Embryonic development of the nervous system is a series of overlapping processes.1 To understand neural development, a traditional view of morphogenesis must be integrated with what we know about molecular genetic programming of the neural tube and fetal brain. Understanding these normal ontogenetic processes is necessary to comprehend neural malformations, which arise as disturbances in one or more of these processes. Malformations of the brain and spinal cord may be caused by genetic mutations or by environmental or acquired influences. Examples of acquired and environmental causes are teratogenic toxins and drugs, fetal ischemia and infarcts, intrauterine trauma from maternal trauma or invasive procedures, cerebral hemorrhages, and infections affecting the fetal brain.
Knowledge of neuroembryology provides insight into the pathogenesis and mechanisms of neural malformations and how the brain determines dysgeneses of many non-neurological structures, such as developmental craniofacial disorders. Molecular genetics has profoundly changed our understanding of the mechanisms of both normal and abnormal development of the nervous system. These advances, combined with medical advances in neuroimaging, clinical neurophysiology, and neuropathologic tissue examination, provide a way to understand malformations of the nervous system. Thus, this chapter is not presented as a traditional list of various known neural malformations, each with their clinical, radiologic, and pathologic findings; nor is it intended as a tabulation of known genetic mutations and deletions associated with specific dysgeneses. It is presented as a modern approach to better understand malformations in the context of development, a new neuroembryology that integrates descriptive morphogenesis and genetic programming.
Neural development is better thought of as a set of simultaneously occurring processes than as a set of sequential steps.
Traditional classifications list a series of developmental processes as a linear sequence of events, one following another. Some processes are indeed step-wise, each dependent upon the preceding one. Among these are:
- Gastrulation
- Establishment of the body axes
- Curling of the primitive neuroepithelium or neural plate to form the neural tube
All of these processes occur before maturation of individual cells and structural organization of the neural tube.
Other neurodevelopmental processes occur late in embryonic development. Synaptogenesis can only occur after the formation of dendrites and dendritic spines, maturation of the neuronal membrane to form synapses, and the synthesis of neurotransmitters and their release at axonal terminals. Myelination of axons does not occur during the projection phase of axonal growth before the axon has reached its target and initiated synapse formation.
However, most processes occur simultaneously, such that they must be considered in relation to one another. Among these are:
- Neuroblast migration
- Maturation of the neuroblast membrane capable of developing a resting membrane potential
- Projection of the axonal growth cone from the migratory neuroblast
- Vascular perfusion of the brain
- The formation of glial cells in relation to capillaries to form a blood–brain barrier
Nevertheless, it is useful both for classification and understanding to list the developmental processes and then consider them individually. This classification must integrate traditional morphogenesis with an understanding of molecular genetic programming of nervous system development. Finally, the relation of nervous system development to the development of non-neural tissues, or neural induction, must also be considered along with development of non-neural cells and tissue within the brain, such as the vascular system for cerebral perfusion, microglia (resident macrophages), and the development of the immune system. Table 26-1 lists the developmental processes considered in the next sections.
|
The concept of germ layers, which give origin to various tissues, as primitive ectoderm, mesoderm, and endoderm, is a traditional and long-accepted basic concept in embryology, conceived in the 19th century and based upon the best embryologic morphologic observations of that time. This concept continued to be accepted until the advent of molecular genetics in the late 1980s and early 1990s, when this deeply ingrained idea began to seem artificial and arbitrary. Gene families are expressed in all three layers, both in initial stages of tissue differentiation and also in maturing and mature tissues. For example, the HOX gene family programs segmentation of the neural tube but also is essential in the development of mesodermal structures of the extremities, endochondral long bones and muscle, and some endocrine organs of endodermal derivation. Neural crest cells of ectodermal origin form many mesodermal structures.
