Changing numbers of neurons in the postnatal rat brain
The literature has it that, when the mammalian cerebral cortex is born, it already holds all the neurons it will have as an adult. The main evidence for this notion is that while prenatal injections of BrdU or 3H-thymidine label copious numbers of newborn neurons, postnatal injections one or two days after birth label only the occasional neuron. All neurons are therefore presumably already born with the animal, and any remaining postnatal neurogenesis in the cerebral cortex would thus be residual.
In contrast to this view, in this study we show that the number of neurons in the rat cerebral cortex actually doubles during the first postnatal week - but only after a quiescent period of about 3 days following birth, which nicely explains why it was not found by previous BrdU-labeling studies. A similar addition of large numbers of neurons occurs in the hippocampus and remaining areas (the ensemble of brainstem, diencephalon and basal ganglia; olfactory bulb not shown in the figure below, adapted from Figure 1 in the paper).
At the end of the first postnatal week, massive numbers of neurons are eliminated from the cortex, hippocampus and remaining areas, simultaneously with the addition of large numbers of nonneuronal cells to the brain.
The diagram below illustrates the changes in the cellular composition of the rat brain (neurons in red, nonneurons in orange) that occur between birth and adulthood.