Chapter 9

• Ectoderm are the cells that develop to form the vertebrate nervous system as well as the epidermis of an organism
• 3 Major Domains


  • Surface
  • Neural Crest
  • Neural Tube

  
• Surface


  Also known as the epidermis, the outer layer of our skin
• Neural Crest


  The region that connects the neural tube and the epidermis

  
• Neural Tube


  Forms the brain and the spinal cord
• Parts of the Surface

  • epidermis
  • hair
  • sebaceous glands
  • olfactory epithelium
  • mouth epithelium
  • anterior pituitary
  • tooth enamel
  • cheek epithelium
  • lens and cornea
• Parts of the Neural Crest

  • peripheral nervous system
  • schwann cells
  • neuroglial cells
  • sympathetic nervous system
  • parasympathetic nervous system
  • adrenal medulla
  • melanocytes
  • facial cartilage
  • dentine of teeth

  
• Parts of the Neural Tube

  • brain
  • neural pituitary
  • spinal cord
  • motor neurons
  • retina

  
• Neural Plate


  A structure that serves as the basis for the nervous system

  
• Notochord


  A flexible rod-shape found in embryos of all chordates composed of mesodermal cells
• Pharynx


  A part of both digestive and respiratory system
• 4 Stages of Pluripotent Development into Neuroblast

  • Competence
  • Specification
  • Commitment
  • Differentiation
• Competence


  Multipotent cells become neuroblast once they are exposed to the appropriate signals, have the ability to respond to the particular signals
• Specification


  The cells received the signals and successfully develop into neuroblasts, but progression along the neural differentiation pathway is repressed by other signals. Specification is a transition between cells to become anything with their own fates.
• Commitment


  The neuroblasts enter the neural differentiation pathway and become neurons even in the presence of signals. Cells will develop into neuroblasts and cannot be reversed.
• Differentiation


  The neuroblasts leave the mitotic cycle and express those genes characteristics of neurons. This is a process in which the unspecialized cells become specialized to carry out distinct functions.
• 2 Ways of Neural Tube Formation

  • Primary Neurulation
  • Secondary Neurulation

  
• Primary Neurulation


  The cells surrounding the neural plate direct the neural plate to proliferate, invaginate, and pinch off the surface to form a hollow tube. The formation of the neural tube direct came from the ectoderm.

  
• Secondary Neurulation


  The neural tube arises from the coalescence of mesenchyme cells into a solid cord that subsequently forms cavities that coalesce to create a hollow tube. Neural tube arises from mesenchyme cells underneath the ectoderm. The anterior portion of the neural tube is made by primary neurulation, the posterior portion is made by secondary neurulation.

  
• 3 Sets of Cells in Ectoderm

  • the internally positioned neural tube
  • the externally positioned epidermis of the skin
  • the neural crest

  
• Neural folds


  Forms when the edges of neural plate thicken and move upward

  
• Neural groove


  Appears in the center of the plate, dividing the future right and left sides of the embryo

  
• 4 Stages of Neurulation

  • Formation and folding of the neural tube
  • Shaping and elevation of the neural tube
  • Convergence of the neural folds, creating a neural groove
  • Closure of the neural groove to form the neural tube

  
• Formation and Shaping of Neural Plate


  Shaped by the movements of the epidermal and neural plate regions. Neural plate lengthens along the anterior-posterior axis and narrows by convergent extension. Convergence and extension movements are critical for shaping the neural plate. In chick, divisions of the neural plate cells are in the anterior-posterior, or break-tail, direction. Epidermis is an important factor in shaping the neural plate.

  
• Bending and Convergence of Neural Plate

  Bending of the neural plate involves the formation of hinge regions where the neural plate contacts surrounding tissue. In birds and mammals, the cells at the midline of the neural plate forms the medial hinge point (MHP) which becomes anchored to the notochord beneath them and form a hinge, which forms a furrow at the dorsal midline. Primary Neurulation is regulated by intrinsic wedging within cells of the hinge regions, bending the neural plate, and extrinsic forces from the migration of the surface ectoderm toward the center of the embryo.

  
• Closure of Neural Plate

  The neural tube closes as the paired neural folds are brought together at the dorsal midline. In some species, the cells at this junction form the neural crest cells. In birds, the neural crest cells do not migrate from the dorsal region of the neural tube until after it has been closed, while in mammals, the cranial crest cells migrate while the neural folds are still elevated, and the spinal region neural crest cells do not migrate until closure has occurred.

  
• 2 Open Ends of Neural Plate

  • Anterior Neuropore
  • Posterior Neuropore

  
• Different Neural Tube Defects

  • Spina Bifida
  • Anencephaly
  • Craniorachischisis
• Spina Bifida


  Failure to close the posterior neuropore around day 27 of development, the severity depends on how much of the spinal cord remains exposed.
• Anencephaly


  A lethal condition in which failure to close sites 2 and 3 in the rostral neural tube keeps the anterior neuropore open, resulting in the forebrain remaining in contact with the amniotic fluid and subsequently degenerating.
• Craniorachischisis


  Failure of the entire neural tube to close over the entire body axis.
• Essential for Neural Tube Formation

  • Pax3
  • Sonic hedgehog
  • Openbrain genes
  • Cholesterol
  • Folate

  
• Folate


  Important in mediating neural tube closure, pregnant women are often advised to take supplements of folic acid due to the role that foliate binding protein exerts on neural tube closure.
• Primary and Secondary Neurulation

  The ectoderm forms a cord, then forms a cavity within the cord, involving the production of mesenchyme cells from the prospective ectoderm and endoderm, with the mesenchyme cells condensing into a medullary cord and the central portion of this cord undergoing cavitation to form hollow spaces called lumens.
• Differentiation in Neural tube

  In Gross Anatomy Level: the neural tube and its lumen bulge and constrict to form the chambers of the brain and spinal cord. In tissue Level: the cell populations in the wall of the neural tube rearrange themselves to form the different functional regions of the brain and spinal cord. In cellular Level: The neuroepithelial cells transform into the various nerve cells (neurons) and supportive cells (glia) found in the body.

  
• Primary Vesicles


  • Prosencephalon (forebrain)
  • Mesencephalon (midbrain)
  • Rhombencephalon (hindbrain)
• Secondary vesicles


  • Telencephalon
  • Diencephalon
  • Mesencephalon
  • Metencephalon
  • Myelencephalon
• The Anterior-Posterior Axis


  The neural tube and its lumen bulge and constrict to form the chambers of the brain and spinal cord.

  
• The Dorsal-Ventral Axis


  The top-bottom differences in the neural tube are caused by signals around it. The ventral side is influenced by the notochord below it, while the dorsal side is influenced by the skin above it. Two important signaling molecules are Sonic Hedgehog and TGF-β protein.

  
• Sonic Hedgehog


  Secreted from the notochord and induces the medial hinge point cells to become the floor plate of the neural tube.