Understanding the development of the brain requires a basic knowledge of ganglia, which are not simple brains, and the components of a simple brain. The differences between brains and ganglia are listed in Table 26-2. A scientific definition of a brain must exclude functional criteria, especially those involving intellect, language, problem-solving, and reasoning. To introduce unique human qualities, or even those of more complex animals, as criteria, removes the definition from the realm of objective science and misrepresents it as philosophy or religion.
| Brain | Ganglion |
|---|---|
| Cephalic site only | Variable sites in body |
| Serves entire body | Serves limited regions or segments |
| Bilobar with commissures | Alobar without commissures |
| Neurons form surface, axons form core | Homogeneous mixture of neurons and axons |
| Interneurons predominate | Interneurons sparse |
| Multisynaptic intrinsic circuits | Monosynaptic relays |
| Specialized local functions | No localized specialization of function |
In many systems of the body, such as the cardiovascular and pulmonary, the abrupt transition from an intrauterine to the extrauterine environment involves profound physiologic changes. The nervous system is not in this category, and the programmed rate of maturation of the central nervous system is not influenced by the time of birth, whether preterm or at term, inthe healthy infant. An infant born at 28 weeks gestation (3 months early) has the same neurological examination and EEG maturation and myelination pattern at postnatal age 3 months as a full-term neonate just born, unless hypoxemia or other pathologic conditions cause a delay in maturation. In assessing neurological maturation of preterm infants, conceptional age (gestational age at birth + chronological age since birth) always must be considered, including head circumference and size of the fontanelles. This is clinically important and makes it crucial that the developmental age of a child be considered when assessing his or her neurological function.
Various developmental genes of the nervous system are expressed as early as gastrulation, the stage of the primitive streak and the Henson node. Others appear later during neural plate development and especially after the neural tube is formed. With the establishment of bilateral symmetry and the formation of the neural tube, three major axes are identified: longitudinal, vertical, and horizontal (Figure 26-1). Genes that are expressed this early are expressed in a gradient along one or more of the three neural axes, with strongest expression at one site and progressively diminishing expression distal to that site. In each of the axes, one of two gradients is possible: in the longitudinal axis, the gradient may either be rostrocaudal or caudorostral; in the vertical axis, the gradient is either dorsoventral or ventrodorsal; in the horizontal axis, it is mediolateral or lateromedial. The bending of the neural tube with the formation of flexures does not alter the axes, which bend with the tube and follow its contour.
Nearly all developmental processes of the neural tube are genetically programmed. Many of the genes that appear at various stages to mediate specific processes are known2-4 and many more remain to be discovered.
A majority of cerebral malformations may be understood in the context of these axes and gradients. In general, up-regulation of a gene results in hypertrophy or duplication of structures at its strongest site of expression; down-regulation results in hypoplasia or noncleavage or midline “fusion” of structures. Many genes are antagonistic to others, and the down-regulation of one may lead to an appearance of up-regulation of the other, but this overexpression of one gene function is not due to increased synthesis of that gene’s product mRNA or protein. This relatively recent molecular genetic insight may be integrated with traditional descriptive morphogenesis to provide a new means of classifying CNS malformations.5
At 6 weeks gestation, the neural tube is fully closed, but the concentric zones of internal architecture are not yet fully developed. At the level of the telencephalon, only two zones are found: a (peri-) ventricular zone of undifferentiated neuroepithelium with cells still in the mitotic/proliferative stage and a marginal zone as a peripheral rim that has relatively few cells (Figure 26-2). The neuroepithelium is densely cellular, with most tightly packed with large nuclei and sparse cytoplasm and very little intercellular neuropil. No ependymal differentiation is present at the ventricular surface.6
Figure 26-2
Section of primitive human telencephalon at 6 weeks gestation. The ventricular surface is formed by undifferentiated neuroepithelium still in the mitotic phase, with most mitoses occurring at the margin of the ventricle (arrows). Ependyma is not yet differentiated. Peripheral to this neuroepithelial “ventricular zone” is a cell-sparse “marginal zone” in which future Cajal-Retzius and subplate neurons and a preplate plexus of neurites are found, but the first wave of radial migration has not yet occurred and the cortical plate is not yet formed. The primitive meninx (primordium of the leptomeninges) is at the surface of the telencephalon. Haematoxylin-eosin. X100.
By 8 weeks gestation, four concentric zones can be distinguished histologically (Figure 26-3):
The ventricular zone is still prominent, forming the ventricular wall and surface.
