The timing of granule cell differentiation and mossy fiber morphogenesis in the opossum.

The timing of developmental events may be important for the orderly formation of neuronal interconnections. In the present study, the timing of granule cell migration is compared with the arrival and maturation of mossy fiber projections. The opossum was chosen as the experimental animal because its protracted postnatal development enables the examination of developmental sequences not as easily recognized in other more commonly used mammalian species. It is shown that all areas that project to the cerebellum as mossy fibers in the adult opossum do so by postnatal day (PD) 30. Their major target, the granule cells begin inward migration from the external germinal layer (EGL) prior to PD 30, but do not form a distinct internal granular layer (IGL) until PD 35. Migrating granule cells penetrate into the IGL deep to granule cells that have begun dendritic differentiation. By PD 50, Golgi impregnations reveal that many granule cells have numerous immature processes, somal spines and dendritic growth cones. After this age these structures are rare and the vast majority of granule cells exhibit short dendrites with digiform endings. Dendritic differentiation subsequent to PD 54 involves an increase in the length of the shaft and the further maturation of terminal digits. Also from Golgi material, immature mossy fiber endings can be identified in the IGL by PD 35 and exhibit mature characteristics at PD 73. Thus, the formation and maturation of granule cell dendrites and their afferents (mossy fibers) occur over an extended period of time (PD 35-73). Moreover, granule cells exhibit a sequence of development similar to that of Purkinje cells: early arrival of their primary afferent projections in the cerebellar anlage; a period of exuberant dendritic growth; and a protracted and overlapping period for dendritic and synaptic maturation.