation of α-fetoprotein, and confirmation is by sonographic imaging as early as 12 weeks’ gestation. The face may show a midline hypoplasia, similar to holoprosencephaly (see following section on Holoprosencephaly), probably because the rostrocaudal gradient of a defective genetic expression extends to the midbrain and interferes with mesencephalic neural crest migration (Sarnat and Flores-Sarnat, 2001a).

Cephalocele (Encephalocele; Exencephaly)

Most encephaloceles are parietal or occipital (Fig. 60.9) and contain supratentorial tissue, cerebellar tissue, or both. Frontal encephaloceles are less common in North America and Europe but are the most frequent variety in Thailand, Vietnam, and surrounding countries. They usually include olfactory tissue. Cases of encephaloceles related to Agent Orange (containing the herbicides, 2,4-dichlorophenoxyacetic acid [2,4-D] and 2,4,5-trichlorophenoxyacetic acid [2,4,5-T]), which was used in the Vietnam War, are still reportedly observed in Cambodia.

Fig. 60.9 Lateral view of the brain of a term neonate with Meckel-Gruber syndrome. This dysplasia is a large occipital encephalocele (e) and lissencephaly. The brain is smooth and shows only a sylvian fissure and a few shallow abnormal sulci near the vertex. The encephalocele contains disorganized neural tissue, angiomatous malformations, focal hemorrhages, and zones of infarction.

Skin may completely cover the encephalocele, or thin, distorted meningeal membranes may be exposed. When the ventricular system also is herniated into the encephalocele sac, hydrocephalus develops. Leaking CSF rapidly leads to infection. Some encephaloceles, particularly those of the occipital midline, may become so large that they exceed the size of the infant’s head. Nasopharyngeal encephaloceles are rare but may be a source of meningitis from CSF leak through the nose. Malformations of the visceral organs often coexist with encephaloceles, and other congenital anomalies of the eyes and face, cleft palate, and polydactyly are also common. The entire brain may be severely hypoplastic (see Fig. 60.1).

Frontal and nasal encephaloceles protrude though bony foramina that normally close in the fetus: the fonticulus frontalis in the case of frontal (forehead midline) encephaloceles and the foramen caecum in the case of intranasal encephaloceles. Nasal encephaloceles might be confused clinically with nasal polyps, and CSF leak in the nose may be confused with benign nasal secretions. Both of these foramina fail to close because of defective prosencephalic neural crest tissue, which migrates from the dorsal part of the lamina terminalis as a vertical sheet of cells in the frontal midline.

Neurological handicaps may be severe because even if the herniated tissue within the encephalocele is small and easily excised, concomitant intracranial malformations of the brain often result in epilepsy, mental retardation, and motor impairment. Cortical blindness often occurs in the case of occipital encephaloceles. The treatment of choice of small encephaloceles is surgical excision and closure of overlying cutaneous defects. Seizures and hydrocephalus are common but treatable complications.

Meningomyelocele (Spinal Dysraphism, Spina Bifida Cystica)

The basis of classification of spina bifida syndromes is on either the bony vertebral deformity or the neurological lesion and associated clinical deficit. No deficits are associated with spina bifida occulta without herniation of tissue or mild spina bifida cystica with herniation of meninges alone. Deficits from herniation of nerve roots include motor, sensory, and autonomic neuropathy (meningomyelocele). Extensive defects occur with herniation of the parenchyma of the spinal cord (myelodysplasia). Most lesions are lumbosacral in location, but meningomyelocele also may occur in the thoracic or even the cervical region, usually as an extension rostrally of lumbosacral lesions. The level of involvement determines much of the clinical deficit. Type II Chiari malformation is consistently present, and aqueductal stenosis coexists in 50% of cases. Hydrocephalus is a common complication involving most patients with meningomyelocele and causing neurological deficit.

The treatment of meningomyelocele is controversial and enters the arena of medical ethics. Surgical closure of small defects in the neonatal period is the rule. Large defects associated with complete paraplegia and flaccid neurogenic bladder, often accompanied by hydronephrosis, severe hydrocephalus, and other cerebral malformations, are associated with poor quality of life. A decision not to treat such infants or not to prolong survival poses a moral question addressed by the physicians in consultation with parents, hospital ethics committees, and other individuals the parents may identify. The most important immediate complications of large meningomyeloceles are hydrocephalus and infection from leaking CSF. Long-term complications include chronic urinary tract infections, decubiti, hydrocephalus, paraplegia, and other neurological deficits. Mental retardation is common but may be mild.

Congenital Aqueductal Stenosis

Another aspect for consideration in the category of disorders of neurulation is the down-regulation of genes in the vertical axis of the neural tube. In the case of the ventrodorsal gradient due to defective SHH expression, sacral agenesis with dysplastic spinal cord at the levels of the deficient vertebrae (and notochord) is the best example. Down-regulation in the dorsoventral gradient of several genes or gene families, including ZIC2, SHH in the forebrain, BMP, and PAX, may result in holoprosencephaly (see following section on Holoprosencephaly) or may cause defective development of the dorsomedial septum of the midbrain with aqueductal stenosis (Sarnat and Flores-Sarnat, 2010b). Box 60.2 lists the various causes of congenital aqueductal stenosis (Sarnat and Flores-Sarnat, 2010b).

