More neurons are born than necessary, and synaptic pruning is the process by which neurons that have not made as many functional synaptic connections with other neurons are preferentially degraded.
Abitz et al counted cells in the medial thalamus of newborn and adult brains using a optical fractionator and Giemsa staining which binds to phosphate groups of DNA. They distinguished small neurons from glial cells on the basis of chromatin pattern, the size / shape of the nucleus, and the visibility of the nucleolus. Here’s an example of the Giemsa stained cells via micrographs:
They found an average of 11.2 million neurons in the newborn MD thalamus, which decreased to an average of 6.43 million neurons in adults, probably as a result of synaptic pruning. On the other hand, they found 36.3 million glial cells in adults, much higher than the 10.6 million they found in newborns, suggesting that glial progenitor cells still have a few proliferation cycles to undergo in development.
Elsewhere, Elston et al measured the number of spines in the average pyramidal cell of macaque brains in the primary visual cortex (V1), the inferior temporal gyrus (TE), and the prefrontal cortex (PFC) at different stages of development. They found an inverted U shaped curve of spine number with log age:
The authors conclude that “synaptic activity thresholds that reinforce synapses and stabilize dendritic spines may vary across cortex.” It is interesting that the regions follow the same general trend in each region, peaking at 3.5 months.
Maja Abitz , Rune Damgaard Nielsen , Edward G. Jones , Henning Laursen , Niels Graem , and Bente Pakkenberg. Excess of Neurons in the Human Newborn Mediodorsal Thalamus Compared with That of the Adult. Cerebral Cortex Advance Access published on January 11, 2007, DOI 10.1093/cercor/bhl163.
Guy N. Elston, Tomofumi Oga, and Ichiro Fujita. Spinogenesis and Pruning Scales across Functional Hierarchies. J. Neurosci. 29: 3271-3275; doi:10.1523/JNEUROSCI.5216-08.2009