PSYC 2410 Chapter Notes - Chapter 9: Radial Glial Cell, Retinal Ganglion Cell, Neural Tube
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19 Apr 2012
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Chapter 9: Development of the Nervous System - From Fertilized Egg to You
The brain is a plastic (changeable) living organ that continuously changes in response
to its genetic program and environment.
Neurodevelopment = neural development
•Early experiences play a critical role in the development of the human cerebral
•The Case of Genie: Limited movement and ability to learn and grow properly as
an infant resulted in unfixable abnormal behaviour for the rest of her life.
9.1 Phases of Neurodevelopment
−Begins as a zygote (fertilized egg) which divides into two daughter cells and so
forth
−Cells must differentiate into specific types of cells, migrate and properly align
in specific spots and establish appropriate functional relations with other cells
−this is accomplished by 5 phases: (1) induction of the neural plate, (2) neural
proliferation, (3) migration and aggregation, (4) axon growth and synapse
formation, and (5) neuron death and synapse rearrangement
Induction of the Neural Plate
−Neural Plate: small patch of ectodermal tissue on the dorsal surface of the
developing embryo that appears 3 weeks after conception
−Development is induced by chemical signals from the mesodermal layer
(organizer)
−The earliest cells of the human embryo are totipotent (able to develop into any
cell type). When the neural plate develops, some cells become multipotent
(able to develop into most types in the mature nervous system, but not other
kinds of cells)
−Neural plate cells are embryonic stem cells - (1) unlimited capacity for self-
renewal, (2) ability to develop into different types of mature cells - some cells
lose their ability to become any cell as the neural plate forms the neural tube
− Unlimited renewal is a result of one daughter cell becoming a body cell and
the other a stem cell following division, however, eventually errors accumulate
during mitosis and the process is disrupted
−The neural plate folds to form the neural grove, where the groove's lips fuse
forming the neural tube where the inside eventually becomes the cerebral
ventricles and the spinal canal. By 40 days following conception, 3 swellings
are visible indicating the forebrain, midbrain and hindbrain
Neural Proliferation
−Beginning once the neural tube is formed, the cells of the tube increase
greatly in number
−Does not occur equally nor simultaneously - most division occurs in the
ventricular zone (region adjacent to the ventricle; Ventricle - fluid-filled center
of the tube)
−Proliferation pattern mediated from two organizer areas of the tube - the floor
plate (along the midline of the anterior surface) and the roof plate (along the
midline of the dorsal surface)
Migration and Aggregation
−Cells of immature form since they lack processes of mature neurons, migrate
to appropriate locations
−Time and location govern migration; in a given region of the tube, neuron
subtypes arise on precise and predictable schedule and then migrate together
to prescribed location
−Cells migrate in two ways: (1) Radial migration: from the ventricular zone in a
straight line outward toward the outer wall of the tube, or (2) Tangential
migration: at a right angle to radial migration (parallel to the tube's wall) - most
cells engage in both
−Two methods by which cells migrate: (1) Somal translocation: an extension
grows from the developing cell in the general direction of the migration where
the cell body moves along the process and it trailing processes retract, or (2)
Glia-mediated migration: walls of the tube thicken with temporary network of
glial cells (radial glial cells) in which most cells engage in radial migration along
this network
−Cortical development occurs in an inside-out pattern where each wave of
cortical cells migrate through already formed lower layers of the cortex to
reach its destination in radial pattern
−Neural Crest: dorsal to the tube, formed from cells that break off from the tube
as it is being formed
−Chemicals guide migrating neuron by attracting or repelling them - some of
these chemicals are released by glial cells
−Aggregation is when these now migrating neurons align themselves
appropriately
−Migration and aggregation mediated via cell-adhesion molecules (CAMs)
located on surfaces of neurons and other cells - they are believed to be factors
in some neurological disorders
−Gap junctions also play a role where the connexins (tubes) allow for exchange
of cytoplasm
Axon Growth and Synapse Formation
−Growth cone: amoebalike structure at the end of each growing tip of an
axon or dendrite, which extends and retracts filopodia extensions to find the
correct route
−Roger Sperry: cut optic nerves in frogs, rotated their eyeballs, and
waited for retinal ganglion cell axons to regenerate (only occurs in frogs).
Provided evidence that each retinal ganglion cell grew back to the same point
of the optic tectum (mammals - superior colliculus). Thus produced the
chemoaffinity hypothesis of axon development: each postsynaptic surface in
the NS releases a specific chemical where each growing axon is attracted to as
a target during neural development and regeneration
−Revised hypothesis: growth cones are influenced by a series of chemical
signals along the pathway, not just one
−Pioneer growth cones: first growth cone to travel along a particular route
in a developing NS - subsequent growth ones follow
−Fasciculation: the tendency of developing axons to grow along the paths
established by preceding axons
−Axonal development is in a topical array to minimize the volume of
neural connections. Topographic gradient 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 that they're arranged on the original
surface
−The ephrin family is strongest in guiding molecules
−Synaptogenesis is the formation of new synapses which deends on the
presence of glial cells, namely astrocytes; Developing neurons need high levels
of cholesterol during synapse formation, provided by astrocytes. They also
process, transfer, and store information supplied by neurons
Neuron Death and Synapse Rearrangement
−About 50% more neurons than required are produced, thus cell death is an
active process
−Necrosis: passive cell death; Apoptosis: active cell death - Necrotic cells spill
their contents into extracellular fluid potentially causing inflammation whereas
apoptotic cells cleave internal structures and package them in membranes
(which attract scavenger microglia) prior to breaking apart. However, blocking
apoptosis can result in cancer