Neurobiology 303 -- Chapter 22 Outline

Development

developmental neurobiology -- the study of the developmental processes that
   underlie the formation of the nervous system
    considers questions concerning the origin of neurons as distinct cell
       types, the rules that determine the size, shape, and location of
       neural structures, and the mechanisms by which growing axons
       find the target cells upon which they synapse

every animal develops from a single fertilized egg (or zygote) and all cells
   are descendants of that egg
    lineage -- sequence of divisions from zygote that produces a given cell
    progeny -- all the cells that are generated by division of a given cell

there are two major types of development:
    determinate development (a.k.a. mosaic development) -- in which
       factors inside a cell mainly determine its progeny
    indeterminate development (a.k.a. regulated development) -- in which
       factors outside a cell mainly determine its progeny

determinate development:
    expect for echinoderms (e.g. starfish) all invertebrates undergo
       determinate development, in which cell lineage is determined
       primarily (if not exclusively) by the genome and cytoplasm of
       each individual cell
    as a consequence, if a cell dies during development, then none of the
       tissues that would have formed from the progeny of that cell
       can develop
    development of the nematode worm C. elegans is a well studied
       example of invertebrate development
    the entire lineage of each of the 959 cells in C. elegans is known!
    experiments have shown that there is no variability in this lineage
       during development
    also, C. elegans shows almost no ability to compensate for the
       experimental deletion of individual cells during development

indeterminate development:
    characteristic of echinoderms and chordates (including vertebrates)
    for indeterminate development, cell lineage is strongly determined by
       the interaction of the genome with epigenetic (non-genetic)
       factors extrinsic to the individual cell, such as molecules
       released by neighboring cells
    epigenetic factors interact with the cell's genome to determine its fate,
       through a process known as induction
    retinoic acid is an example of an epigenetic factor -- its role is to
       accelerate the differentiation of the posterior part of the neural
       tube (i.e. it makes the neural tube grow longer)
    because any cell's fate is the consequence of induction, removal of a
       few cells will not adversely affect development -- other cells
       will simply be induced to take over for the missing ones
    thus, lineage is probabilistic for indeterminate development
    for example, the Mauthner cells, which are unique, can originate from
       any of several progenitor cells, depending upon which one is
       induced first

in both determinate and indeterminate development, there is a specific
   pattern to the development of the nervous system
    in C. elegans, for example, the entire nervous system develops from
       an initial set of 12 nerve progenitor cells distributed along the
       length of the ventral body wall
    in insects, each segmental ganglion develops from 60 to 68
       neuroblasts, each of which gives rise to several cells known as
       ganglion mother cells, which in turn produce all of the neurons
       in each segmental ganglion
    in vertebrates, the nervous system develops from a developmental
       structure called the neural tube, which forms from a groove in
       the gastrula called the neural plate -- the anterior and posterior
       parts of the neural tube develop into the brain and spinal cord

at one time it was thought that cell division in the brain ceases before birth
    recent research has shown that cells in the hippocampus and probably
       elsewhere in cerebral cortex continue to divide and produce
       new neurons throughout life -- wonderful news!!!!!
    newly formed neural cells migrate away from the area in which they
       are born and move past cells that were born first -- thus the
       youngest neural cells are in the outermost layers of cortex

specific connections between neurons must be made in the developing brain
    birth and migration of neural cells is an important part of brain
       development
    an even more important part is the formation of specific connections
       between neurons in the developing brain
    no pathway or circuit in the brain can function properly without
       specific connections between the neurons that compose it

