In forty years, more than four hundred thousand scientific articles have been written on stem cells. Fifty thousand concern the brain, ten thousand the generation of new neurons in adulthood.
Stem cells are undifferentiated cells which, if appropriately stimulated, reproduce themselves indefinitely and, at the same time, a cell which, as it matures, specializes and replaces its adult and dead similar ones. So at the end of the process we will have another stem and a mature cell going to do its job. Biologists speak of asymmetric reproduction because cells usually only make copies of themselves (symmetrical reproduction). The entire epidermis regenerates in 3-4 weeks, the red blood cells in four months, the bones of the skeleton in ten years. Salamanders, lucky them, even regenerate an entire amputated limb.
Regarding the human brain, there was a dogma: as we age, the supply of neurons received at birth can only be depleted. No regeneration, otherwise the experiences accumulated over time and therefore the identity of the person itself will evaporate. This was authoritatively affirmed by Santiago Ramòn y Cajal, founder of modern neuroscience and Nobel laureate in 1906 together with Camillo Golgi.
A dogma falters
Seventy years later, at Rockefeller University, Fernando Nottebhom discovered that in the brains of certain songbirds the cells that direct the nuptial chirp are renewed at each mating season, a phenomenon he documented in 150 works. It was the beginning of a revolution. The hunt for brain stem cells was unleashed in all neuroscience laboratories around the world. Ezio Giacobini, a pharmacologist and researcher of Alzheimer’s disease, divulged the discovery in “Tuttoscienze”, and since the fall of a dogma makes the news, stem cells – real or alleged – won headlines in all the newspapers, including those you read at the barber shop.
It is therefore not surprising that today, even at the popular level, the idea is widespread that the brain – at least in part – is renewed. In essence, Cajal was right, but he was exaggerating. The brain is not rigid like a crystal and not even gaseous as it appears in some people, but plastic like the Pongo. Therefore, brain plasticity is now an established fact. But there are several types of plasticity. We have 86 billion neurons and each has thousands of contacts with other neurons: the synapses. Contacts reconfigure for life or almost. After reading a book or meeting a person, we feel that we have changed, sometimes forever. Memory is a plasticity of connections, it modifies the structure of the network (the connectome) and does not involve new neurons or stem cells.
Trust the Swiss?
Instead, the plasticity due to stem cells is not limited to the restructuring of the existing: it introduces new bricks. Many neuroscientists have observed it in mice throughout its life (two years) and also in humans, in which, however, neurogenesis seems limited to the peri-ventricular area – where it disappears at the age of two – and to the hippocampus, where ends with adolescence.
But randomness also plays its part in science. In 2010, due to a fatal distraction, the Swiss researcher Hans Lipp missed a zero in the typescript of his article and led to an estimate of 20 times the neurogenesis in the hippocampus of the mouse. Incredibly, the mistake was never retracted. Swiss precision is trusted and not only when it comes to watches. This led a Swedish team into the same misunderstanding regarding adult humans. The misprint helped fuel the neuroscientist community’s disagreement over the stem cell issue of the brain. How are things really?
Two intertwined passions
Luca Bonfanti, who teaches veterinary anatomy at the University of Turin and does research at the Neuroscience Institute “Cavalieri Ottolenghi” (Nico), has been studying brain plasticity since 1990 and is also an irreducible connoisseur of detective stories. Writing “The enigma of the young neuron” (Edizioni Dedalo, 90 pages, 11.50 euros) he intertwined his two great passions. The result is that the reader learns a lot and has fun. Anyone wishing to know in depth the background of the “mystery” can read Bonfanti’s previous book, “The invisible cells” (Bollati Boringhieri, 2009), also built as a thriller.
The matter is tangled, we cannot untangle it here. In summary, between 2018 and 2019 three articles confirmed the genesis of new neurons in the hippocampus of adults and elderly people. Too bad that in 2011 another, more reliable study, published in “Nature”, had reached the opposite conclusion. The work was by Arturo Alvarez-Buylla, who in 1994 had demonstrated adult neurogenesis in the lateral ventricle of mice, from where they would migrate to the olfactory bulb: so we’re certainly not talking about a conflicting source of interest. The picture is enriched by the previous discovery of a “molecular sign of immaturity” by the Japanese Tatsunori Seki and by Luca Bonfanti himself, who thus enters the thriller scene and will see his role expand with the discovery of many immature neurons (photo in high) in the neocortical brain of sheep and other mammals (fertility of comparative research!).
Exemplary case study
Hence the enigma of the “young neuron”, an exemplary case study for epistemologists because it involves on the one hand the difficult demolition of the ancient dogma of Cajal and on the other hand the modern and fierce competition between two schools of neuroscientists. Today we know that synaptic plasticity is flanked by stem cell neurogenesis and “immature neurons”, which can remain so for very long. Advocates of adult neurogenesis have probably made the mistake, this time not in print, of considering neurogenesis to be the presence of immature neurons.
Finally, Mikhail Semenov, a professor at Boston University, insinuated a radical denial suspicion: according to him the phenomenon of adult neurogenesis (real in mice but not in humans) “cannot be considered on a par with the renewal present in other stem systems , such as skin or blood, because the hippocampal cells are not replaced several times and massively “.
Final exhortation
From the narrative point of view, the yellow of the young (or immature) neuron has an open ending. The soap opera makes people want new episodes. Who knows that one day these searches will not lead to finding a cure for the 300 million Alzheimer’s patients around the world.
But for us mere mortals, the most important message is another: the plasticity that we really must care about is not so much that of stem cells but of synaptic, because it is structural, lasts a lifetime and is intertwined with the destiny of eight billion people. , how much is the world population. “Seniors, socialize, read, walk, do not shut yourself up in your solitude in front of the TV” exhorts Bonfanti. As for young neurons, “we can cultivate them with lifestyles, making them work to actively solve the problems and difficulties that the world presents to us every day. It seems that evolution has given them to us not so much to repair the damage as to have an extra gear in our brains when we are still well “. That is when we can increase the “cognitive reserve” which will help us to counteract the decline and make it a little sweeter and more acceptable.