Box 60.2

Causes of Congenital Aqueductal Stenosis

Genetic or Presumed Genetic Causes

Holoprosencephaly

Chiari II malformation

X-linked hydrocephalus with aqueductal stenosis and pachygyria

Autosomal recessive hydrocephalus with aqueductal stenosis

Mutation of dorsalizing gene in vertical axis of neural tube

Agenesis of mesencephalic and metencephalic neuromeres

Primary defective ependymal and choroid plexus epithelia (?)

Acquired Causes in Utero

Intraventricular hemorrhage with thrombus in aqueduct

Congenital infections (e.g., cytomegalovirus infection, mumps)

Ependymitis/ventriculitis with gliosis around and within aqueduct

Chronic arachnoiditis

Hydranencephaly

Aqueductal membrane across lumen

Amnion rupture sequence

Aneurysms, venous angiomas, and other vascular malformations

Cystic dilatation of perivascular Virchow-Robin spaces in midbrain

Tumors of aqueduct (e.g., ependymoma, astrocytoma, glioneuronal hamartoma, neurepithelial tumor of subcommissural organ)

Tumors that compress the midbrain tectum from above (e.g. pineal tumors and cysts, arachnoidal cysts, lipomas)

From Sarnat, H.B., Flores-Sarnat, L., 2010. Congenital aqueductal stenosis associated with aplasia or hypoplasia of the dorsal median septum of the midbrain. A disorder of genetic gradients along the axes of the neural tube. [To be submitted.]

Midline Malformations of the Forebrain (4 to 8 Weeks’ Gestation)

Several developmental malformations of the prosencephalon relate embryologically to failure of the lamina terminalis to differentiate into telencephalic structures. The lamina terminalis is the rostral membrane of the primitive neural tube that forms with closure of the anterior neuropore. The expression of such disorders is mainly as midline defects, not only because of its location in the midline but also because of impaired lateral growth of the cerebral hemispheres due to deficient or abnormal cellular migration centrifugally to form the cerebral cortex. The series of midline prosencephalic malformations relates to the embryological time of the beginning of each and includes alobar, semilobar, and lobar holoprosencephaly, arhinencephaly, septo-optic dysplasia, colpocephaly, and agenesis of the corpus callosum.

The lamina terminalis, after differentiating the forebrain structures, becomes the anterior wall of the third ventricle in the mature brain. It extends between the optic chiasm ventrally and the rostrum of the corpus callosum dorsally. Some authors contend that a defective cephalic notochord induces midline forebrain defects. Understanding of the complex embryological relationship of neuroectoderm and mesoderm in early ontogenesis is incomplete.

Holoprosencephaly

Holoprosencephaly (HPE) is a malformation in which the two cerebral hemispheres appear fused in the midline but is really a failure of cleavage in the midsagittal plane of the embryonic cerebral vesicle at 33 days’ gestation and thus a paramedian hypoplasia of the forebrain. HPE has a frequency of 1 in 16,000 live births but in 1 in 250 spontaneously aborted fetuses in the first trimester; hence it is among the most common of the major cerebral malformations.

Traditionally, HPE was a single malformation with three variants: alobar, semilobar, lobar. A fourth was added later: the middle interhemispheric variant (Hahn and Pinter, 2002; Simon et al., 2002). Another variant recently described is septopreoptic holoprosencephaly, demonstrated as noncleavage restricted to the preoptic and septal region by MRI in 7 patients (Hahn et al., 2010); we have now recognized two additional cases (unpublished). Recent molecular genetic data redefine HPE as a common end-stage malformation with six known different genetic mutations demonstrated in various cases (Golden, 1998) (Table 60.3). Other chromosomal defects (in loci 3p26, 4,5, 6, 14q13, 14q21.1-q21.2, 20, 21q22.3) are known in which the specific genetic mutation is not yet identified. All six known defective genes together account for only about 20% of cases, so many more gene defects remain undiscovered. Furthermore, each of the traditional anatomical variants of HPE is demonstrable in each of the six known genetic forms, signifying that these merely represent degrees of severity without etiological implication. A defect in the ZIC2 gene is associated with chromosome 13q deletions, and HPE is frequent in infants with trisomy 13 (Brown et al., 1998). One of the most studied of the genetic mutations is the strong ventralizing gene, sonic hedgehog (SHH); lack of expression of this gene in the prechordal mesoderm ventral to the rostral end of the neural tube results in no neural induction (Roessler et al., 1996). Abnormal SHH expression also may be altered in metabolic diseases with impaired cholesterol synthesis and high serum levels of the cholesterol precursor molecule, 7-dehydrocholesterol, as in the Smith-Lemli-Opitz syndrome associated with HPE (Kelley et al., 1996).

Table 60.3 Best Documented Genetic Mutations in Holoprosencephaly

Chromosomal Locus	Defective Gene	Vertical Gradient Effect
2p21	SIX3	Dorsoventral
7q36	SHH	Ventrodorsal (spinal cord, hindbrain), dorsoventral (midbrain, forebrain)*
13q32	ZIC2	Dorsoventral
18q11.3	TGIF	Ventrodorsal
9q22.3	PTCH	Ventrodorsal
10q11.2	DKK	Ventrodorsal

* Although SHH is a powerful ventralizing gene in the embryonic spinal cord and hindbrain, recent evidence indicates that at the level of the midbrain and most rostral regions of the neural tube, it changes its gradient and becomes dorsalizing in the vertical axis.

