Veterinary Histology and Embryology

NEUROGLIA

• This is the interstitial tissue of the nervous system. In the brain and spinal cord the connective tissue is limited to the enveloping membranes the meninges and a slight amount accompanies the blood vessels. The supportive framework the interstitial tissue is formed by a special group of cells and fibres called neuroglia.
• In ordinary preparations of CNS many nuclei are seen which belong neither to nerve cell nor vascular tissue. These nuclei which do not possess nucleoli belong to Neuroglia cells. The neuroglia (often simply termed glia) may be divided into the following classes.
• Astroglia or Astrocytes,
• Oligodendroglia,
• Microglia or mesoglia and
• Ependyma.
• Except microglia all of the above are of ectoderm origin.
• Special and often difficult methods should be employed to study details neuroglia. What are described below will not be seen in ordinary preparations.
• Astrocytes
• These are stellate cells with many processes. They are of two varieties ;
• Protoplasmic astrocytes, of numerous processes which extend from the cell body in all directions and branch frequently. The nucleus is oval and has scan chromatin and has no nucleolus. Small granules present in the cytoplasm of cell body and processes are called Gliosomes. They are found principally in the great matter of brain and spinal cord. They partly envelop the neuron bodies to form satellite cells. One or more of the processes of these astrocytes have peculiar terminal expansions called foot plates or perivascular feet which are applied walls of capillaries or small blood vessels.
• The fibrous astrocytes differ from the protoplasmic astrocytes having fewer processes. But these are most straight and longer. They possess gliosomes and coursing through their cytoplasm are fit straight, unbranched fibres the neuroglia fibres. The fibrous astrocytes have perivascular feet and are found chiefly in the white matter.
• Oligodendroglia 
  • These are smaller than astrocytes. Their processes are few smaller and more delicate. They have no perivascular feet but their cell body may be applied to surface of a capillary as perivascular satellite.
  • They are present both in grey and white matter. In grey matter they form perinueuronal satellite to the neuron bodies. In white matter they lie in row between myelinated nerve fibres to form an investment around the myelin. They are known as inter-fascicular glia. Their relation to myelin sheath in CNS is similar to that of Schwann cells myelinated fibres in PNS.
• Microglia 
  • These are small cells with deeply staining small nuclei often elongated or irregular shape they resemble fibroblast nuclei.
  • They have neither gliosomes nor fibres. They are found both in the grey and white matter but a less in numbers in the latter.
  • They form in the grey matter perineuronal satellite and although they never have perivascular feet they form perivascular satellites.
• Ependyma 
  • In ordinary preparations appears to consist of closely packed elongated nuclei lining the central canal of spinal cord and ventricles of brain. Their long axes are perpendicular to the cavity and they present the appearance of columnar epithelium.
  • Their process ramifies more or less deeply in the wall to the neural tube. They show neuroglia fibres and are to the regarded as neuroglial cell. They may have in certain places, cilia which protrude into the neural cavity.