The following images are from a sagittally-sectioned specimen of an E14 rat embryo exposed to tritiated thymidine on E13–24 hr survival. Note the heavily-labeled cells just outside the germinal zones in the brain and spinal cord, while the germinal zones contain more lightly-labeled cells. This embryo was embedded in methacrylate. Methacrylate causes excess background noise, but the heavily-labeled migrating cells still stand out. Many of these cells are postmitotic young neurons generated ON E13. Also note that many UNLABELED cells are outside the germinal and migratory zones–in these areas, we observe only randomly-scattered black specs–the background noise level. Many of the cells here are settling neurons generated BEFORE E13; they are especially prominent in the midbrain tegmentum (floor), pons, medulla, and ventral spinal cord. There are also a few unlabeled areas in the basal telencephalon (the forebrain).