Data from Blaess, S., Corrales, J.D., Joyner, A.L., 2006. Sonic hedgehog regulates Gli activator and repressor functions with spatial and temporal precision in the mid/hindbrain region. Development 133, 1799-1809.

After chromosomal defects, the most common association of HPE is maternal diabetes mellitus; sacral agenesis is another common malformation in infants of diabetic mothers. Both involve down-regulation of SHH. A defect at the same chromosome 7p36.2 locus associated with an autosomal dominant form of HPE also affects SHH at the posterior, rather than the anterior, end of the neural tube and results in sacral agenesis (Lynch et al., 1995). Disturbed insulin metabolism may affect SHH in programming the neural tube.

Olfactory bulbs and tubercles differentiate at 41 days, a few days after forebrain cleavage, but olfactory agenesis usually accompanies all but the mildest forms of HPE; therefore, the term arhinencephaly, often used interchangeably, is incorrect. Callosal agenesis also is a uniform feature except in the mildest forms, and the cerebral mantle shows gross disorganization with multiple heterotopia, poorly laminated cortical gray matter, and heterotopic neurons and glial cells in the overlying meninges. Extensions of germinal matrix into the lateral ventricles through gaps in the ependyma are common. Thus, although HPE can be dated to about 33 days’ gestation at onset, the pathological process extends throughout most of fetal life.

Four different anatomical variants of HPE reflect different degrees of abnormal cerebral architecture. Characteristic of alobar HPE is a brain with a single midline telencephalic ventricle rather than paired lateral ventricles and continuity of the cerebral cortex across the midline frontally. The roof of the monoventricle balloons into a dorsal cyst. The corpus striatum and thalamus of the two sides are uncleaved, and the third ventricle may obliterate with rudiments of ependymal rosettes in its place. In semilobar HPE, an incomplete interhemispheric fissure forms posteriorly, and the occipital lobes, including the occipital horns of the ventricular system, may approach a normal configuration despite noncleavage of the frontal lobes across the midline. Lobar HPE is a less severe dysgenesis; the hemispheres form well but are in continuity through a band of cortex at the frontal pole or the orbital surface, and the indusium griseum and cingulate gyri overlying the corpus callosum are in continuity. The corpus callosum incompletely forms but is not totally absent as in alobar and semilobar HPE. The middle interhemispheric variant consists of hypoplasia of the middle part of the corpus callosum and associated structures of the medial side of the hemispheres. In the more severe forms of HPE, the optic nerves are hypoplastic or fused to enter a single median eye. Midline cerebellar defects, absent pyramidal tracts, and malformed brainstem structures accompany the more severe forms of this malformation. Meningeal heterotopia or marginal glioneuronal nodules commonly result from overmigration, perhaps associated with the absence of the transitory external granular layer of the fetal brain in HPE.

The diagnosis of HPE often occurs at the time of delivery, because 93% of patients exhibit midline facial dysplasias. Midfacial hypoplasia is present in most patients with HPE, but others have a normal face. The facial dysmorphism ranges from mild hypotelorism and vomer bones to severe forms including cebocephaly with a single naris, severe hypotelorism and absence of the premaxilla and vomer bones to produce a midline cleft lip and palate, or cyclopia with a midline proboscis dorsal to the single median eye. The severity of the facial dysmorphism does not correlate as well with the anatomical variant as originally expressed in the often cited statement “the face predicts the brain.” Midfacial hypoplasia does correlate, however, with the rostrocaudal extent of the defective genetic expression. If the gradient extends to the embryonic mesencephalic neuromere and causes hypoplasia of the midbrain, neural crest formation and migration are affected (Sarnat and Flores-Sarnat, 2001a). The mesencephalic neural crest is the most rostral origin of neural crest, and this tissue forms not only peripheral neural structures such as the ciliary ganglion but also most of the membranous bones of the face, globe of the eye (except the retina and choroid), and much of the facial connective tissue.

The various forms of HPE are well demonstrated by most imaging techniques (Fig. 60.10), including prenatal ultrasound. The imaging features of each anatomical variant are distinctive (Hahn and Pinter, 2002) and correspond well to the gross neuropathological findings (Golden, 1998). The anterior cerebral artery is usually a single azygous vessel coursing just beneath the inner table of the skull, a pathognomonic finding. The sagittal sinuses, deformed or replaced by a network of large abnormal veins, resemble the early embryonic pattern of venous drainage.

Fig. 60.10 Unenhanced computed tomographic scan of a 6-year-old boy with semilobar holoprosencephaly. The lateral ventricles are fused, particularly frontally, but show some division into two occipital horns. A deep abnormal sulcus is seen across the fused frontal lobes (arrowheads). This is one of several radiological variants of holoprosencephaly.

