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Lecture 7

Lecture 7, Nov 30th, 2011.docx

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Dalhousie University
NESC 2470

Lecture 7, Nov 30th, 2011 November-30-11 6:12 AM Neuronal structure develops in three major stages  Cell proliferation o A cell in the ventricular zone extends a process that reaches upward toward the pia o The nucleus of the cell migrates upward from the ventricular surface toward the pial surface; the cell's DNA is copied o The nucleus, containing two complete copies of the genetic instructions, settles back to the ventricular surface o The cell retracts its arm from the pial surface o The cell divides in two (new daughter cell is formed)  Cell migration  Cell differentiation o The process in which a cell takes on the appearance and characteristics of a neuron o Neuronal differentiation occurs before astrocyte differentiation Ventricular zone  Lines the inside of each vesicle Marginal zone  Faces the overlying pia Gene expression  Regulates fate of cells produced through cell division during development  Regulated by transcription factors Neural stem cells  Can give rise to many different types of tissue Dorsal ventricular zone  Give rise to o Cortical pyramidal neurons o Astrocytes Venral telencephalon  Gives rise to o Inhibitory neurons o Oligodendroglia Sub-plate  Contains the first cells to migrate away from the dorsal ventricular zone Radial glial cells  Provide scaffolding on which the cortex is built  Some neurons actually derive from these  Withdraw their radial process once cortical assembly is complete Neuroblasts  Immature neurons  Follow the radial path from ventricular zone toward surface of brain  First ones to migrate are destined to become sub-plate  Next ones to migrate form the cortical plate o Layer VI first o Then V, IV, III, II, I Semaphorin 3A  Secreted by cells in the marginal zone  Acts first to repel growing pyramidal cell axons, forcing them to stream away from the pial surface  Attracts growing apical dendrites Growth cone  The growing tip of a neuron  Specialized to recognize the correct path of growth for neurite elongation  Lamellipodia o Leading edge of growth cone, flat sheets of membrane  Filopodia o Thin spikes extending from the lamellipodia, constantly probe the environment o Move in and out of lamellipodia to do so o Axonal growth occurs when a filopodium grabs hold of something instead of retracting Extracellular matrix Needed for axonal growth to occur   Must contain the right proteins  Example interaction o Growing axons express surface molecules called integrins, which bind laminin, and promotes axonal elongation Fasciculation  A mechanism that causes axons growing together to stick together  Due to expression of surface molecules called cell-adhesion molecules (CAMs) o Bind tightly to one another, causing cells to grow in unison Pioneer axons  These axons stretch as the nervous system expands during development  Used to guide their later-developing neighbor axons to the same targets Guidance cues  Determine the direction and amount of growth  Can be attractive or repulsive  Chemoattractant o A diffusible molecule that acts over a distance to attract growing axons toward their targets o Netrin  First discovered chemoattractant  Secreted by neurons in the ventral midline of the spinal cord  Gradient of netrin attracts axons of dorsal horn neurons across midline to form the spinothalamic tract  Chemorepellent o A diffusable molecule that chases axons away o Slit  Example of chemorepellent  Cause axons to grow away from midline Chemoaffinity hypothesis  The idea that chemical markers on growing axons are matched with complementary chemical on their targets to establish precise connections  First tested in 1940s by
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