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Chapter 9

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 9: Development of the Nervous System • The brain is a plastic (changeable), living organ that continuously changes in response to its genetic programs and environment • Three general ideas are emphasized: 1. The amazing nature of neurodevelopment 2. The important role of experience in neurodevelopment 3. The dire consequences of neurodevelopmental errors • Because there is so little variation in most people’s early experience, the critical role of experience in human cerebral and psychological development is not always obvious PHASES OF NEURODEVELOPMENT • In the beginning, there is a zygote, a single cell formed by the amalgamation of an ovum and a sperm • The zygote divides to form two daughter cells • Three things other than cell multiplication occur: • First, cells must differentiate some must become muscle cells, some must become multipolar neurons, and so on • Second, cells must make their way to appropriate sites and align themselves with the cells around them to form particular structures • Third, cells must establish appropriate functional relations with other cells • Developing neurons accomplish this in five phases: • Induction of the Neural Plate • Three weeks after conception, the tissue that is destined to develop into the human nervous system becomes recognizable as the neural plate • Neural plate is a small patch of ectodermal tissue on the dorsal surface of the developing embryo • The ectoderm is the outermost of the three layers of embryonic cells: ectoderm, mesoderm, and endoderm • The development of the neural plate seems to be induced by chemical signals from an area of the underlying mesoderm layer - an area that is consequently referred to as an organizer • Tissue taken from the dorsal mesoderm of one embryo (the donor) and implanted beneath the ventral ectoderm of another embryo (the host) induces the development of an extra neural plate on the ventral surface of the host • The earliest cells of the human embryo are totipotent - that is, they have the ability to develop into any type of cell in the body if transplanted to the appropriate site • As the embryo develops, the destiny of various cells becomes more specified • When the neural plate develops its cells lose some of their potential to become different kinds of cells • Each cell of the early neural plate still has the potential to develop into most types of mature nervous system cell, but it cannot normally develop into other kinds of cells are said to be multipotent • The cells of the neural plate are often referred to an embryonic stem cells • Stem cells are cells that meet two specific criteria •They have a seemingly unlimited capacity for self-renewal if maintained in an appropriate cell culture •They have the ability to develop into different types of mature cells Chapter 9: Development of the Nervous System • Because these cells still have the capacity for unlimited self-renewal and are still multipotent, these cells are termed glial stem cells and neural stem cells respectively • This capacity results from the fact that when a stem cell divides, two different daughter cells are created: one that eventually develops into some type of body cell and one that develops into another stem cell • Eventually errors accumulate, which can disrupt the process, that is why stem cell cultures do not last forever • The neural plate folds to form the neural groove, and then the lips of the neural groove fuse to form the neural tube • The inside of the neural tube eventually becomes the cerebral ventricles and spinal cord • Neural Proliferation • Most cell division in the neural tube occurs in the ventricular zone - the region adjacent to the ventricle • In each species, the cells in different parts of the neural tube proliferate in a particular sequence that is responsible for the pattern of swelling and folding that gives the brain of each member of that species the characteristic shape • The complex pattern of proliferation is in part controlled by chemical signals from two organizer areas in the neural tube: the floor plate, which runs along the midline of the anterior surface of the tube, and the roof plate, which runs along the midline of the dorsal surface of the tube • Migration and Aggregation • Migration • Once cells have been created through cell division in the ventricular zone of the neural tube, they migrate to the appropriate target location • During this period of migration, the cells are still in an immature form, lacking the processes that characterize mature neurons • Two major factors govern migration in the developing neural tube: time and location • Cell migration in the developing neural tube: •Radial Migration - proceeds form the ventricular zone in a straight line outward toward the outer wall of the tube •Tangential Migration - occurs at a right angle to radial migration, that is, parallel to the tubes walls • Most cells engage in both radial and tangential migration to get form their point of origin in the ventricular zone to their target destination • There are two methods by which developing cells migrate, one is somal translocation • In somal translocation, an extension grows from the developing cell in the general direction of the migration; the extension seems to explore the immediate environment for attractive and repulsive cues as it grows • Then, the cell body itself moves into and along the extending process, and trailing processes are retracted Chapter 9: Development of the Nervous System •Glial-mediated migration, once the period of neural proliferation is well underway and the walls of the neural tube are thickening, a temporary network of glial cells called radial glial cells, appears in the developing neural tube •At this point, most cells engaging in radial migration do so by moving along the radial glial network •Orderly waves of migrating cells, progressing from deeper to more superficial layers, referred to as an inside-out pattern •Cortical migration patterns are more complex than was first thought: many cortical cells engage in long tangential migrations to reach their final destinations, and the patterns of proliferation and migration are different for different areas of the cortex •The neural crest is a structure that is situated just dorsal to the neural tube •It is formed from cells that break off form the neural tube as it is being formed •Neural crest cells develop into the neurons and glial cells of the peripheral nervous system, and thus many of them must migrate over considerable distances •Numerous chemicals that guide migrating neurons by either attracting and repelling them •Some of these guidance molecules are released