The EEG in HPE shows multifocal spikes that often evolve into hypsarrhythmia. In the neonatal period, the characteristic feature of the waking EEG is almost continuous high-voltage alpha-theta monorhythmic activity, becoming discontinuous in sleep. Visual evoked potentials also are abnormal or altogether absent. The characteristic clinical course of HPE is severe developmental delay and a mixed pattern of seizures that often are refractory to antiepileptic drugs. The presence or absence of seizures does not correlate with the anatomical severity or variant of the defective forebrain and correlates poorly with the genetic mutation (Hahn and Pinter, 2002). A better correlation may be with the degree of mediolateral extension of genetic expression in disrupting the histological architecture of the cortex, or it may relate to an abnormal sequence of maturation of axosomatic (inhibitory) and axodendritic (excitatory) synapses in relation to the maturation of the neuron innervated by these axonal terminations (Sarnat and Flores-Sarnat, 2001a). Some patients develop hydrocephalus that requires a ventriculoperitoneal shunt. This condition is paradoxically more common in the less severe anatomical forms of the malformation. In the severe alobar form, a “dorsal cyst” occupies the entire posterior one-half to two-thirds of the intracranial space and occasionally even protrudes through the anterior fontanelle as a unique encephalocele that may be larger than the rest of the head. No other type of encephalocele occurs at the anterior fontanelle. The dorsal cyst seems to originate from a dilated suprapineal recess of the third ventricle and later is a dorsal membrane that includes the roof of the forebrain, extending from the hippocampi (Sarnat and Flores-Sarnat, 2001a).

Endocrine dysfunction may be present, associated with hypothalamic or pituitary involvement, and vasopressin-sensitive diabetes insipidus occurs in about 86% of cases, other hypothalamic-pituitary dysfunction being much less frequent (Plawner et al., 2002). The basis of this specific involvement of the paraventricular and supraoptic hypothalamic nuclei may be hypoplasia in some cases in which the midline hypoplasia involves the diencephalon as well as the forebrain (most patients), but it also occurs in some children without hypothalamic noncleavage. One hypothesis is that the primary gene defect suppresses expression of the gene, orthopedia (OTP). OTP and downstream genes such as SIM1 and BRN2 are essential for terminal differentiation of neuroendocrine cells of these hypothalamic nuclei (Sarnat and Flores-Sarnat, 2001a).

The treatment of HPE symptoms entails treating the complications (e.g., seizures, hydrocephalus, endocrine disturbances). Educational potential and needs depend on the degree of mental retardation, speech, and visual impairment.

Isolated Arhinencephaly and Kallmann Syndrome

Absence of olfactory bulbs, tracts, and tubercles commonly accompanies extensive malformations such as HPE and septo-optic dysplasia but may occur with callosal agenesis or as an isolated cerebral anomaly. Kallmann syndrome is an X-linked autosomal dominant condition limited to males, in which anosmia secondary to arhinencephaly without other forebrain malformations is associated with lack of secretion of gonadotropic hormones. The defective gene is KAL1 at the chromosome Xp22.3 locus. Also implicated is the EMX2 gene, though schizencephaly does not occur with Kallmann syndrome (Taylor et al., 1999). Olfactory reflexes may be elicited in the neonate consistently after 32 weeks’ gestation and provide a useful supplement to the neurological examination of newborns suspected of cerebral dysgenesis.

Septo-Optic–Pituitary Dysplasia

De Mosier first recognized the association of a rudimentary or absent septum pellucidum with hypoplasia of the optic nerves and chiasm in 1956. Underdevelopment of the corpus callosum and anterior commissure and detachment of the fornix from the ventral surface of the corpus callosum are additional features. Patients with this combination of anomalies overlap others with semilobar HPE, and some children with septo-optic dysplasia have arhinencephaly as well.

Disturbances of the hypothalamic-pituitary axis often occur in septo-optic dysplasia, ranging from isolated growth hormone deficiency to panhypopituitarism and deficient secretion of antidiuretic hormone. Hypothalamic hamartomas, gliosis, and the absence of some hypothalamic nuclei may be associated with a histologically normal pituitary. Absence of the neurohypophysis is demonstrable postmortem in some cases.

Midline cerebellar defects and hydrocephalus occur inconsistently in septo-optic dysplasia. One cerebellar lesion, called rhombencephalosynapsis is aplasia of the vermis and midline fusion of the cerebellar hemispheres and of the dentate nuclei, probably the down-regulation of a dorsalizing gene at the level of rhombomere 1 (Sarnat, 2000).

Clinical manifestations relate mainly to the endocrine deficiencies and vision impairment. Ataxia may be compensable if the cerebellar vermis is mildly involved. Seizures are uncommon. Intellectual development usually is normal. Hypertelorism is not a constant finding. Chromosome analysis is invariably normal. The gene, HESX1, is defective in at least some cases (Dattani et al., 1998). No reports of familial cases exist. However, a high incidence of teenage pregnancy and drug abuse in early gestation occurs in mothers of affected infants. Septo-optic–pituitary dysplasia has occurred in an infant of a diabetic mother.

Rhombomeric Deletions and Ectopic Genetic Expression

Rare patients with absence of certain parts of the brain appear in the literature. Only recently, by understanding the families of genes responsible for neural tube segmentation (e.g., HOX, WNT, PAX), have these conditions been understood at the level of molecular embryology. Agenesis of the midbrain and upper pons (metencephalon) with cerebellar hypoplasia are attributable to the EN2 gene, which produces an almost identical malformation in the knockout mouse model (Sarnat et al., 2002). EN1 and WNT1 genes also are essential for development of the mesencephalic and rhombomere 1, but the animal models of these genetic defects produce total agenesis of the cerebellum. Sonic hedgehog also regulates the temporal and spatial precision of the midbrain-hindbrain junction, mediated through Gli activator (Blaess et al., 2006). Absence of the corpus striatum might be due to mutation of the EMX1 gene, which is essential in the programming of the basal telencephalon but not the cerebral cortex (Sarnat and Flores-Sarnat, 2001a). The Chiari malformations, particularly type II, were incompletely explained by mechanical theories of pathogenesis, but a molecular genetic hypothesis of ectopic expression provides a more complete and reasonable explanation (see section on Chiari Malformations) (Sarnat and Flores-Sarnat, 2001b, 2004). Despite documentation of many of these genetic malformations in experimental animal models, definitive confirmation in humans has not occurred.

