Postnatal development of the cerebellar cortex in the rat. II. Phases in the maturation of Purkinje cells and of the molecular layer

The growth and synaptic maturation of Purkinje cells and of the molecular layer were studied in the cerebellar cortex of rats aged 0, 3, 5, 7, 10, 12, 15, 21 and 30 days with histological (including Golgi), histochemical, autoradiographic and electron microscopic techniques. Five phases were distinguished in the maturation of Purkinje cells. During the first phase, Purkinje cells become dispersed and aligned in a monolayer but as yet few or no synapses are formed. Next, two transient structures appear: a hypertrophied apical cone composed of „reticular” cytoplasm, and lateral perisomatic processes which establish conspicuous asymmetrical synapses with climbing fibers. During the third phase the perisomatic processes disappear; the „reticular” cytoplasm streams upward into the growing dendrites; the soma is invaded by permanent inconspicuous, symmetrical synapses of basket cells; and, finally, it is surrounded by glial processes, which marks the end of the synaptic maturation of the soma. During the fourth phase parallel fibers form synapses with dendritritic spines in the lower half of the molecular layer. During the fifth phase, which occurs after the disappearance of the external germinal layer, parallel fibers establish synapses with dendritic spines in the upper molecular layer. The „march” of synaptogenesis in the molecular layer from the bottom upward is characterized by three successive events: an initial gradient in the appearance and disappearance of coated vesicles, heralding synaptogenesis; a similar subsequent trend in the formation of synapses; and finally, the interposition in the same sequence of glial processes between Purkinje cell dendrities and parallel fibers, marking the cessation of synaptogenesis.