Migratory paths of neurons and glia in the embryonic chick spinal cord.

To study the migration of chick spinal cord neurons, we labeled individual cells in the ventricular zone with recombinant retroviruses, then identified their progeny histochemically. First, we analyzed cell mixing in the ventricular zone. Some clones labeled at early neural tube stages spread widely along both the dorsoventral and rostrocaudal axes. However, clones labeled later were confined to narrow domains along both axes. These results imply that displacement of cells within the ventricular zone becomes progressively restricted. Second, we studied the migration of cells out of the ventricular zone by infecting embryos at a fixed stage and varying the time of analysis. At first, most clones consisted of radial arrays of cells, suggesting that the initial migration is predominantly radial. In many clones, however, neurons turned orthogonally from parental radial arrays and migrated along the path of circumferentially oriented axons. By hatching, clonally related cells in the gray matter were usually distributed in narrow transverse slabs, but some white matter glial cells had migrated longitudinally for up to several segments. We conclude that the dispersal of clonally related cells results from (1) early mixing of progenitors within the neural tube; (2) radial stacking of progeny in the ventricular zone; (3) migration of progeny from the ventricular zone in spoke-like routes; (4) circumferential migration of some neurons along axons; (5) short-distance dispersal of differentiating neurons; and (6) a late, longitudinal migration of glia through white matter tracts. Finally, we show that floor plate cells differ from other spinal cord cells in both their lineage and migration patterns.