Neurophilosophy has this nice post reviewing work by Curtis and colleagues in Science. The main idea of the paper is that a neural migratory pathway in rats and mice, which enables neurons to migrate into the olfactory bulb from the subventricular zone, also exists in humans -- even though nobody noticed it before.
To me, the interesting part is the description of how they noticed the new neurons. In this case, cancer treatment had left the new cells with a flag showing that their DNA had been recently synthesized:
Maurice Curtis and his colleagues examined the brains of deceased cancer patients who had previously been injected with bromo- deoxyuridine (BrdU), a chemical which is incorporated into newly-synthesized DNA, and which is therefore used by oncologists to visualize and monitor the growth of tumours. To their surprise, they found BrdU-positive cells in the olfactory bulbs of the patients' brains, suggesting that it contained newly-generated neurons. Curtis's team then used antibody staining to show that the cells begin to differentiate into olfactory neurons while migrating through the rostral migratory stream. Upon arriving at the bulb, the cells continued to differentiate, forming mature olfactory neurons. Using electron microscopy, they also showed that this 'tube' is 3.5 mm long and 1.5 mm in diameter.
I like the "natural experiment" aspect of this. We'll have to see if other instances of adult neurogenesis can also be highlighted in this way, particularly among younger adults (this is a group where the minimum age was 38), but also in older adults with neurodegeneration.
Curtis, M. A. et al. (2007). Human neuroblasts migrate to the olfactory bulb via a lateral ventricular extension. Science (Online early) doi:10.1126/science.1136281