The subventricular zone, also known as the germinal matrix is peripheral to the ventricular zone. It should be noted that this is an imprecise but traditional term used by neuropathologists but not by strict neuroembryologists. Radial glial cells differentiate within the subventricular zone and send their long, slender process centrifugally to the pial surface, but do not have attachments to the ventricular surface as do neuroepithelial cells of the ventricular zone.
The intermediate zone lies peripheral to the subventricular zone and is the future white matter of the centrum semiovale.
The cortical plate forms from the migratory neuroblasts in the middle of the marginal zone, thus dividing the marginal zone into three layers:
- A superficial part that persists as the molecular layer, which will be layer 1 of the mature cortex
- A middle layer that is the cortical plate proper
- A deep layer between the cortical plate and the intermediate zone that is called the subplate zone
- A superficial part that persists as the molecular layer, which will be layer 1 of the mature cortex
With maturation of the cortical plate, the subplate zone becomes incorporated into layer 6; hence it is a transitory lamina.
Figure 26-3
Section of human telencephalon at 8 weeks gestation. Four layers of the parenchyma are now defined. The innermost periventricular layer is the “ventricular zone” (vent z). Just distal to it is the “subventricular zone” (subvent z), consisting of postmitotic, premigratory neuroblasts and glioblasts and the site of the radial glial cell bodies. The middle layer is called the “intermediate zone” (int z), through which radial glial fibres and migratory neuroblasts and glioblasts pass; it is the future subcortical white matter. The previous marginal zone is now divided into thirds by the formation of the cortical plate (cort plate) in the middle of this zone by neuroblasts that have completed their radial migration. The peripheral part of the original marginal zone becomes the “molecular layer” (mol) or layer 1 of the mature, laminated neocortex. The area deep to the cortical plate from the original marginal zone becomes the “subplate layer” (subp), a transitory fetal layer that later becomes incorporated into the deep cortical laminae and loses its distinction.
A fundamental principle of development is the overproduction and subsequent reduction of excessive cells, axonal collaterals, dendritic spines, and synapses. Redundancy of neuroblasts in every part of the brain and spinal cord, mediated by apoptosis, may be modulated by the need for, and provides for the potential repair or replacement of, cells lost early in gestation by physical, metabolic, or toxic injury to the developing brain.
During development, two cellular arrangements are present throughout the neuraxis. A nucleus is a cluster of the same type of neurons for a common, though not necessarily identical, purpose. Cranial nerve nuclei are a good example. Most of the thalamus and the basal telencephalic nuclei (ie, “basal ganglia”) are organized with nuclear architecture. Also, more than one type of neuron may be included in nuclei. Many nuclei include interneurons as well as primary sensory or motor neurons. Cortical architecture is a laminar arrangement of neurons of the same type in each layer, with synaptic connections between layers. Examples of cortical architecture are the cerebral neocortex, hippocampus (paleocortex), lateral geniculate body of the thalamus, superior colliculus (optic tectum of the rostral midbrain), cerebellar cortex, olfactory bulb, and retina. The cerebral cortical plate at mid-gestation is not yet a laminated structure, and has a fetal architecture that is more columnar than laminar, corresponding to the recently completed phase of neuroblast radial migration (Figure 26-4). In one type of focal cortical dysplasia, this columnar architecture persists (Figure 26-5).
Figure 26-4
Sections of cerebral cortex of human fetuses at (A) 20 weeks gestation, (B) 28 weeks, and (C) 38 weeks. The arrangement of cells within the cortical plate at 20 weeks is radial columnar more than horizontal laminar, and the immature neurons are tightly packed because they have little cytoplasm and the neuropil is very sparse. By 28 weeks there is distinctive histological layering of the cortex superimposed on the earlier fetal columnar architecture, but the distinctive six layers of the mature cortex are not evident until near-term. Note the prominent thin layer of cells at the surface of the brain at 20 weeks and more disperse at 28 weeks and only occasional persistent cells at 38 weeks; this is the subpial granular glial layer of Brun that is transitory and disappears with maturation. Haemotoxylin-eosin X100.