Agenesis of the Corpus Callosum

A commissural plate differentiates within the lamina terminalis at day 39 of embryonic life. The plate acts as a bridge for axonal passage and provides a preformed glial pathway to guide decussating growth cones of commissural axons. Microcystic degeneration in the commissural plate and physiological death of astrocytes precedes the interhemispheric projection of the first axons. The earliest callosal axons appear at 74 days in the human embryo, the genu and the splenium are recognizable at 84 days, and achievement of the adult morphology is by 115 days.

The pathogenesis of callosal agenesis relates to the commissural plate; if this plate is not available to guide axons across, the corpus callosum does not develop. Failure of physiological degeneration of a portion of the plate results in a glial barrier to axonal passage and the disappearance or deflection of primordial callosal fibers posteriorly to another destination within their hemisphere of origin (bundle of Probst). Other destinations of callosal axons that are unable to cross the midline at their expected site include passage into the anterior commissure, which can become enlarged as much as four times its normal volume by the addition of these axons; aberrant sites of crossing of individual fibers not forming large bundles; and occasionally, callosal axons descend within the internal capsule with the corticospinal tract as far as the spinal cord, where their termination and function remain unknown (Sarnat 2008).

Agenesis of the corpus callosum is a common malformation, having a 2.3% prevalence in computed tomographic (CT) scans in North America and 7% to 9% prevalence in Japan. Most cases are isolated malformations, but callosal agenesis is an additional feature of many other prosencephalic dysplasias; it also occurs with aplasia of the cerebellar vermis and anomalous pyramidal tract. Simple callosal agenesis may involve the entire commissure or may be partial, usually affecting only the posterior fibers. Hypoplasia or partial agenesis of the commissure is much more common than total agenesis. In callosal agenesis, the anterior and hippocampal commissures are always well formed or large.

A rare genetic form of callosal agenesis is associated with defective neural crest migration causing aganglionic megacolon (Hirschsprung disease). The cause is a defective human gene, Smad-interacting protein 1 (SMAD1), at the chromosome 2q22-q23 locus (Cacheux et al., 2001). In the absence of a corpus callosum, the lateral ventricles displace laterally, and the third ventricle rises between them (Fig. 60.11). Often the ventricles dilate mildly, but intraventricular pressure is normal. The anomaly may be demonstrable by most imaging techniques. The varying degrees of partial callosal agenesis produce several radiographic variants.

Fig. 60.11 Pneumoencephalogram (from the preimaging period) of an 18-month-old boy with agenesis of the corpus callosum associated with an interhemispheric arachnoidal cyst (arrowhead), a complication of some cases of callosal agenesis. The lateral ventricles are widely separated from the medial side of each hemisphere by the bundle of Probst; the third ventricle rises between them. The brainstem, cerebellum, and cerebral cortical convolutions appear normal. The patient has mental retardation and epilepsy.

Clinical symptoms of callosal agenesis may be minimal and unrecognized in children of normal intelligence. Detailed neurological examination discloses deficits in the interhemispheric transfer of perceptual information for verbal expression. Mental retardation or learning disabilities occur in some cases. Epilepsy is common, particularly in patients diagnosed early in life. Seizures may relate more to minor focal cortical dysplasias than to the callosal agenesis itself. Hypertelorism is present in many and often is associated with exotropia and inability to converge.

The EEG characteristically shows interhemispheric asynchrony or poor organization, with or without multifocal spikes, but is not specific enough to establish the diagnosis. Asynchronous sleep spindles after 18 months of age are a good clue to the diagnosis. Several hereditary forms of callosal agenesis occur besides its occurrence as an additional anomaly in some cases of tuberous sclerosis and other genetic syndromes. Andermann syndrome is an autosomal recessive syndrome of callosal agenesis, mental deficiency, and peripheral neuropathy. Aicardi syndrome consists of agenesis of the corpus callosum, chorioretinal lacunae, vertebral anomalies, mental retardation, and myoclonic epilepsy. This disorder is found almost exclusively in girls and is thought to be X-linked dominant (Xp22) and generally lethal in the male fetus. The EEG shows a typically asymmetrical asynchronous burst-suppression pattern. Neuropathological findings in Aicardi syndrome include a variety of minor dysplasias in addition to agenesis of the corpus callosum and anterior commissure, and nonlaminated polymicrogyric cortex with abnormally oriented neurons. Callosal agenesis is a common component in many chromosomal disorders, particularly trisomies 8, 11, and 13. Interhemispheric lipoma replacing part of the corpus callosum is associated with a high incidence of epilepsy.

Colpocephaly

Colpocephaly is a selective dilatation of the occipital horns, not due to increased intraventricular pressure but rather due to loss of white matter. Colpocephaly occurs in three conditions:

1. It appears as a primary malformation, histologically associated with poorly laminated striate cortex, subcortical heterotopia, and defective ependymal lining the occipital horns.