by glial cells • Aggregation •Once developing neurons have migrated they must align themselves with other developing neurons that have migrated to the same area to form the structures of the nervous system called aggregation •Migration and aggregation are though to be mediated by cell-adhesion molecules (CAMs), which are located on the surface of neurons and other cells •Cell-adhesion molecules have the ability to recognize on other cells and adhere to them •Elimination of just one type of CAM in a knockout mouse has been shown to have a devastating effect on brain development •This finding suggests that abnormalities of CAM function may be causal factors in some neurological disorders •Evidence that gap junctions play a role in migration and aggregation • Axon Growth and Synapse Formation • Axon Growth •Once neurons have migrated to their appropriate positions and aggregated into neural structures axons and dendrites begin to grow from them •At each growing tip of an axon or dendrite is an amoebalike structure called growth cone, which extends and retracts fingerlike cytoplasmic extensions called filopodia, as if searching for the correct route •Chemoaffinity hypothesis of axonal development hypothesized that each postsynaptic surface in the nervous system releases a specific chemical label and that each growing axon is attracted by the label to its postsynaptic target during both neural development and regeneration •The chemoaffinity hypothesis fails to account for the discovery that some growing axons follow the same circuitous route to reach their target in every member of a species, rather than growing directly to it Chapter 9: Development of the Nervous System • Guidance molecules are not the only signals that guide growing axons to their targets • Pioneer growth cones - the first growth cones to travel along a particular route in a developing nervous system - are presumed to follow the correct trail by interacting with guidance molecules along the route • Subsequent growth cones embarking on the same journey follow the routes blazed by the pioneers • The tendency of developing axons to grow along the paths established by preceding axons is called fasiculation • Much of the axonal development in complex nervous systems involves growth from one topographic array of neurons to another • In most species, the synaptic connections between retina and optic tectum are established long before either reaches full size • Then, as the retinas and the optic tectum grow at different rates, the initial synaptic connections shift to other tectal neurons so that each retina is precisely mapped onto the tectum, regardless of their relative sizes • The topographic-gradient hypothesis has been proposed to explain accurate axonal growth involving topographic mapping in the developing brain • According to this hypothesis, axons growing from one topographic surface to another are guided to specific targets that are arranged on the terminal surface in the same way as the axons’ cell bodies are arranged on the original surface • The key part of this hypothesis is that the growing axons are guided to their destinations by two intersecting signal gradients • Synapse Formation • A single neuron can grow an axon on its own, but it takes coordinated activity in at least two neurons to create a synapse between them • Synaptogenesis (the formation of new synapses) depends on the presence of glial cells particularly astrocytes • Contribution of astrocytes to synaptogenesis emphasized a nutritional role: developing neurons need high levels of cholesterol during synapse formation and the extra cholesterol is supplied by astrocytes • Current evidence suggests that astrocytes play a much more extensive role in synaptogenesis by processing, transferring, and storing information supplied by neurons • In-vitro studies suggest that any type of neuron will form synapses with any other type • However, once established, synapses that do not function appropriately tend to be eliminated • Neuron Death and Synapse Rearrangement • Neuron Death • Many more neurons - about 50% more - are produced than are required and large scale neuron death occurs in waves in various parts of the brain throughout development • Cell death during development is usually active • Genetic programs inside neurons are triggered and cause them to actively commit suicide Chapter 9: Development of the Nervous System • Passive cell death is called necrosis; active cell death is called apoptosis • Apoptosis is safer than necrosis • Necrotic cells break apart and spill their contents into extracellular fluid, and the consequence is potentially harmful inflammation • Apoptotic cell death, DNA and other internal structures are cleaved apart • These membranes contain molecules that attract scavenger microglia and other molecules that prevent inflammation • If genetic programs for apoptotic cell death are blocked, the consequence can be cancer; if the programs are inappropriately activated, the consequence can be neurodegenerative disease • There appear to be two kinds of triggers • First, some developing neurons appear to be genetically programmed for an early death - once they have fulfilled their functions, groups of neurons die together, in the absence of any obvious external stimulus • Second, some developing neurons seem to die because they fail to obtain the life-preserving chemicals that are supplied by their targets • Evidence that life-preserving chemicals are supplied to developing neurons by their postsynaptic targets comes from two kinds of observations: grafting an extra target structure to an embryo before the period of synaptogenesis reduces the death of neurons growing into the area, and destroying some of the neurons growing into an area before the period of cell death increases the survival rate of the remaining neurons • Several life-preserving chemicals that are supplied to developing neurons by their targets have been identified • Neurotrophins class of these chemicals • Nerve Growth Factor (NGF) was the first neurotrophin to be isolated • The neurotrophins promote the growth and survival of neurons, function as axon guidance molecules, and stimulate synaptogenesis • Synapse Rearrangement • As they die, the space they leave vacant on postsynaptic membranes is filled by the sprouting axon terminals of surviving neurons • Thus, cell death results in a massive rearrangement of synaptic connections • This phase of synapse rearrangement tends to focus the output of each neuron on a smaller number of postsynaptic cells, thus increasing the selectivity of transmission POSTNATAL CEREBRAL DEVELOPMENT IN HUMAN INFANTS • The human brain develops more slowly than those
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