Mustafa Sahin
The human brain consists of 100 billion neurons and over 100 trillion synapses. The ability of the neurons to form precise connections with one another is critical for the proper functioning of the nervous system. Malformation or dysfunction affecting a subset of cells and/or connections in the nervous system can manifest in diverse ways through different neurologic symptoms. Examining a patient who presents with a neurologic problem should lead to the understanding of two main aspects of the dysfunction: What part(s) of the nervous system is affected? What is the nature of the dysfunction? This approach requires an understanding of the localization of function within the nervous system. From a pediatric neurology perspective, this may be best achieved by reviewing the development of nervous system in early life.
Work over the last few decades has shed light on the remarkable cellular and molecular bases of neural development. While much more needs to be elucidated, this chapter will highlight some of the major advances in our understanding of normal human brain development. The chapter is organized into 3 parts. The first part will review the process of neural tube formation and regional patterning that determines the major subdivisions of the human central nervous system. The second part will provide an overview of the specific steps required for proper wiring of neurons in the cerebral cortex, the part of the brain responsible for most of human behavior. The final part will review the major subdivisions of the human central nervous system and their most prominent functions.
Normal neural development can be considered in several steps. In chronological order, these include: neurulation, regionalization, neurogenesis, migration, differentiation, apoptosis, axon guidance, synapse formation/pruning, and myelination. In these 9 steps, cells go from unspecified ectodermal constituents to being fine-tuned at the synapse level.
PATTERNING OF THE BRAIN
NEURULATION
The nervous system develops from a region of the ectoderm called the neural plate. Underlying notochord and adjacent mesoderm induce the overlying ectoderm to differentiate into the neural plate during the third week of gestation. Induction is followed by invagination of the neural plate along its central axis to form a neural groove that has neural folds on each side. By the end of the third week, the neural folds start to fuse at the hindbrain/cervical boundary and form the neural tube, which gives rise to the brain and the spinal cord.1 Fusion then proceeds from this level in both rostral and caudal directions. Closure of the neural tube is finally completed by the closure of the neuropore at the sacral region at day 26 to 28. As the neural folds fuse, some neuroectodermal cells detach from the folds and migrate ventrally, forming the neural crest. The neural crest gives rise to the dorsal root ganglia, autonomic nervous system ganglia, Schwann cells, meninges, adrenal medulla, and several key skeletal and muscular components of the face.
REGIONALIZATION
During this stage, segmentation and positional identity are determined both in the rostral-caudal axis and also in the dorsal-ventral axis. At the rostral end of the neural tube, 3 brain areas are formed: the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). The prosencephalon further develops into the telencephalon (cortex, basal ganglia, olfactory system) and diencephalon (thalamus and hypothalamus) and the rhombencephalon becomes subdivided into the metencephalon (pons and cerebellum) and myelencephalon (medulla). The telencephalon evaginates to form the 2 cerebral hemispheres. Each hemisphere is further divided into subdomains, such as the functional and anatomical subdivisions of the neocortex (motor, somatosensory, auditory, visual, etc). Programs of regional identity and morphogenesis in the telencephalon are directed in part by various molecules secreted by at least 3 patterning centers. Ventral telencephalon and hypothalamus express Sonic hedgehog (Shh). Along the dorsal midline, secreted molecules such as bone morphogenetic protein and Wnt families control patterning of the hippocampus and neocortex. At the anterior margin of the telencephalon, Fgf8 and Fgf17 promote telencephalic growth and rostral regionalization. The patterning centers operate in part through generating graded expression of the transcription factors that control cellular programs for proliferation, migration, and connectivity. Several genes encoding transcription factors, including Foxg1 (BF1), COUPTF1, Emx2, Lef1, Lhx2, Sp8, and Pax6, are expressed in gradients along the dorsoventral and rostrocaudal axes of the cerebral cortex.2,3
NEUROGENESIS
During early development of the cerebral hemispheres, a progressive restriction of cell fate occurs, with stem cells playing a major role in differentiation. Coincident with regionalization of the cerebral cortex, cortical neurons are generated from proliferating cells in the telencephalic ventricular zone. After their terminal mitotic division, young neurons migrate outward along radial glial fibers into the cortical plate to form the 6-layered neocortex. Quantitative studies of proliferation suggest that prior to neurogenesis, ventricular zone cells divide symmetrically to expand the progenitor pool. As neurogenesis starts, progenitors increasingly divide asymmetrically, and the daughters of asymmetric divisions can differentiate into neurons. As neurogenesis ends, progenitors begin to produce glia. Radial glia are present during neurogenesis and serve as neuronal precursors as well as guides for radial migration.
MIGRATION
Once born in the ventricular zone, neurons have to migrate to their final position within the nervous system. Excitatory (glutamatergic) neurons of the neocortex are generated in the dorsal ventricular zone and migrate outward along radial glia, assembling in an inside-out manner to establish the 6 layers that characterize the mature neocortex. In other words, layer VI and layer V cells migrate first. Layers II to IV migrate later and pass through the early generated cells to get to their final location in the neocortex. In contrast, both inhibitory (GABAergic) neurons and oligodendrocytes are generated in the germinal zone of the ventral telencephalon and follow several tangential migratory routes to their final destinations in the neocortex. On the whole, neurons born on the same birth date reach the same layer of cerebral neocortex, regardless of their site of origin and neurotransmitter type. Most neurons destined for the cerebral cortex migrate between 10th and 18th gestational weeks, and the full complement of neurons is essentially completed by week 20.