2. It is common in many cases of agenesis of the corpus callosum because of absence of the splenium and hypoplasia of white matter.

3. It may be the acquired result of periventricular leukomalacia, especially in premature infants, because of loss of periventricular white matter in the posterior half of the cerebral hemispheres.

Clinical findings are usually those of mental retardation, spastic diplegia, epilepsy, and vision loss, but it does not always cause complete blindness. CT in the neonatal period or early infancy demonstrates most cases. Isotope cisternography shows normal CSF dynamics in most. Colpocephaly is associated with several syndromes and systemic disorders including cerebrohepatorenal (Zellweger) disease, hemimegalencephaly, and several chromosomal disorders. The EEG in colpocephaly ranges from normal in mild cases to near-hypsarrhythmia in infants who develop myoclonic epilepsy. Bilateral posterior slowing of low voltage with occipital spikes is common. Colpocephaly also develops late in fetal life because of infarction and cystic degeneration of the deep white matter of the posterior third of the cerebral hemispheres, rather than as a developmental disorder of neuroblast migration. It is often confused with hydrocephalus.

Disorders of Early Neuroblast Migration (8 to 20 Weeks’ Gestation)

Lissencephaly (Agyria, Sometimes with Pachygyria)

Lissencephaly is a failure of development of convolutions in the cerebral cortex because of defective neuroblast migration. The cortex remains smooth, as in the embryonic brain (see Fig. 60.9). The migrations of the cerebellum and the brainstem also usually are involved, but the thalamus and basal ganglia form properly. Structural and metabolic abnormalities of the fetal ependyma may be supplementary factors in disturbing the normal development of radial glial cells.

The cytoarchitecture of the neocortex in lissencephaly takes one of two forms. In the first, a four-layered sequence develops. The outermost layer is a widened molecular zone; layer 2 contains neurons corresponding to those of normal laminae III, V, and VI; layer 3 is cell-sparse; and layer 4 contains heterotopic neurons that have migrated incompletely. Decreased brain size leads to microcephaly with widened ventricles, representing a fetal stage rather than pressure from hydrocephalus, and an uncovered sylvian fossa representing lack of operculation. The second form of cortical architectural abnormality in lissencephaly is disorganized clusters of neurons with haphazard orientation, forming no definite layers or predictable pattern. Type 2 lissencephaly is associated with several closely related genetic syndromes: Walker-Warburg syndrome, Fukuyama muscular dystrophy, muscle-eye-brain disease of Santavuori, and Meckel-Grüber syndrome, the latter often associated with posterior encephalocele (see Fig. 60.9).

Miller-Dieker Syndrome (Type 1 Lissencephaly)

Miller-Dieker syndrome is a familial lissencephaly characterized clinically by microcephaly and a peculiar facies that includes micrognathia, high forehead, thin upper lip, short nose with anteverted nares, and low-set ears (Fig. 60.12). Neurologically, the children are developmentally delayed in infancy and mentally retarded, lack normal responsiveness to stimuli, initially exhibit muscular hypotonia that later evolves into spasticity and opisthotonos, and develop intractable seizures. Death before 1 year of age is common. The EEG often shows focal or multifocal spike-wave discharges that later become bisynchronous bursts of diffuse paroxysmal activity, and extremely high-voltage diffuse rhythmic theta and beta activity. At autopsy, the original cases showed lack of gyral development in the cerebral cortex. Later patients with the typical craniofacial features and clinical course showed gyral development, although the convolutions were abnormal, and pachygyria predominated. The term Miller-Dieker syndrome as originally proposed was to distinguish this syndrome from other cases of lissencephaly without the clinical and dysmorphic facial features. A microdeletion at the chromosome 17p13.3 locus is demonstrable by high-resolution studies in most patients with Miller-Dieker syndrome, and family members of the original patients show the defect (Chong et al., 1997). The responsible gene is LIS1. Histological examination of the brain in Miller-Dieker syndrome confirms the presence of a severe disorder of neuroblast migration, as in other cases of lissencephaly.

Fig. 60.12 A, This 7-month-old boy has Miller-Dieker syndrome. He had severe developmental delay, tetraparesis, and epilepsy. Note he required a gastrostomy. B, He has the typical facies of this genetic syndrome, with a high forehead with temporal hollowing, upturned nares, and long philtrum (upper lip). His gaze is dysconjugate, but there is no paresis of extraocular muscles. Sagittal (C) and parasagittal (D) views of T1-weighted magnetic resonance images, showing type 1 lissencephaly with very thick cortex and only mild ventriculomegaly. The cerebellum and brainstem, including the basis pontis, are grossly well formed. The corpus callosum is very thin. Extraaxial (i.e., subarachnoid) spaces are wide over the convexities of the cerebral hemispheres and in the cisterns surrounding the brainstem.

Walker-Warburg and Related Syndromes (Type 2 Lissencephaly)

Type 2 lissencephaly/pachygyria includes several distinctive disorders of different genetic origin. All involve the terminal organization and architecture of the cortical plate and most of which include a dystrophic myopathy. In Fukuyama muscular dystrophy, a congenital muscular dystrophy is associated with cerebral dysgenesis of this type and due to mutation in the gene, fukutin. Though common in Japan, where it is the second most common muscular dystrophy (after Duchenne-type dystrophy), it is rare in other ethnic populations. The muscle-eye-brain disease of Santavuori is most common in Finland but also exists in other northern European ethnic groups. Walker-Warburg syndrome is another congenital muscular dystrophy found in diverse ethnic groups. An autosomal recessive type 2 lissencephaly associated with cerebellar hypoplasia is due to defective expression of reelin, and X-linked congenital hydrocephalus (usually due to aqueductal atresia) is associated with pachygyria and mutation of the cell adhesion gene, L1CAM.

