Obstructed neuronal migration along radial glial fibers in the neocortex of the reeler mouse: a Golgi-EM analysis.

The interrelationship of radial glial fibers (RGF) and young neurons migrating to the neocortex of normal and reeler mutant mice at 17 days of gestation are reconstructed from serial and from closely spaced thin sections. The glial fibers are identified unequivocally by correlated light and electron microscopy by means of the Golgi-gold toning method of Fairén and associates. The migrating cell in the normal animal is closely apposed to and coiled about the RGF throughout most of its ascent. In the terminal few microns of its movement, however, it begins rapidly to differentiate and at the same time surrenders its close attachment to the RGF. In the reeler, by contrast, the migrating cell maintains normal apposition to the RGF only until it enters the cortex. There its leading process is unable to pass between the surfaces of the RGF and those of postmigratory elements. Abnormally extensive contact between the glial fiber and the somata of postmigratory cells appears to be sustained in the mutant. The upward migration of the young neuron is terminated in the depths of the cortex and the cell soma gives rise to a profusion of small processes. This study affirms the critical role served by RGF as guides to neuronal migration and provides evidence that abnormal adhesions between postmigratory cells and the RGF obstruct neuronal migration in the reeler mouse.