Figure 26-5
Cerebral cortex with abnormal persistent radial columnar architecture in a 6-week-old male infant born at 29 weeks gestation (conceptional age 35 weeks at death) with DiGeorge syndrome, genetically confirmed. This cortex is an example of maturational arrest. He also had absence of the anterior commissure and hypoplasia of the corpus callosum. Haematoxylin-eosin. X100.
Because the mammalian brain has cortices with laminar architecture, but a limited cranium in which the brain must fit, an increase in surface area without a corresponding volume increase is necessary. This is accomplished in the mammalian brain with folding of the cortex to form gyri and sulci in the cerebrum and folia in the cerebellum. Fissures and sulci of the cerebral cortex are similar but not identical because they form differently (see section “Fissures, Sulci, and Gyri of the Cortex”).
Almost all primary sensory systems—somatosensory, proprioceptive, auditory, vestibular, visual, and taste—have synapses in the thalamus for modulation before relay to the sensory cortices. The notable exception is the olfactory system, in which olfactory nerve impulses are received in the olfactory bulbs and relayed via mitral and tufted neurons directly to neocortex, the entorhinal cortex (parahippocampal gyrus); local circuitry within the olfactory bulb functions as its own thalamus.7
Gastrulation occurs at 16 days postconception. The formation of the Henson node and primitive streak establish bilateral symmetry as the basic body plan of all vertebrates, and the three axes of the body and of the CNS:
Longitudinal (cephalic and caudal ends
Vertical (dorsal and ventral surfaces)
Horizontal (medial and lateral in relation to the midline of the longitudinal axis)
Gastrulation also is the “birthday” of the nervous system, as it is the earliest time when neuroepithelium can be distinguished from other tissues.
Neurulation is the bending of the neural plate to form a tube and changes the orientation of surfaces and edges of the neural plate. The dorsal surface becomes the inner surface facing the lumen of the central canal and ventricles; it is lined by neuroepithelium and later by ependyma. The lateral margin of the neural plate closes the tube in the dorsal midline and neural crest arises from most of this edge.
Closure of the neural tube begins in the cervical region and progresses rostrally and caudally. The anterior neuropore closes at 24 days, and the posterior at 28 days. There are multiple sites of closure in the position of the anterior and posterior neuropore.
Secondary neurulation is the formation of the most caudal sacral segments of spinal cord, posterior to the site of closure of the posterior neuropore. This part of the spinal cord does not develop as a folding of the neural plate, but rather is a solid cone of neuroepithelium with an ependymal central canal forming within its core.
Induction is the influence of one embryonic tissue upon another, each of which differentiates into different mature tissues. Neural crest cells separate from the dorsal midline of the neural tube at the time of its closure. The incipient neural crest or progenitor cells develop at the lateral margin of the neural plate. Neural crest arises from three sites of the neural tube and each migrates to give rise to specific structures (Table 26-3):
Prosencephalic neural crest is generated in the dorsal part of the lamina terminalis, the same place that gives origin to the bridge for the corpus callosum. It migrates rostrally as a thin vertical sheet of cells in the midline to the nose and forehead, and forms the intercanthal ligament between the embryonic orbit.
Mesencephalic neural crest arises from the dorsal midline of the mesencephalic neuromere and rhombomeres r1 and r2. It migrates as streams of cells in the horizontal plane to form most of the craniofacial structure, including the orbits, all cartilage and membranous bone of the face and cranial vault (but not endochondral bone of the cranial base, basioccipital, exoccipital and supraoccipital bones), dura mater including falx and tentorium, leptomeninges over the brain (but not over the spinal cord), the globe of the eye (except the retina, optic nerve, choroid, lens, and cornea), blood vessels, nerve sheaths (including Schwann cells, but not axons), and melanocytes.
