Brain Development


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Brain Development

  1. 1. PRE LECTURE QUIZ WILL BE GIVEN ON: <ul><li>BRAIN DEVELOPMENT </li></ul><ul><li>CRANIAL NERVES </li></ul>
  2. 2. The Human Embryo <ul><li>Zygote </li></ul><ul><li>Embryo (first 8 weeks) </li></ul><ul><ul><li>endoderm (bodily linings) </li></ul></ul><ul><ul><li>mesoderm (connective tissues) </li></ul></ul><ul><ul><li>ectoderm (nervous system) </li></ul></ul><ul><li>Fetus (after 8 weeks) </li></ul>
  3. 3. Emergence of the Nervous System <ul><li>Neural plate becomes neural tube </li></ul>
  4. 4. … Emergence of the Nervous System <ul><li>Walls of neural tube become neurons and glial cells </li></ul><ul><li>Cavity of neural tube becomes ventricular system </li></ul>
  5. 5. … Emergence of the Nervous System <ul><li>Neural tube defects </li></ul><ul><ul><li>anencephaly </li></ul></ul><ul><ul><li>spina bifida </li></ul></ul>
  6. 6. Anencephaly <ul><li>cephalic disorder that results from a neural tube defect </li></ul><ul><li>occurs when the cephalic (head) end of the neural tube fails to close </li></ul><ul><li>usually between the 23rd and 26th day of pregnancy, </li></ul><ul><li>resulting in the absence of a major portion of the brain, skull, and scalp. </li></ul>
  7. 7. <ul><li>Children with this disorder are born without a forebrain, the largest part of the brain consisting mainly of the cerebral hemispheres (which include the neocortex, which is responsible for higher-level cognition, i.e., thinking). </li></ul><ul><li>The remaining brain tissue is often exposed—not covered by bone or skin. </li></ul>Anencephaly
  8. 8. <ul><li>A baby born with anencephaly is usually blind, deaf, unconscious, and unable to feel pain. </li></ul><ul><li>Although some individuals with anencephaly may be born with a main brain stem, the lack of a functioning cerebrum permanently rules out the possibility of ever gaining consciousness. </li></ul><ul><li>Reflex actions such as breathing and responses to sound or touch occur. </li></ul>
  9. 9. Spina bifida <ul><li>developmental birth defect caused by the incomplete closure of the embryonic neural tube </li></ul><ul><li>cleft spine, which is an incomplete closure in the spinal column. </li></ul>
  10. 10. <ul><li>Some vertebrae overlying the spinal cord are not fully formed and remain unfused and open. </li></ul><ul><li>If the opening is large enough, this allows a portion of the spinal cord to protrude through the opening in the bones. </li></ul><ul><li>There may or may not be a fluid-filled sac surrounding the spinal cord. </li></ul><ul><li>Other neural tube defects include anencephal y encephalocele , which results when other parts of the brain remain unfused. </li></ul>
  11. 11. The Developing Brain <ul><li>Rostral part of neural tube becomes brain </li></ul><ul><li>Caudal part of neural tube becomes spinal cord </li></ul><ul><li>Differentiation of brain into 3 parts: </li></ul><ul><ul><li>prosencephalon (forebrain) </li></ul></ul><ul><ul><li>mesencephalon (midbrain) </li></ul></ul><ul><ul><li>rhombencephalon (hindbrain) </li></ul></ul><ul><li>Further differentiation of these 3 parts </li></ul>
  12. 12. The Developing Spinal Cord <ul><li>2 major zones of cells: </li></ul><ul><ul><li>alar plate (dorsal) </li></ul></ul><ul><ul><ul><li>sends sensory projections to brain </li></ul></ul></ul><ul><ul><li>basal plate (ventral) </li></ul></ul><ul><ul><ul><li>motor neuron development </li></ul></ul></ul>
  13. 13. Formation of Neurons and Glial Cells <ul><li>Single layer of cells within neural tube…ventricular layer </li></ul><ul><li>Daughter cells </li></ul><ul><ul><li>Migration first to marginal (outer) layer </li></ul></ul><ul><ul><li>Migration then to intermediate layer </li></ul></ul><ul><ul><li>Migration to cortical plate & subventricular layer </li></ul></ul><ul><ul><li>Some remain in ventricular layer (ependymal cells) </li></ul></ul>
  14. 14. Cell Formation and Migration <ul><li>Neurogenesis : formation of new neurons </li></ul><ul><li>Neural migration guided by: </li></ul><ul><ul><li>radial glial cells </li></ul></ul><ul><ul><li>glycoproteins </li></ul></ul>
