A quantitative lifespan study of changes in cell number, cell division and cell death in various regions of the mouse forebrain.

A quantitative study of changes in total cell number was carried out in the indusium griseum and anterior commissure from fetal life to old age in the mouse brain. The changes in the number of mitotic and pyknotic cells were recorded in the indusium griseum, anterior commissure, subependymal and ependymal layers over the same period. The number of neurons which are produced and which migrate to the indusium griseum are in excess of the number eventually required and the surplus neurons are lost by cell death in late gestation and early postnatal life while synaptogenesis and neuronal differentiation is taking place. This neuronal loss is associated with a rapid turnover of glia. Most first generation glia, or their immediate precursors, are produced prenatally, in parallel but one day behind neurons. There is no large burst of mitotic activity in the postnatal brain which gives rise to the myelination gliosis which is probably largely a migratory phenomenon. Cell division continues throughout life in all parts of the brain studied. The greatest mitotic activity is centred in the subependymal layer where mitotic cells substantially outnumber pyknotic ones. There is a gradual decrease in mitotic activity in the subependymal layer up to 9 months of age with fairly constant mitotic activity thereafter. Mitotic activity in the indusium griseum levels out at 3 months postnatum with mitotic and pyknotic cells present in roughly equal numbers thereafter. Mitotic activity in all parts of the anterior commissure levels out at 6 months postnatum and remains constant thereafter. Mitotic and pyknotic cells are present in similar numbers except for a peak in pyknotic cells at 9 months. Cell number in the indusium griseum and anterior commissure is fairly constant between 3 and 9 months, but glial number begins to decrease in all parts of the anterior commissure from 12 to 22 months. In the indusium griseum the number of glia increased slightly between 6 and 22 months. The number of neurons fluctuated during the first week after birth then remained constant until 18 months. There was a significant decrease in the number of neurons between 18 and 22 months.