Histo.11

• Neuroglia (Glial cells)

  Supporting cells in the nervous system in addition to neurons
• Glial cells

  • Support neuronal survival and activities
  • 10 times more abundant than neurons in the brain
  • Surround neuronal cell bodies and processes
  • Create microenvironments optimal for neuronal activity
• The fibrous intercellular network of CNS tissue resembles collagen but is actually the network of fine cellular processes emerging from neurons and glial cells
• Neurons and neuroglia are separated by a very narrow extracellular space
• Neuroglial cells present in the parenchyma of brain and spinal cord

  • Astrocytes
  • Oligodendrocytes
  • Ependymal cells
  • Microglia

  
• Oligodendrocytes

  • Rounded or pear shaped bodies with relatively few processes
  • Each process becomes sheetlike and wraps repeatedly around a portion of a CNS axon
  • Provide myelin sheaths to nerve fibers in the brain and spinal cord
  • Relationship to nerve fibers is similar to Schwann cells in the peripheral nervous system
  • One oligodendrocyte may enclose several axons

  
• Types of oligodendrocytes

  • Classified based on the number of neurons they provide sheaths to
  • Those related to large diameter axons provide sheaths to fewer axons than those related to small diameter axons
• Astrocytes

  • Small star-shaped cells that give off a number of processes
  • Processes are often flattened into leaflike laminae that may partly surround neurons and separate them
  • Processes frequently end in expansions in relation to blood vessels or brain surface
  • Have small swellings called gliosomes that are rich in mitochondria

  
• Types of astrocytes

  • Fibrous astrocytes (in white matter)
  • Protoplasmic astrocytes (in grey matter)

  
• Fibrous astrocytes

  Processes are thin and asymmetrical
• Protoplasmic astrocytes

  • Processes are thicker and symmetrical
  • Surround nodes of Ranvier

  
• The processes of astrocytes are united to those of other astrocytes through gap junctions
• Ependymal cells

  • Columnar or cuboidal cells that line the fluid-filled ventricles of the brain and central canal of the spinal cord
  • Have cilia and long microvilli on their apical ends
  • Joined apically by junctional complexes but lack a basal lamina
  • Have elongated basal ends that extend branching processes into the adjacent neuropil
• Microglia

  • Smallest neuroglial cells with flattened cell bodies and short processes
  • Frequently seen in relation to capillaries
  • Do not originate from neural progenitor cells like other glia, but from circulating blood monocytes
• Neuroglial cells present in PNS

  • Schwann Cells
  • Satellite Cells of Ganglia

  
• Schwann cells

  • Found only in the PNS
  • Counterparts to oligodendrocytes in the CNS, having trophic interactions with axons and forming their myelin sheaths
  • Unlike oligodendrocytes, a Schwann cell forms myelin around a portion of only one axon
• Satellite cells of ganglia

  • Small cells that form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS
  • Exert a supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments
• Types of nerve fibers

  • Myelinated fibers
  • Unmyelinated fibers
• Myelin sheath

  • Seen outside the axolemma
  • Provided by Schwann cells for axons in the PNS
  • Provided by oligodendrocytes for axons in the CNS
• Formation of myelin sheath

  1. Axon invaginates into the cytoplasm of Schwann cell

  2. Fold of Schwann cell membrane (mesaxon) becomes greatly elongated and spirally wound around the axon

  3. Lipids are deposited between adjacent layers of the membrane to form the myelin sheath

  4. Thin layer of Schwann cell cytoplasm persists to form the neurilemma
• The presence of a myelin sheath increases the velocity of conduction
• Nodes of Ranvier

  Short gaps in the myelin sheath where the axon is exposed
• Internode

  Segment of myelin sheath between two nodes of Ranvier
• Myelin contains protein, lipids, and water. The main lipids are cholesterol, phospholipids, and glycosphingolipids
• Unmyelinated fibers

  • Axons that are devoid of myelin sheaths
  • Invaginate into the cytoplasm of Schwann cells but the mesaxon does not spiral around them
  • Several such axons may invaginate into the cytoplasm of a single Schwann cell