Rhombencephalic neural crest arises from rhombomeres r3 through r8 including the spinal cord. It migrates as blocks of cells in streams and forms most of the peripheral autonomic nervous system (including sympathetic chain and parasympathetic ganglia in GI tract), chromaffin tissue (adrenal medulla; carotid body), nerve sheaths, dorsal root ganglia, stria vasculosa over the hair cells of the cochlea, melanocytes, and adipocytes.
| Type of Neural Crest | Origin | Migration | Structures Generated |
|---|---|---|---|
| Prosencephalic | Dorsal part of lamina terminalis | Rostrally as thin sheet of cells to midline of face | Intercanthal ligament |
| Mesencephalic | Dorsal midline of mesencephalic neuromere and rhombomeres 1 and 2 | As a stream of cells in the horizontal plane | Most of craniofacial structures: orbits, cartilage, and bone of face; dura mater, leptomeninges over brain; parts of the eye globe; blood vessels and nerve sheaths |
| Rhomboencephalic | Rhombomeres 3-8 | As blocks of cells in streams | Most of the peripheral and autonomic nervous system, nerve sheaths, dorsal root ganglia, stria vasculosa over the hair cells of the cochlea; melanocytes adipocytes |
Migratory neural crest cells do not differentiate into their mature identity until they reach their terminal site. Abnormalities of neural crest, known in human medicine as “neurocristopathies,” include many neurocutaneous syndromes and aganglionic megacolon (Hirschsprung disease). The mesencephalic origin of craniofacial structures also explains hereditary neurosensory deafness and craniofacial dysmorphisms also associated with many cerebral malformations.8
Segmentation of the neural tube or the formation of transitory physical compartments (neuromeres) restricts the longitudinal movement of cells during development.9 Segmentation is mediated by a group of genes called homeoboxes, which are restricted DNA sequences composed of exactly 183 DNA base pairs. Homeobox genes encode a class of proteins that share a common or very similar 60-amino acid motif, the hom-eodomain.10 Mammalian homeoboxes are identical to the segment polarity genes of invertebrates, and program the embryonic segmentation of the vertebrate neural tube.11 For example, HOX genes are essential for limb development and the anterior and posterior orientation of the arms and legs. The human EGR2 (Krox-20 in the mouse) is expressed only in rhombomeres r3 and r5 in the neural tube, and is an essential gene for myelination in the peripheral nervous system (PNS).12
Neural development is redundant and generates more neurons than are needed. There is a subsequent reduction of this abundance via the mechanism of apoptosis (programmed cell death). Most mitoses occur in the neuroepithelium at the ventricular wall (m-phase).13,14 Neuroepithelial cells have long radial processes that span the neural tube, with end feet at the ventricular surface and the pial surface; nuclei travel to and fro within this thin cytoplasmic extension. When distal to the ventricular wall, the s-phase enables replication of DNA in preparation for the next mitosis.
If the mitotic spindle is oriented perpendicular to the ventricular surface (symmetrical mitosis), the two daughter cells each reenter the mitotic cycle. If the mitotic spindle is parallel to the ventricular wall, the cell next to the ventricle reenters the mitotic cycle and the other has completed its last mitosis and prepares for migration as a neuroblast. In the human, 33 mitotic cycles produce all the neurons of the cerebral cortex.15
There are 50% more motor neuroblasts produced in the spinal cord than needed, and this number is later reduced through apoptosis.16 The Bcl-2 family of proteins plays an integral role in regulating the time and rate of apoptosis,17 which usually occurs in undifferentiated or only incipiently differentiated neuroblasts. However, a second phase of apoptosis may involve more mature neurons during late development. Hormones and growth factors may influence the rate of apoptosis. This process is more complex than most developmental processes because there is no single gene or gene combination that intiates the cascade of events comprising apoptosis, which is programmed into every cell, but only expressed when trophic factors secreted by nearby cells remove the inhibitors of this process that normally preserve metabolic integrity.
Until recently, it was thought that the mature brain has no capacity to generate new neurons; however, potentially neurogenic stem cells within the brain have now been identified in the periventricular regions. These are most numerous in the region of the dentate gyrus of the hippocampus and in the olfactory bulb. In fact, there is a constant turnover of some neurons in both locations. Primary olfactory receptor neurons are continuously renewed, but interneurons (granule cells) in the olfactory bulb are also regenerated.18,19