  15. 15. Cell Differentiation <ul><li>Creation of different cell types </li></ul><ul><li>2 types: </li></ul><ul><ul><li>cell-autonomous differentiation (genetic) </li></ul></ul><ul><ul><li>induction (dependent upon influence of other cells) </li></ul></ul>
  16. 16. Glial Cell Development <ul><li>Begins in embryonic period </li></ul><ul><li>Most intense glial production after birth </li></ul><ul><li>Myelination (wrapping axon in myelin sheath) </li></ul><ul><ul><li>spinal cord then hindbrain then midbrain then forebrain </li></ul></ul><ul><ul><li>sensory then motor </li></ul></ul>
  17. 17. Formation of Neural Connections <ul><li>Axons grow towards target cells </li></ul><ul><li>Axon emerges from growth cone (containing filopodia) </li></ul><ul><li>Filopodia are attracted to target cell through chemical signal </li></ul><ul><ul><li>neurotrophins </li></ul></ul><ul><li>Abrupt changes in direction of growth due to guidepost cells </li></ul>
  18. 18. Importance of Neural Activity <ul><li>Axonal remolding (Shatz, 1992) </li></ul><ul><ul><li>neurons grow to many addresses </li></ul></ul><ul><ul><ul><li>correct address...remain </li></ul></ul></ul><ul><ul><ul><li>incorrect address...eliminated </li></ul></ul></ul><ul><li>Neurons that fire together wire together </li></ul>
  19. 19. Neuron Cell Death <ul><li>Theories: </li></ul><ul><ul><li>Competition to attain connections with target cells </li></ul></ul><ul><ul><li>Need to receive certain qualitity of neurotrophins for survival </li></ul></ul><ul><li>Examples: </li></ul><ul><ul><li>Motor neuron-muscle fiber connections (PNS) </li></ul></ul><ul><ul><li>Purkinje cells of cerebellum (CNS) </li></ul></ul>
  20. 20. Failures of Neural Development <ul><li>Genetic defects </li></ul><ul><ul><li>Down Syndrome - trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. </li></ul></ul><ul><ul><li>Phenylketonuria (PKU) - autosomal recessive metabolic genetic disorder characterized by a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH) </li></ul></ul><ul><ul><ul><li>PAH : converts phenylalanine ('Phe') to the amino acid tyrosine. </li></ul></ul></ul><ul><ul><ul><li>hyperphenylalaninemia (HPA) was associated with mental retardation . </li></ul></ul></ul><ul><ul><li>Fragile X Syndrome </li></ul></ul>
  21. 21. Fragile X Syndrome <ul><li>genetic syndrome which results in a spectrum of characteristic physical and intellectual limitations and emotional and behavioral features which range from severe to mild in manifestation. </li></ul><ul><li>expansion of a single trinucleotide gene sequence ( CGG) on the X chromosome, and results in a failure to express the protein coded by the FMR1 ge ne, which is required for normal neural development. </li></ul>
  22. 22. <ul><li>External factors </li></ul><ul><ul><li>Malnutrition </li></ul></ul><ul><ul><li>Fetal Alcohol Syndrome - next slide </li></ul></ul>Failures of Neural Development
  23. 23. <ul><li>(Andrew Syndrome) is a pattern of mental and physical defects that can develop in a fetus when a woman drinks alcohol during pregnancy. </li></ul>
  24. 24. Redevelopment of the Nervous System <ul><li>Damage to cell body of nerve...death; damage to axon...degenerative changes, sometimes death </li></ul><ul><li>anterograde degeneration (from point of injury to axon terminals) </li></ul><ul><li>retrograde degeneration (between point of injury and cell body) </li></ul><ul><li>chromatolysis (degeneration of cell body) </li></ul><ul><li>transneuronal degeneration (damage to connecting neurons) </li></ul>
  25. 25. ...Redevelopment of the Nervous System <ul><li>Regeneration occurs within PNS only </li></ul><ul><li>Collateral sprouting within CNS </li></ul><ul><li>Research on recovery of CNS functioning </li></ul><ul><ul><li>collateral sprouting </li></ul></ul><ul><ul><li>suppression of growth-inhibiting chemicals </li></ul></ul><ul><ul><li>fetal tissue transplantation </li></ul></ul><ul><li>Compensatory strategies after CNS damage </li></ul><ul><ul><li>rehabilitation </li></ul></ul>