DNA replication is followed by division of the neuronal nucleus and chromosomes.

Thymidine, one of the four main components of DNA can be made radioactive (a marker) and observed with the electron microscope.

When Thymidine shows incorporated in neuronal DNA, it indicates, that a new neuron has emerged. 

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Incorporation of Thymidine marker in the DNA of neurons, indicates  newly synthesised DNA.

Incorporation of Thymidine marker