A quantitative study on the postnatal development of the cerebellar vermis of mouse

Abstract The postnatal changes in the cross‐sectional area of the cortex and white matter in the median sagittal section of the cerebellum of mouse were studied quantitatively. That of the cortex increases about 20 fold from birth to maturity, increasing slowly during the first five days following birth, then sharply during the next five days to resume slow increase again after the tenth day. There is very little increase after the fifteenth day. That of the white matter increases only four‐fold during the same period. By the end of the second week it has attained only half of its adult value and continues to increase even after the thirtieth day. The increase in the cross‐sectional area of the cerebellar cortex is mainly due to rapid expansion of the cortical surface which forces the white matter to be arranged into medullary rays (actually ridges) to keep in contact with the rapidly expanding cortex, thus cerebellar fissures are formed. The number of mitotic figures (uncorrected for fragments) in the superficial granular layer in a median sagittal section 10 m̈ thick increases from about 22 at birth to a peak of 230 at seven days then falls to less than 36 at 15 days of age, after which the entire layer disappears.