Age related changes in cellularity, mitotic activity and pyknotic cell number in the mouse subependymal layer.

The mouse subependymal layer shows a decrease in cellularity between 22 and 31 months of age after having remained fairly constant between 6 and 22 months. The mitotic index falls from 2.32% at 6 months to 1.19% at 18 months before rising to 2.37% at 28 months. The variation in the mitotic index prevents the number of mitotic cells decreasing between 22 and 28 months along with decreasing cellularity. There is no statistical correlation between the number of mitotic cells and the number of pyknotic cells in the subependymal layer although, at all ages, mitotic cells out-number pyknotic cells.