An ultrastructural and radioautographic study of the migrating neuroblast in hamster neocortex

Abstract

Newborn hamsters were injected with [ 3H]thymidine and sacrified on postlabel day 2 through 6. Samples of parietal cerebral cortex were fixed and embedded according to conventional electron microscopic techniques. Alternate 1 μm thick sections and 100 nm thin sections were mounted on glass slides and grids, respectively. The glass slides were processed for radioautography and the grids stained for electron microscopy. Labeled neuroblasts were identified with the light microscope on the radioautographs. The same cells were identified with the electron microscope for detailed study of their organelles. Two days after injection, the first cells to arrive in the cortex were found in the deepest laminae of the cortical plate. They were pear-shaped cells with most of the organelles located at the base of the apical or leading process. The nuclei were irregular in shape and had heterogeneous chromatin. The cytoplasm contained free ribosomes, mitochondria, Golgi complexes, vesicles, and microtubules. At later stages, the labeled cells were distributed throughout the cortical plate, but by the 5th day all were situated immediately below the molecular layer. At this time the nuclei were oval to round with homogeneously distributed chromatin. The apical or leading processes had the appearance of dendrites and contained an increasing number and complexity of organelles. Most outstanding was the increase in RER and microtubules during migration. Migrating neuroblasts thus labeled and positively identified were found to differentiate into cortical neurons as they pass through the cortical plate to layers II and III.