X-Linked Recessive Lissencephaly with Abnormal Genitalia

The most recently defined genetic form of lissencephaly is due to a mutation in the ARX gene in both the mouse and human (Kitamura et al., 2002). It generally is associated with microencephaly and global cerebellar hypoplasia but also with abnormal genitalia. The brain malformation is the most severe of the lissencephalies, and affected children are profoundly mentally retarded and have epilepsy.

Subcortical Laminar Heterotopia (Band Heterotopia) and Bilateral Periventricular Nodular Heterotopia

Subcortical laminar heterotopia and bilateral periventricular nodular heterotopia both result from X-linked recessive traits occurring almost exclusively in females. Both disorders present clinically as severe seizure disorders in childhood, although they are often associated also with mental retardation and other neurological deficits. In subcortical laminar heterotopia, a band of gray matter heterotopia within the subcortical white matter lies parallel to the overlying cerebral cortex but separated from it by white matter. Histologically, it lacks lamination, as is the normal cortex, and consists of disoriented neurons and glial cells and fibers with poorly organized architecture. The few male fetuses that have not spontaneously aborted have been born with lissencephaly, in addition, and even more severe neurological deficits. The defective gene and its transcription product in subcortical laminar heterotopia are known; the latter is called doublecortin (Gleeson et al., 1999). In bilateral periventricular nodular heterotopia, islands of neurons and glial cells occur in the subependymal regions around the lateral ventricles and are neuroepithelial cells that have matured in their site of origin without migrating (Eksioglu et al., 1996). The gene responsible is filamin-1. MRI best demonstrates both conditions, but they are also detectable by CT.

Schizencephaly

Schizencephaly is a unilateral or bilateral deep cleft (usually in the general position of the sylvian fissure) but is not a sylvian fissure. This cleft is the full thickness of the hemispheric wall, and no cerebral tissue remains between the meninges and the lateral ventricle (the pial-ependymal seam). If the cerebral cortical walls on either side of the deep cleft are in contact, the condition is closed lip, and if a wide subarachnoid space separates the two walls, it is open lips, but these two variants do not provide a clue to pathogenesis. Schizencephaly is often classified as a neuroblast migratory disorder, but this mechanism is only partially true; it is primarily a disorder of development of the telencephalic flexure (see Fig. 60.6). It may occur either as a Mendelian or sporadic genetic trait or as a fetal deformation of the telencephalic hemisphere at the time of development of the telencephalic flexure (Sarnat and Flores-Sarnat, 2010a); in some cases, it results from porencephaly due to fetal cerebral infarction.

Schizencephaly was thought to be associated with defective expression of the gene, EMX2 (Granata et al., 1997), but this is not the case (Tietjen et al., 2007), and the genetic basis remains unknown. It may be associated with a variable degree of lissencephaly/pachygyria, may be bilaterally symmetrical, or may be asymmetrical and more severe on one side. Schizencephaly is unilateral in half of cases.

Disorders of Cerebellar Development (32 Days’ Gestation to 1 Year Postnatally)

The cerebellum has the longest period of embryological development of any major structure of the brain. Neuroblast differentiation in the cerebellar plates (rhombic lips of His) of the dorsolateral future medulla oblongata and lateral recesses of the future fourth ventricle are recognizable at 32 days. Neuroblast migration from the external granular layer is not complete until 1 year postnatally. Because of this extended ontogenesis, the cerebellum is vulnerable to teratogenic insults longer than most parts of the nervous system. Malformations of the cerebellum may occur alone or be associated with other brainstem or cerebral dysplasias. The cerebellar cortex is especially susceptible to toxic effects of many drugs, chemicals, viral infections, and ischemic-hypoxic insults.

Selective Vermal Aplasia

Selective hypoplasia or aplasia of the vermis, with intact lateral hemispheres, occurs in some genetic disorders in association with other midline defects involving the forebrain, as in some cases of HPE and callosal agenesis. Characteristic of Joubert syndrome, a specific autosomal recessive trait, is episodic hyperpnea, abnormal eye movements, ataxia, and mental retardation. Joubert syndrome has a variable but often progressively worsening course, with improvement in some cases. Anomalies of visceral organs and polydactyly may be associated. Joubert syndrome is one of several disorders now recognized as “ciliopathies.”

Selective Cerebellar Hemispheric Aplasia

Selective agenesis of the cerebellar hemispheres is much less common than aplasia of the vermis alone. Other components of the cerebellar system, such as the dentate and inferior olivary nuclei, may also be dysplastic. The lateral hemispheres and the inferior olivary and pontine nuclei more commonly are selectively involved in certain degenerative diseases of genetic origin, such as olivopontocerebellar atrophy and other spinocerebellar degenerations.

