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: P4 through P7.
Nearly all cells in the GRANULAR LAYER are heavily labeled in all lobules. An occasional granule cell is unlabeled in the lingula, centralis, and nodulus; these are the oldest granule cells. Between P2 and P4, the external germinal layer is just beginning to produce granule cells. Unlabeled cells in the lower half of the molecular layer are now commonly seen in the lingula, centralis, and nodulus. Most of these are basket cells that were generated between P2 and P4. A fair number of unlabeled cells are in the upper half of the molecular layer in these same lobules; these are the oldest stellate cells. Thus, by the morning of P4, neurogenesis is rapidly proceeding in the basket cell population, is already at a good start in the stellate cell population, and is just beginning in the granule cell population--at least in the lingula, centralis, and nodulus. As always, the Purkinje cells and Golgi cells are all unlabeled because they are generated before birth.
An artifact in this section is that the primary fissure is pulled open, and there is a gap between the anterodorsal and central lobes.
Most likely composition of the external germinal layer (egl) at P4:
1. The egl is rapidly proliferating and thickening due to the generation of precursor cells for stellate and granule cells; it is still "stockbuilding" precursors of these populations.
2. Some of the precursors of the basket cell population already have entered their final neurogenetic division, and the number of basket cell precursors has reached its maximum--just before most of these neurons are being generated.