Development of CNS.docx

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Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Kem Rogers

Development of CNS Neural Crest Cell Derivatives: • Sensory (dorsal root) ganglia **neuroganglia outside CNS o Neuroepithelial cells – structures of CNS o Neurocrest cells – form ganglia • Sympathetic neurons and ganglia • Schwann cells • Cranial nerve ganglia • Glial cells Embryonic Development of Nervous Tissue • Ectoderm starts to close over neural tube once neural tube has closed • Neural tube & neural crest form from invaginating ectoderm • Neuroepithelial cells of the neural tube divide & migrate externally to become neuroblasts (neurons) & neuroglia (oligodendrites – myelin sheath & astrocytes – great BBB & support neuron) – CNS • Neuroblasts cluster just external to neuroepithelium into o Alar plate  Dorsally – neuroblasts become interneurons – remain in CNS – sprout axons form white matter by growing outward along the length of CNS o Basal plate (future gray matter)  Ventrally – neuroblasts of basal plate become motor neurons & sprout axons that grow out to effectors • Sensory neurons with cell bodies in the sensory ganglia or dorsal root ganglia – peripheral to CNS – processes that extend towards CNS and combine with interneurons from alar plate & other process goes towards receptors • Neurons continue to form rapidly until 6 month – then slows down • Before neuron formation slows – early neuroglial cells differentiate into astrocytes & oligodendrocytes • As division of cells slow – neuroepithelium differentiates into the layer of ependymal cells • Sensory neurons arise not from neural tube but from neural crest o Cell bodies of sensory neurons lie outside CNS • Motor neurons, interneurons & sensory neurons stop dividing during the fetal period • As growing axons elongate at growth cones they are attracted by chemical signals such as neurotrophins released from other neurons & astrocytes • Receiving dendrites send out thin, wiggling extensions to reach approaching axons & form synapses • Which synaptic connections are made & which persist depends on: o Amount of neurotrophin initially received by an axon o Degree to which a synapse is used after being established • Neurons that make bad connections are signaled to die via apoptosis - ½ of neurons die before birth • Initial overproduction of neurons ensures that all necessary neural connections will be made • Some dividing cells in subependymal zone are shown to form new neurons (past fetal period) in hippocampus & olfactory bulb Embryonic Development of Brain • Brain arises as rostral part of neural tube in the 4 week of development • Starts to expand and 3 primary brain vesicles are established o Prosencephalon (forebrain – anterior) o Mesencephalon (midbrain) o Rhombencephalon (hindbrain - posterior) • Caudal portion of the neural tube becomes the spinal cord • In week 5 – 3 primary brain vesicles give rise to 5 secondary brain vesicles o Prosencephalon  Telencephalon (endbrain)  Diencephalon (through-brain) o Mesencephalon (remains undivided) o Rhomencephalon  Metencephalon (afterbrain)  Myelencephalon (brain most like spinal cord) • Formation of 2 flexures by week 5 causes telencephalon & diencephalon to angle toward brain stem o Midbrain flexure o Cervical flexure – around the neck of the future fetus • Each secondary brain vesicle develops producing major structures o Telencephalon: 2 lateral swellings become cerebral hemispheres – cerebrum – neocortex o Diencephalon: thalamus, hypothalamus & epithalamus o Mesencephalon: midbrain o o Mentecephalon: ventral  pons & dorsal  cerebellum o M
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