The Evolution of Myelinating Cells

The development, maintenance, and functioning of the complex glial ensheathment of axons in vertebrates (phylum Chordata) is a classic manifestation of neuron—glia interrelations. Oligodendrocytes form the sheath in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS)—cells that differ in their origin and in their nature. Complex glial ensheathment of axons is not restricted to vertebrates, however, but is found also in a number of invertebrate species in the phyla Annelida and Arthropoda. In all the organisms in which they are found, myelin and myelin-like sheaths enhance the conduction velocity of action potentials along the axons by making saltatory conduction possible. In all sheaths, vertebrate and invertebrate alike, there are interruptions that mediate the jumping of the potential along the nerve fiber. These interruptions may be simply the point of origin of branches or well-developed nodes, such as the nodes of Ranvier in vertebrates. In this chapter the occurrence of myelin and myelin-like sheaths in the three phyla will be surveyed, the sheaths and the myelinating cells that give rise to them will be compared, and their evolutionary significance will be discussed.