WIRING OF THE BRAIN
DIFFERENTIATION
The second half of the gestation is characterized by formation of connections between postmitotic neurons. After completion of migration, the neurons become polarized and axons and dendrites become discernible. Neurons are highly polarized cells, typically extending a single long axon to propagate action potentials and several shorter dendrites to receive synaptic input. The initial step in establishing this polarized structure is the specification of one of the neurites as the axon. This has been studied in detail using dissociated hippocampal neuron cultures and has been divided into several stages.4 Immediately after plating, neurons extend short lamellipodia, and then they extend short processes called neurites. Within 24 hours, one of the neurites begins to elongate much more rapidly than others and will eventually become the axon. Over the next few days, the remaining neurites transform into dendrites. Dendrites, axons, and synapses form at a tremendous rate and result in a rapid increase in brain size.
APOPTOSIS
Curiously, half of the neurons that were generated up until this point naturally die off, as another planned step in the normal development process. Cell death is mediated through an active and programmed cellular suicide, called apoptosis. It is not yet clear why the central nervous system overproduces cells in the first place. It is thought that apoptosis may represent the response of some neurons that cannot compete with others for the limited amount of target-derived trophic factors. Through this restrictive process, only the neurons that have established proper synaptic connections survive.
AXON GUIDANCE
Axons reach their final targets in the central nervous system by interacting with either attractive or repulsive cues.5 Some of these cues are secreted molecules that act at a distance and others are membrane-associated molecules that act upon contact between the axon and the guiding cells. Growth cones are specialized regions at the tips of growing axons and are vital for formation of the precise connections between neurons and their targets. An axon tip extending is flattened at its tip into a fan-shaped sheet (lamellipodia) interrupted with spikelike protrusions (filopodia), both of which are made up of the actin cytoskeleton. Navigating growth cones are guided toward their targets by a complex set of extracellular guidance molecules including, but not limited to, netrins, slits, semaphorins, and ephrins. According to studies on human neocortex using antibodies to markers of axonal growth, axons grow dramatically between 21 and 64 weeks after conception and reach their adultlike state by 17 postnatal months.6,7
SYNAPSE FORMATION AND PRUNING
The transmission of information between neurons occurs at specialized sites of contact called synapses. The majority of synapses in the vertebrate central nervous system are chemical synapses, which convert a change in the membrane potential of the presynaptic neuron into a chemical signal capable of altering the activity of the postsynaptic neuron. Chemical synapses are asymmetric cellular junctions comprised of presynaptic bouton, synaptic cleft, and postsynaptic reception apparatus. Synapse can be detected in the visual cortex as early as 28 weeks gestation. However, the synaptic density at this time is only 2% of adult value and by birth is only 17% of adult value. The greatest increases in synaptogenesis occur during 2.5 to 8 postnatal months, with the most rapid increases between 2 and 4 months. Similar to the overproduction of neurons, synapses are also overproduced.8,9 It is thought that excess numbers of synapses are first generated to establish the initial wiring pattern of the brain, but the formation of mature, precise neural circuits requires the selective elimination and pruning of inappropriate synapses.
MYELINATION
Myelin is the fatty sheath that insulates axons. It is produced and wrapped around axons by the Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. Recent use of diffusion tensor imaging has offered a sensitive, noninvasive window into myelination in the human brain and confirmed the previous findings from autopsy specimens.10 Primary sensory and motor areas are myelinated first in the brain starting at 30 weeks of gestation.11,12 There is rapid myelination within the first year of life; however, myelination in association areas occurs well after birth and proceeds over decades.
ANATOMY OF THE MATURE BRAIN
The spinal cord consists of segments, each of which is connected to the body via dorsal (sensory) and ventral (motor) roots. The anterior horn of the spinal cord contains the motor neurons, which send axons to the skeletal muscles and are responsible for the spinal reflexes. The white matter occupying the periphery of the cord includes various longitudinal fiber tracts such as the corticospinal tract, spinothalamic tract, posterior columns, and spinocerebellar tracts. The fiber tracts of the spinal cord continue in the medulla. In addition, the medulla contains several nuclei, such as the inferior olive and the lower cranial nerve nuclei. The pons contains nuclei of some of the cranial nerves (V, VI, VII), motor and sensory tracts, and the tracts connecting the cerebellum to the rest of the central nervous system (cerebellar peduncles). The midbrain contains nuclei of the cranial nerves III and IV, motor and sensory tracts, the tectum, substantia nigra, and the red nucleus. The tectum is involved in visual and auditory processing and the red nucleus and the substantia nigra are nuclei involved in motor circuits. The cerebellum is involved in balance, muscle tone, and locomotion, as well as higher cognitive functions such as speech.
The thalamus, the largest component of the diencephalon, contains the sensory relay nuclei that receive input from sensory systems and project to specific sensory areas of the neocortex. Other nuclei in the thalamus are involved in motor circuits connecting the striatum, neocortex, and cerebellum. The hypothalamus is the central controller of the autonomic and endocrine systems. It regulates sympathetic and parasympathetic systems and secretes regulatory hormones to influence the pituitary gland.
The telencephalon contains the neocortex, which is particularly enlarged during mammalian evolution. It is divided into specialized areas such as the primary motor, somatosensory, auditory, visual cortices, and many association areas that receive input from the primary sensory areas. A small but important portion of the telencephalon is made up of the olfactory cortex, hippocampus, and entorhinal cortex, which are involved in the limbic system. Finally, the striatum is a large gray matter structure that controls motor function and is implicated in the movement disorders and some psychiatric diseases.
Various disease mechanisms (genetic, infectious, vascular, etc) can affect different parts of the developing central nervous system, resulting in a plethora of neurologic and behavioral symptoms in children, and these symptoms and their management will be discussed in the upcoming chapters.