Meeting Abstract
Evolution of the myelin sheath surrounding neuronal axons was pivotal for the success of vertebrates. This remarkable innovation facilitates increased electrical conductivity along axons, allowing evolution of large body size and enabling animals to both respond rapidly to danger and become successful predators. Myelin is absent in the few remaining basal jawless vertebrates, lampreys and hagfish, suggesting that myelination arose after the divergence of agnathans and gnathostomes. The myelin sheath is formed by elongated and flattened glial cells. In the central nervous system (CNS), the myelin sheath is formed by oligodendrocytes that originate from epithelial cells in the ventral neural tube of most jawed vertebrates (a recent study shows mouse has a small number of oligodendrocytes of dorsal origin), and migrate to the marginal zone dorsally and laterally and myelinating the CNS axons. Since motor neurons and oligodendrocytes both originate along the ventral neural tube, this raises the possibility that oligodendrocytes first evolved to accompany motor axons, facilitating a rapid response to the environment. Our investigation of gliogenesis in a basal vertebrate—the sea lamprey, provides evidence to support this hypothesis. Lampreys possess glial cells in the ventral neural tube that share a similar origin and regulatory mechanism with gnathostome oligodendrocytes. This suggests oligodendrocytes were present in the ancestral vertebrate prior to the myelinating function of glial cells. Instead of being restricted to the CNS as in jawed vertebrates, lamprey oligodendrocytes emerge from the neural tube along with motor axons, suggesting they may have evolved in the ancestral vertebrate to support motor axons both inside and outside the neural tube. After the divergence of agnathan and gnathostome vertebrates, oligodendrocytes were restricted to ensheath the axons in the gnathostome CNS, while Schwann cells evolved to support peripheral neurons.