the most well-studied example of the formation of connections is the
   projections of retinal ganglion cells onto the tectum in frogs
    the tectum has a topographic representation of the external world
    topographic representation -- essentially a brain map; a brain structure
       in which neighboring locations in the world are represented by
       neurons that are neighbors in the brain structure
    because the tectum is topographically organized, each retinal ganglion
       cell must project to a particular part of the optic tectum
    studies by Roger Sperry have shown that quite specific connections
       between neurons can be made during development of the brain,
       and he called this the principle of neural specificity
    Sperry showed that these connections, once established in an adult
       animal, can be permanent
    he rotated the eye of a frog without damaging the optic nerve, so that
       up became down and left became right in its visual experience
    for example, retinal ganglion cells that still projected to the "down"
       part of the tectal map received input from the "up" part of the
       visual world
    following the surgery, responses driven by the tectum (e.g. approach)
       were made in the wrong direction, and the frog could never
       learn to correct these incorrect responses
    next Sperry rotated the eye and also disconnected the optic nerve from
       the tectum, knowing that the connection can regenerate in frogs
    he wanted to see whether the optic nerve fibers would grow back to
       locations on the tectal map that are correct for the new, rotated
       position of the eye, or whether they would grow back to their
       old locations, in which case the frog's view of the world would
      still be reversed
    experiments showed clearly that after the optic nerve regrew the frog's
       view of the world was still reversed
    apparently the optic nerve fibers grew back to their original locations
       on the tectum

similar experiments were conducted in other animals and brain structures
    the results suggested that specific synaptic connections between
       neurons can become established during development, and that
       the pattern so established persists into adulthood
    Sperry suggested that this developmental specificity comes about
       because target structures secrete chemical substances to which
       the neurons seeking that target would respond
    chemoaffinity hypothesis -- the idea that the attraction between any
       neuron and the target neuron to which it projects is guided by
       chemical cues released by the target neuron
    later experiments showed that the idea of specific chemical attractions
       between individual pairs of neurons is not correct
    however, chemical cues can help guide bundles of axons in more
       general ways, and this is clearly important for development

current research in developmental neurobiology is focused on the formation
   of specific connections and pursues three main questions:
        what regulates the growth of an axon?
        how does a growing axon select the correct path?
        how does an axon identify its specific target?

the growth of axons and dendrites
    neural cells are born without axons and dendrites
    extrinsic factors are important for the sprouting and direction of
       growth of axons, but not dendrites
    axons tend to grow in the proper directions even if their parent neural
       cells are oriented in the wrong directions
    this suggest that chemical attractants guide the axons in particular
       directions, independent of the orientation of the parent neuron
    in one example, pyramidal cells in rat sometimes fail to properly
       orient during development, but their axons turn around and
       grow in the correct direction
    in contrast the dendrites always grow out in the same direction form
       the neuron, regardless of its orientation
    in another example, Mauthner cells transplanted from one goldfish to
      another during development send their axons down the
       appropriate side of spinal cord regardless of their orientation
    apparently, chemical cues from target tissues guide axon growth, but
       other factors (electrical, physical) may also play a role

axonal pathfinding
    chemicals released by target tissues can provide general directional
       cues, but often more specific local cues are also needed
    growing axons actively probe the developing brain for cues as to the
       direction in which they should grow
    growth cone -- enlarged tip at the end of every developing axon
    filopodia -- fine, threadlike projections from the growth cone that
       probe the environment of the developing brain for cues
    chemical cues can either be:
          long range -- diffusing out from the point of release
          short range -- attached to other cells or the extracellular matrix
    chemical cues can either attract or repel the growth cone and axon
    growth of an axon is amoeboid, as the growth cone slithers about with
       filopodia extending and retracting in response to cues

growth of axons of peripheral sensory neurons to target neurons in the locust
   CNS provides a good illustration of axon pathfinding
    two special types of cells are involved in this process
       pioneer cells -- sensory neurons that differentiate before the
          others and forge an axonal path from the periphery to the
          CNS for the other (follower) cell axons to follow
       guidepost cells -- cells located along the path from the
          periphery to the CNS that express unique surface
          molecules that are recognized by the filopodia of the
          pioneer cells
    pioneer cells use other types of cues in addition to those provided by
       the guidepost cells
          cell adhesion molecules (CAMs) -- expressed in the membranes
             of epithelial cells, they guide the pioneer cells along the
             inner surface of the epithelium
          diffusible chemicals -- provide chemical gradients that the
             pioneer cell can follow or be repelled by (as in the case
             of the marker semaphorin I, which is a repellant)
    similar cues are used by growing axons in the insect CNS, particularly
       markers expressed on the surfaces of other, existing axons

