ABSTRACT

Glial cells are divided into macroglia and microglia. Several distinct populations of macroglia are recognized; astrocytes, oligodendrocytes and Schwann cells. Astrocytes are the largest and most numerous of glial cells. Astrocytes fill most of the space between neurons leaving gaps only about 20 nm across. Astrocytes processes surround synapses and some form end feet which butt onto capillaries or onto the pia mater to produce a layer covering the surface of peripheral nerves and central nervous system (CNS) called the glial membrane. Oligodendrocytes in the CNS and Schwann cells in the peripheral nervous system have the common function of providing the myelin sheath, an electrically insulating covering around many axons. Schwann cells line up along the axon and surround the axon with a pseudopodium-like structure, the mesaxon. Myelination proceeds in a similar way in the CNS except that each oligodendrocyte extends several processes so that it can contribute to the myelination of several adjacent axons.