Cell proliferation and migration in the external granular layer of the mouse cerebellum were studied with autoradiography after cumulative labeling with H3-thymidine. The germinative cells in the external granular layer were considered as externally dislocated matrix cells. Their generation time, presynthetic time, duration of DNA synthesis, postsynthetic time and mitotic time were determined in one-, three-, seven-and ten-day-old mice. The entire sequence of the ontogeny of the external granular cell-system was separated into three consecutive stages; stage 1 or stage of pure external matrix cell proliferation, stage 2 or stage of neuroblast production, and stage 3 or stage of neuroglia differentiation. Production of neuroblasts in the external granular layer at seven and ten days of life and their migration into the internal granular layer were demonstrated by means of autoradiography. Transit times of the neuroblasts migrating across the external mantle layer and the molecular layer of ten-day-old mice were estimated at 21 and four hours, respectively. More than 50% of the inner granular cells migrated from the external granular layer later than ten days of life and almost 81 to 92% were produced later than seven days of postnatal life. In conclusion, on the basis of the matrix cell concept, the authors tried to uniiy observations of previous and present investigators and presented a scheme of pre-and postnatal histogenesis of the mouse cerebellum.

ICR-JCL strain mice were injected subcutaneously with 30 mg/kg body weight of cytosine arabinoside at 2, 3, and 4 days of age. This chemical prevented the production of the basket cells, stellate cells, and granule cells in the external granular layer of the cerebellum. Decrease in number of these microneutrons affected the noraml synaptic connections between the Purkinje cells and the microneurons, thus causing the disarrangement and abnormal arborization of the Purkinje cells. Of the three types of microneurons, the basket and a few stellate cells played a more important role in the disarrangement of the Purkinje cells and abnormal arborization of their primary dendrites than the granule cells did. Abnormal outgrowing directions of other smooth dendrites of the Purkinje cells were caused mainly by the diminution of stellate cells. Although parallel fibers were grossly decreased in number in the treated cerebellums, spines of the spiny dendrites of the Purkinje cells sprouted considerably in the 15-day-old mice, and then their morphological features remained even after 100 days of age.