target recognition and synapse formation
    this requires the presence on the target cell of specific chemical cues
    additional chemical cues initiate formation of synaptic specializations

a bewildering array of molecules has been implicated in axon guidance
    an effective way of classifying them is by their chemical structure
    all axon guidance molecules are proteins, and proteins occur in
       families (and super-families) defined by structural similarities
    axon guidance molecules are currently grouped into
       four families: cadherin, integrin, netrin, and semaphorin
       and one super-family: immunoglobulin (similar to antibodies)
    guidance molecules work in several ways:
       by causing the growth cone to adhere to the surfaces of cells on
          which they are expresses
       by influencing the production and placement of actin strands in
          the developing growth cone that in turn influence its
          direction of growth
       by inhibiting growth of the axon altogether

monoclonal antibodies
    studying molecular markers in development is facilitated by
       immunohistochemical techniques, in which specific molecules
       in the developing brain are located by marking them with
       antibodies made specifically against them
    the monoclonal technique can be used for making specific antibodies:
    a mouse is injected with a homogenate of the tissue containing the
       molecule to be studied
    the immune system of the mouse will treat the molecules in the
       homogenate as antigens and its lymphocytes will make
       antibodies against them -- each lymphocyte will make one
       antibody against one antigen, and some of these lymphocytes
       will make antibodies against the molecule to be studied
    lymphocytes are removed from the mouse and hybridized with
       myeloma cells, which are cancerous and so multiply
       indefinitely -- the resulting hybridoma cells will multiply in
       great numbers and produce only one antibody
    hybridomas producing antibodies against the molecule to be studied
       are selected and grown up in large quantities (cloned)
    after binding the antibodies with visible histological markers, they can
       be harvested and used in immunohistochemical studies
       immunohistochemical studies have shown that many guidance molecules are
       expressed only at specific times during development
       (sometimes only for a few hours) and in specific places
    for example in the locust CNS, fasciclin I and fasciclin II are
       expressed only on horizontally and vertically oriented axons,
       respectively, and only during a brief developmental period

nerve growth factor
    some molecules are important not only for axon guidance but also for
       supporting the functioning and even survival of certain cells
    nerve growth factor (NGF) -- causes explosive growth of neurites
       (young axons) from certain peripheral cells such as dorsal root
       ganglion cells and sympathetic ganglion cells
    NGF was discovered by an Italian named Rita Levi-Montalcini
       (she pronounces it "enna-g-effa")!
    NGF can be found in all vertebrate species but in different forms
    NGF is necessary for the normal development of sympathetic and
       unmyelinated sensory neurons especially nociceptive neurons
    NGF does not influence the development of fast-conducting,
       myelinated sensory neurons, however
    NGF binds with tyrosine kinase A receptors on the membranes of the
       neurites of developing neurons
    the receptor/NGF complex is internalized and may remain in the distal
       part of the neurite or may be transported back to the soma,
       where it stimulates DNA transcription and promotes growth of
       the neuron

other neurotrophic factors
    since the discovery of NGF, other neurotrophic factors have been
      identified and have been classified into several families
    neurotrophin family (includes NGF) -- needed for growth of sensory
       neurons (and sympathetic neurons in the case of NGF);
       neurotrophin receptors are tyrosine kinases A, B, and C
    ciliary neurotrophic factor family -- may play some role in recovery
       from injury
    transforming growth factor super-family
    neurotrophic factors may promote growth, survival, and
       differentiation of neurons, and recovery from injury
    neurotrophic factors work together, but exactly how they work is still
       under investigation

programmed cell death -- death of selected neurons that
   occurs during the course of normal development
    programmed cell death occurs for two reasons:
       overproliferation -- production of more neurons that necessary
          leads to competition for resources like neurotrophic
          factors; neurons that lose the competition die
       changing conditions -- as the brain develops, some classes of
          neurons are no longer necessary and so die off; for
         example, when a tadpole develops into a frog, certain
          neurons important for swimming (like Mauthner
          neurons) are no longer necessary and so they die
    apoptosis -- a form of cellular suicide that is the main mechanism of
       programmed cell death
    apoptosis involves activation of specific cell-death genes whose
       produces destroy the cell