This is a slice of the midline vermis in the cerebellum of a 60-day-old rat that was injected with 3H-thymidine on 4 consecutive days: Birth (P0) through P3.
Nearly all cells in the GRANULAR LAYER are lightly to heavily labeled in all lobules. That indicates that the external germinal layer, the place where granule cell precursors are proliferating, is mainly producing more cells that stay there and increase its thickness. No granule cells have been generated by the time of birth. Likewise, nearly all neurons in the MOLECULAR LAYER are labeled, indicating that most of them have not yet been generated; the precursors to all these neurons are also proliferating in the external germinal layer. The large cells at the interface of the granular layer and the molecular layer are in the thin PURKINJE CELL LAYER; without exception, these cells are unlabeled because they were generated before birth. There are some slightly larger cells in the granular layer that are unlabeled, the Golgi cells, that were also generated before birth.
Most likely composition of the external germinal layer (egl) at birth:
1. The egl is rapidly proliferating three populations of precursor cells, those for basket, stellate, and granule cells.
2. Since neurogenesis has not started in any of these populations, the egl is in its "stockbuilding" stage.