Dandy-Walker Malformation

The Dandy-Walker malformation consists of a ballooning of the posterior half of the fourth ventricle, often but not always associated with lack of patency of the foramen of Magendie. In addition, the posterior cerebellar vermis is aplastic, and there may be heterotopia of the inferior olivary nuclei, pachygyria of the cerebral cortex, and other cerebral and sometimes visceral anomalies. Hydrocephalus from obstruction usually develops, but if treated promptly, the prognosis may be good. Neurological handicaps such as spastic diplegia and mental retardation probably relate more to the associated malformations of the brain than to the hydrocephalus. Various incomplete forms are described as Dandy-Walker variants, particularly in MRI studies, but the classification of these is debated.

Chiari Malformation

The Chiari malformation is a displacement of the tonsils and posterior vermis of the cerebellum through the foramen magnum, compressing the spinomedullary junction; this simple form is termed Chiari type I malformation. Type II involves an additional downward displacement of a distorted lower medulla and dysplasia of medullary nuclei and is a constant feature in lumbosacral meningomyelocele. Chiari type III malformation, a rare form, involves cervical spina bifida with cerebellar encephalocele. Chiari originally identified a type IV in 1896, but this type is actually cerebellar hypoplasia with no relation to the other types, and the term Chiari malformation type IV is now used only in its historical context. Hydrocephalus is commonly associated with Chiari malformations.

The pathogenesis has been a matter of controversy for many years. Mechanical theories have dominated since the time of Chiari: (1) the traction theory, a result of a tethered spinal cord with traction as the vertebral column grows; (2) the pulsion theory of fetal hydrocephalus pushing the cerebellum and brainstem from above; and (3) the crowding theory in which a small posterior fossa provides insufficient room for the growth of neural structures and causes a “toothpaste tube effect.” The torcula is indeed too low and the volume of the posterior fossa too small, so that this latter explanation is probably a true contributory factor, but only in late gestation as a superimposed secondary influence. A molecular genetic hypothesis of ectopic expression of a segmentation gene in the rhombomeres explains not only the Chiari malformation but also the brainstem anomalies, the myelodysplasia, and the defective basioccipital and supraoccipital bone formation that results in a too-small posterior fossa (Sarnat, 2004a).

Global Cerebellar Hypoplasia

Global cerebellar hypoplasia has diverse causes that include chromosomal and genetically determined diseases, Tay-Sachs disease, Menkes kinky hair disease, some cases of spinal muscular atrophy, and sporadic cases of unknown cause. Histologically, there may be a selective depletion of granule cells or a loss of Purkinje cells and other neuronal elements in addition to granule cells (Fig. 60.13). In selective granule cell depletion, the axons and dendrites of Purkinje cells are deformed.

Fig. 60.13 Cerebellar cortex of infant with cerebellar hypoplasia shows extensive gliosis and loss of all neuronal elements. This histological appearance resembles that of cerebellar sclerosis secondary to acquired injury, but in the latter condition there are usually a few neurons still surviving. Some cases of cerebellar hypoplasia show selective loss of granule cells and preservation of Purkinje cells. (Hematoxylin-eosin stain. Bar = 100 µm.)

Clinically, the most constant features of cerebellar hypoplasia in infancy are developmental delay and generalized muscular hypotonia. Truncal titubation and ataxia become evident after several months, and nystagmus and intention tremor may appear in severe cases. Tendon stretch reflexes usually are hypoactive but may be hyperactive if corticospinal tract deficit is also present because of cerebral involvement.

Focal Cerebellar Dysplasia

Focal dysplasias and hamartomas of the cerebellar cortex (Fig. 60.14) are often incidental findings at autopsy and are often clinically asymptomatic. More extensive lesions present abnormal cerebellar findings clinically. These small focal malformations are a disorder of neuronal migration programmed as genetic defects or, more commonly, acquired from brief insults during the long period of cerebellar development. Focal ischemic insults and exposure to cytotoxic drugs or viruses are among the more common causes. The granule cells of the cerebellar cortex retain a regenerative capacity lost early in gestation by most other neurons, but the regenerative pattern of lamination in the cerebellar cortex may be imperfect.

Fig. 60.14 Focal dysplasia of cerebellar cortex. The normal laminar architecture is disrupted, and granule and Purkinje cells show a haphazard orientation and array. Some granule cells are spindle shaped, resembling the shape assumed during transit from the external granular layer in normal development. This dysplasia is due to faulty neuronal migration and probably occurred at midgestation. (Hematoxylin-eosin stain. Bar = 15 µm.)

Craniosynostosis

The development of the cranium and sutures closely relates to neuromeric formation in the neural tube. Mesencephalic neural crest induces most of craniofacial development and is involved in the formation of the rostral two-thirds of the cranial vault and underlying meninges. More than 100 syndromes are identifiable that include a component of craniosynostosis. Several have a genetic basis, including the syndromes of Apert, Crouzon, Pfeiffer, and Saethre-Chotzen; these are related to fibroblast growth factor receptor defects, and in some, specific causative genes such as TWIST are recognized (Flores-Sarnat, 2002b). The genetic bases of the more common isolated craniostenoses of the coronal and sagittal sutures are unknown. Although once thought to be only a cosmetic defect, some believe that untreated isolated synostosis results in neurological impairment. Advances in imaging have enhanced both prenatal and postnatal diagnosis, and parallel advances in craniofacial surgery makes treatment more effective from both a neurological and cosmetic perspective. Distinguishing true craniosynostosis from positional deformities of the head resulting from abnormal prenatal compression by the maternal pelvis and postnatal effects of head position, particularly in premature infants, is important for management.