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Chapter 9

Chapter 9.pdf

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University of Toronto St. George
Junchul Kim

Chapter 9 Development of the Nervous System • Brain is a plastic (changeable) living organ • Case Study) Genie: raised in abnormal environment • demonstrates critical role of experience in neurodevelopment 9.1) Phases of Neurodevelopment • Zygote (single cell formed by ovum-sperm) divides to form daughter cells • Cells must: differentiate (muscle cells, glial cells etc); locate/form particular structures; establish appropriate functions • 5 Phases of Neurons: • 1) Induction of Neural plate • 2) Neural proliferation • 3) Migration and aggregation • 4) Axon growth & Synapse Formation • 5) Neuron death & Synapse rearrangement • Fertilized eggs are totipotent (able to develop into any class of cell in the body); then begin to specialize (~4 days into embryological development) • pluripotent- ability to develop into many but not all classes of cells • multipotent- ability to develop into limited number of types • unipotent- ability to develop into one type of cell • most developing cells eventually become unipotent (ex. bipolar neurons) Induction of the Neural Plate • 3 weeks after conception, tissue for NS becomes recognizable as neural plate(patch of ectodermal tissue on dorsal surface of embryo) • ectoderm is the outermost of 3 layers of embryonic cells: ectoderm, mesoderm, endoderm • Development of neural plate= first major stage of neurodevelopment (vertebrates) Development is INDUCED by chemical signals from mesoderm layer(“organizer”) • • Cells of neural plate referred to as stem cells • Stem Cells- cells that meet 2 criteria: 1) unlimited capacity for self-renewal 2) pluripotent • Stem cells-unlimited capacity for self renewal because it divides into 2 different daughter cells: body cell & another stem cell In theory, stem cells can keep dividing forever but eventually errors accumulate • • Growing neural plate folds to form neural groove • Lips of neural groove form neural tube: cells specified as they develop into neural tube • Inside of neural tube becomes cerebral ventricles & spinal canal • ~40 days after conception- 3 swelling become visible at anterior neural tube: These swellings develop into forebrain, midbrain, hindbrain • Neural Proliferation • Cells of neural tube proliferate (increase greatly in number) • Proliferation occurs mostly in ventricular zone- adjacent to ventricle(fluid filled center of tube) Cells in different parts of neutral tube proliferate in particular sequence responsible for pattern of • swelling/folding • Pattern of proliferation partially controlled by chemical signs • From 2 organizer areas in neural tube: floor plate & roof plate • floor plate runs along midline of ventral surface of tube roof plate runs along midline of dorsal surface of tube • Migration & Aggregation • Migration- Once cells are created via cell division in ventricular zone of neural tube, they migrate to appropriate target location Cells still in immature form, lack processes that characterize mature neurons (axons/dendrites) • • 2 Major factors govern migration in neural tube: time & location Chapter 9 Development of the Nervous System • In given region of tube, subtypes of neurons arise on predictable schedule then migrate to their prescribed destinations • 2 Types of Cell Migration: Radial Migration & Tangential Migration • Radial: proceeds from ventricular zone in straight line outward toward outer wall of tube • Tangential: occurs at right angle to radial- parallel to tube’s walls • Most cells engage in both types to get to their target destination • 2 Methods of Migrating: Somal translocation & Glia Mediated Migration • Somal: • extension grows from developing cell in the direction of migration and explore immediate environment for attractive/repulsive cues • Then cell body itself moves into the extending process; trailing processes are retracted • Glia-mediated: • once neural proliferation starts and walls of neural tube start thickening, temporary network of glial cells (radial glial cells) appear in developing neural tube • immature neurons move out from central canal via radial glial cells • Many radial glial cells eventually develop into neurons • Some are pluripotent but many specialize • Research on migration ~cortex: • Migrating cells progress from deeper to more superficial layers in waves • Inside-out pattern: radial pattern of cortical development- each wave of cortical cells migrate through already formed lower layers of cortex before reaching its destination • Many cortical cells engage in long tangential migrations to reach their final destinations • Patterns of proliferation and migration- different for different areas of cortex • Neural crest- structure situated dorsal to neural tube • Formed from cells that break off from neural tube as it is being formed • Neural crest cells develop into neurons and glial cells of peripheral NS thus many migrate over long distance Many chemicals guide various classes of migrating neurons by either attracting/repelling them • • Guidance molecules play critical role in neurodevelopment because brain cannot function normally unless each class of developing neurons arrive at correct location • Aggregation: Once developing neurons have migrated,they must align themselves with other developing neuron in same area to form the structures of the NS • Both migration & aggregation thought to be mediated by cell-adhesion molecules (CAMs) located on surface of neurons and other cells • CAMS- able to recognize molecules on other cells and adhere to them • Elimination of just one type of CAM-detrimental to brain development Gap junctions(between adjacent cells) play a role in migration and aggregation (brain development) • • Recall: Gap junctions are points of communication (neurons and glia) • Gaps are bridged by narrow tubes (connexins) through which cells exchange cytoplasm Axon Growth & Synapse Formation Axon Growth: After migration-aggregation into neural structures, axons and dendrites begin to grow • from them (projects grow to appropriate targets) • At each growing tip of axon/dendrite is growth cone- extends/retracts fingerlike cytoplasmic extensions called filopodia • Most growth cones reach their correct targets Sperry) cut optic nerves of frogs, rotated their eyeballs 180degrees and waited for axons of retinal • ganglion cells to regenerate • Once regeneration was complete, tested its visual capacities • Results: RGC grew back to same point of optic tectum to which it had originally been connected • Chemoaffinity hypothesis of axonal development- each postsynaptic surface in the NS releases specific chemical label. Each growing axon is attracted by the label to its postsynaptic target during neural development/regeneration Chapter 9 Development of the Nervous System • Fails to explain how some growing axons follow same route to reach their target in every member of species, rather than growing directly to it • Revised Hypothesis: growing axon is not attached to its target by single attractant (released by target) rather: growth cones seem to be influenced by series of chemical signals along the route • These guidance molecules are similar to those that guide neural migration in the sense that some attract and others repel the growing axons • Guidance molecules are not the only signals that guide growing axons to their targets • Other signals come from adjacent growing axons • Pioneer growth cones- first growth cones to travel along particular route in developing NS- presumed to follow correct trail by interacting with guidance molecules along the route • Then, subsequent growth cones follow the routes blazed by pioneers • Fasciculation- developing axons tend to grow along paths established by preceding axon • Much of axonal development involves growing from one topographic array of neuron to another • Neurons on one array project to another, maintaing same topographic relation they had on the first array. Ex) Topographic map of retina is maintained on the optic tectum • Topographic gradient hypothesis- accurate axonal growth involving topographic mapping in the developing brain • Axons growing from one topographic
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