September 19 - Development of NS.docx

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Western University
Anatomy and Cell Biology
Anatomy and Cell Biology 3319

September 19, 2013 Development of Nervous System Pg. 53-54, 367-369, 379-381 Objectives: - Be able to describe the stages of development of the nervous system - Determine the embryonic origins of adult brain structures The Mesoderm Begins to Differentiate – see Sept 12 – Embryology - 17 Days: neural tube being formed in a caudal direction from primitive node o Have tri-laminar layer and form neural plate - 19 Days: start to get groove o Neural fold and beginning of neural tube o Mesoderm starts to differentiate and form somites (go along the length of embryo) - 20 Days: have neural crest cells that start to migrate and leave ectoderm to move down where the mesoderm is o Neural groove continues to deepen and neural folds get closer together - 22 Days: neural tube starts to close around neck region and then zips itself up (zips in caudal and rostral direction from the middle) o Neural crest cells continue to migrate and further differentiate o Rostral/head fails to close  anencephaly  No sensory, cant feel pain, won’t live for long o Caudal/tail fails to close  spina bifida  May not be able to walk o Take folic acid (B vitamins) during pregnancy to reduce neural-tube defects Nervous Tissue Throughout Life: Embryonic Development of Nervous Tissue - 28 Days: Nervous system develops from the dorsal ectoderm, which invaginates to form the neural tube and the neural crest - Week 5: The neural tube, whose walls begin as a layer of pseudostratified neuroepithelial cells, becomes the CNS - Neuroepithelial cells of the neural tube divide and migrate externally to become neuroblasts (future neurons) and neuroglial cells (supporting cells – glial cells like astrocytes and oligodendrocytes) - The ectoderm moves over the closed neural tube - Week 6: - Neural crest cells form the sensory neurons (and other things…see below)  This explains why the cell bodies lie outside the CNS  Their cell bodies are located in the dorsal root ganglia - Dorsal neuroblasts form the alar plate (future interneuron’s)  Long axons extending from the interneuron’s form the white matter  In adult spinal cord become dorsal horn  In adult medulla oblongata become sensory nuclei  Stay in CNS - Ventral neuroblasts form the basal plate (future motor neurons)  Axons grow out to the effector organs – innervate muscles  In adult spinal cord becomes ventral horn  In adult medulla oblanagata become motor nuclei (nuclei contribute to cranial nerve) - The alar and basal plates are the future gray matter - Neurons continue to form rapidly until about the 6 month, and then slow down - Just before neuron formation slows, the early neuroglial cells differentiate into astrocytes and oligodendrocytes - How do forming neurons meet and form synapses with one another? - As growing axons elongate at growth cones, they are attracted by chemical signals such as neurotrophins released from other neurons and astrocytes - The receiving dendrites send out thin, wiggling extensions to reach the approaching axon and form synapses - Which synaptic connections are made, and which persist, are determined by 2 factors: - The amount of neurotrophin initially received by an axon - The degree to which a synapse is used after being established  Neurons that make “bad” connections are signaled to die via apoptosis  about
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