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: P2 through P5.
Nearly all cells in the GRANULAR LAYER are still labeled in all lobules, even more heavily than when the injections began on the day of birth (P0). Between P0 and P2, the external germinal layer is mainly producing more cells that stay there and increase its thickness. Few to no granule cells have been generated by the morning of P2. Likewise, nearly all neurons in the MOLECULAR LAYER are labeled, indicating that most of them have not yet been generated by the morning of P2. There is an occasional unlabeled cell in the lower half of the molecular layer. These are probably the oldest basket cells that were generated between P0 and P2. 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 P2:
1. The egl is rapidly proliferating and thickening due to its generation of 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 still in its "stockbuilding" stage.