Adult neurogenesis was found in humans again
Stem nerve cells and newborn neurons were again found in the memory center of adults
Kirill Stasevich, "Science and Life"
The story of adult neurogenesis in humans is becoming more and more like a detective series: less than a month after the article in Nature, which stated that there are no new neurons in the adult human brain, another article appeared in Cell Stem Cell with the statement exactly the opposite – that they exist (Boldrini et al., Human Hippocampal Neurogenesis Persists through Aging).
A newborn neuron in the senile brain. Image: Maura Boldrini / Columbia University Vagelos College of Physicians and Surgeons, Even Old Brains Can Make New Neurons, Study Finds.
Adult neurogenesis is the appearance of new neurons in the adult brain. New neurons in the brain of adult mammals began to be found in the 90s of the last century; then eventually they were found in humans. But due to the fact that the human brain is more difficult to study than the animal brain (not all "animal" experiments can be transferred to humans), there was always some ambiguity regarding human adult neurogenesis.
The intrigue here is all the more interesting because both groups of researchers went the same way in general. Different cells differ in their molecular portrait, which means that, for example, in young neurons, you can find specific proteins that old ones do not have. Maura Boldrini from Columbia University and her colleagues – the authors of the article in Cell Stem Cell– used much the same molecular features as the authors of the "anti-neurogenesis" article in Nature. But here the nuances begin.
Firstly, the authors of the article in Cell Stem Cell took samples from the brains of deceased people (a total of 28 people aged 14 to 79 years) no later than 26 hours after death. The authors of the article in Nature used samples taken no later than two days after death, respectively, some changes could occur in them, erasing their characteristic features in new neurons.
Secondly, in the second work, samples were taken so as to cover the entire hippocampus – the memory center of the brain, where, as it is believed, new neurons are born in us. (In the first work, some areas of the hippocampus were left behind the scenes.) Finally, the authors of the article in Cell Stem Cell used stereological methods to understand how many cells of a particular type there are in the entire tissue as a whole.
The cellular composition of a tissue or organ is studied by extremely thin sections, and, of course, the question always arises how such two-dimensional sections correspond to a three-dimensional organ. Stereological methods, which rely on statistical processing of data obtained from multiple slices, just allow you to restore a three-dimensional picture. For various reasons, stereology is not always possible to use, but now it has been used for the first time to count certain cells in the entire hippocampus.
The researchers were looking for four types of cells. Firstly, mature granular neurons (this is the name of one of the varieties of brain neurons that can be found in the hippocampus and in some other brain regions). Secondly, young granular neurons that have formed recently.
Thirdly and fourthly, there are two types of progenitor cells, or stem cells, which can only turn into cells of a certain type – in our case, nerve cells.
What turned out to be? Progenitor cells of one variety really disappear from the brain with age (although we managed to count about a thousand of these disappearing in adults). However, the progenitor cells of another kind - the so–called intermediate progenitor cells - apparently do not go away from the brain: despite their age, their number remains more or less constant, and many thousands of them were counted in the hippocampus. The same applies to young (or immature) neurons – there were also several thousand of them in the samples. So the researchers concluded that neurogenesis in humans still lasts a lifetime.
On the other hand, young neurons in some parts of the old hippocampus differed in structure – judging by their appearance, they were worse at contacting other neurons, it was more difficult for them to form interneuronal synapses. So if neurogenesis does not fade with age, new neurons are not so useful because of weak plasticity (that is, because of a weak willingness to form new connections) - at least in some areas.
True, the authors of the first, "anti-neurogenetic" article say that they saw exactly the same cells and in the same quantities, but after they tested them with additional methods, it turned out that these were not stem cells and not young neurons, but simply mature neurons of a different type. And if we looked at the newborn cells from the new article with an electron microscope, or analyzed the activity of genes in them, we would be convinced that the authors of the publication in Cell Stem Cell considered something wrong...
In general, it's too early to draw any conclusions here - at least until specialists get rid of such methodological ambiguities in the study of the human brain.
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