Elizabeth Norton has an interesting write-up in Science Now. Some years ago, after a long period of suspicion, it was seemingly demonstrated that neurogenesis (the formation of new neurons) happened in the human nose. This research was based on the identification of proteins that would be associated with the early formation of baby neurons. Therefore, it was not possible to prove that full grown and functioning neurons were being grown in the nose, but it was assumed to be a reasonable possibly.
However, it really isn't a reasonable possibility. If there was an Intelligent Designer, then sure, why would baby neurons pop up and then not turn into functioning adult neurons? But if there is no Intelligent Designer, and instead, things evolved, then it is quite possible that the lack of novel fully formed and hooked up neurons in an adult human (which seems to be the general rule of thumb, for whatever reason) is not necessarily achieved via some highly sensible planned out feature. Rather, it is most likely that an evolved feature is a kludge. If it turns out that neurogenesis occurs in the adult human nose but that those nascent neurons never enervate, well, that is what we might expect evolution, which is not intelligent but, rather, pragmatic, to come up with.
The method of testing this idea, applied by Jonas Frisén of the Karolinska Institute in Stockholm, is just as interesting as the finding itself. The idea is to date the neurons in the nose. One way to date organic tissue might be to use C-14 dating like archaeologists use, but that method is not precise enough. The neural tissue in a living human might be something like "50 years old plus or minus 80 years" which would not be too useful. But there is a way to use C-14 after all. Since atomic testing started, there has been a LOT more C-14 pushed into the atmosphere, and the added radiocarbon allows for a more precise atomic clock, if the clock is properly calibrated. This method was initially pioneered a few years ago in the forensic case of two sisters who were found dead, long after they had expired, in their home in Vienna. Both sisters had considerable wealth, and the one who died first would have passed on that wealth to the second, living sister. The relatives of the second-to-die sister would therefore receive a considerably larger inheritance than the relatives of the first-to-die sister. The two sisters' bodies were found semi-mummified, and a couple of years after death, in their apartment which was surrounded by neighbors who never noticed they were no longer around.
The post-A-bomb calibrated C-14 method was used to determine that the sisters had in fact died about a year apart. This method has subsequently been used for other fine-tuned post atomic dating. (There is a write-up of this here.)
OK, now back to the nose.
In the new study, published this week in Neuron, Frisén, Spalding, and colleagues measured levels of 14C in olfactory bulb tissue taken during autopsy from the brains of 15 subjects who were born either before or after the atomic testing period. The researchers found that the neurons in the olfactory bulb were all the same age: the age of the individual they came from. "[That's] evidence that in humans, in this area, neurogenesis doesn't occur," says Frisén.
There is still evidence, i.e. from mice, that neurogenesis of useful neurons does happen in some mammals. The question of novel nose neurons is not entirely settled. But, when the question comes up "Do humans generate new neurons as adults" please make sure that the assumption that they do is not based on this earlier nose research, or on any studies that merely looked for new neuron proteins.
In addition, Macklis points out that the tissue samples may have biased the results. The donors in the study died at the Karolinska Institute, he notes, and some had a history of substance abuse or psychiatric illness, both of which have been shown to decrease neurogenesis. He says that a better test would be to repeat the experiment in healthy people constantly exposed to new scents—chefs, sommeliers, perfumers, or travelers to exotic locales.
Face it: there is still some head scratching going on. We will need to keep an eye on this nose research before sealing our lips on it, and in the mean time, keep your chin up.