The following images are from a sagittally-sectioned specimen of an E17 rat embryo exposed to tritiated thymidine on E16–24 hr survival. Note the heavily-labeled cells just outside the germinal zones in the brain and spinal cord, while the germinal zones themselves contain more lightly-labeled cells. Germinal zones in the brainstem continue to shrink and transform to an ependymal layer. The heavily-labeled migrating cells are postmitotic young neurons generated ON E16. By now, UNLABELED cells (generated before E16) take up most of the space outside the germinal and migratory zones. The pattern of heavily-labeled cells outside the cerebral cortex neuroepithelium deserves special mention. Notice that the labeled cells are in BANDS. We have postulated that these bands contain layer-specific neurons destined to settle in Layers V and VI in the mature cerebral cortex. We call this area the Stratified Transitional Field. See the neocortical development page in neurondevelopment.org for more information. The cerebellum now contains a prominent superficial band of labeled cells–the external germinal layer that will generate all short-axon neurons in the cerebellar cortex.
