The author studied the cytokinetics in the early postnatal histogenesis of mouse brain by autoradiography after repeated injections of tritiated thymidine.
1. The cellular proliferation is especially remarkable in the external granular layer of the cerebellum and in the subependymal layer of the cerebrum. The proliferative activity of the cells in the external granular layer is the highest in the first postnatal week, then decreases rapidly with thinning of this layer and is lost within 20 days of age. The proliferative activity of the cells in the subependymal layer is remarkable for a week or two after birth followed by gradual decrease thereafter, but the cellular multiplication can be noticed even in the vestigial subependymal layer of adult mouse cerebrum.
2. The external granular layer is divided from the functional point of view into two zones: outer zone composed of actively proliferating cells and inner zone consisting of cells which have lost the proliferative activity. The author named the former “germinal zone” and the latter “transitional zone”.
3. The generation time and duration of each phase in the mitotic cycle of the germinal cell in the external granular layer and of immature cell in the subependymal layer were calculated by the cumulative labeling method. The t_G, t₁, t_S, t₂ and t_M of the germinal cells of the mice between 1 to 10 days of life were 15 to 29, 7 to 20, 5.6 to 8.0 and 1 to 2 hours and 0.4 to 0.7 hour, respectively and those of the immature cells in the subependymal layer of 1 and 3-day-old mice were 63 to 65, 52.6 to 54.6, 7 to 10 and 1 to 2 hours and 0.4 hour, respectively.
4. Almost all of the transitional cells in the external granular layer migrate through the molecular layer into the internal granular layer to mature into granule cell neurons; and even after 10 days of age, about 48 to 49per cent of the cells in the internal granular layer are provided from the external granular layer.
5. Many of immature cells in the subependymal layer migrate into the olfactory lobe. Besides the olfactory lobe, the corpus callosum, internal and external capsules and other fiber tracts depend on the subependymal layer for the abundant supply of cells. Some of the cells which have migrated into olfactory lobe differentiate into the granule cells, whereas the majority of the immature cells in the subependymal layer differentiate into the oligodendrocytes and the astrocytes.