ORIGINS OF SPINAL-CORD OLIGODENDROCYTES - POSSIBLE DEVELOPMENTAL AND EVOLUTIONARY RELATIONSHIPS WITH MOTOR-NEURONS

Spinal cord oligodendrocytes develop from migratory glial progenitor c ells that are generated by a small subset of neuroepithelial cells in the ventral part of the neural tube. Specification of these neuroepith elial oligodendrocyte precursors, in common with other ventral cells s uch as motor neurons, depends on morphogenetic signals from the notoch ord and/or floor plate. The ventrally derived signals can be mimicked in vitro by purified Sonic hedgehog (Shh) protein. Oligodendrocytes an d motor neurons are induced over the same range of concentrations of S hh, consistent with the idea that Shh might specify a common precursor of motor neurons and oligodendrocytes. A lineage relationship between motor neurons and oligodendrocytes has previously been suggested by c lonal analysis in the embryonic chick spinal cord. We propose a lineag e diagram that connects oligodendrocytes and motor neurons and that ta kes into account the fact that motor neurons and oligodendrocyte precu rsors are generated at different times during development. Oligodendro cytes might originally have evolved to ensheath motor axons and facili tate a rapid escape response. If so, oligodendrocyte ontogeny and phyl ogeny might share a common basis.