During pregnancy, some women find that their sense of smell changes. Researchers have now found a possible explanation for this using mice. Accordingly, dormant stem cells in the brain are activated during pregnancy and form new nerve cells that migrate to the olfactory bulb in time for the birth of the offspring. This enables the mice to recognize their own young by smell. Similar processes could also occur in humans.
During pregnancy, various hormones are released in the mother’s body, which prepare the body and brain for the birth and the upcoming role of mother. The pelvis loosens, milk production is stimulated and changes also take place in the brain. Previous studies have shown that pregnancy hormones promote parental behavior in humans and animals and support bonding with offspring. Some women also report changes in their sensory perceptions: some develop preferences for foods during pregnancy that they previously disliked, and some find that they perceive smells differently than they did before pregnancy.
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Dormant stem cells activated
A team led by Zayna Chaker from the University of Basel has now gotten to the bottom of this phenomenon. To do this, they examined how the nerve cells in the brain change in pregnant mice. The team focused on the so-called ventricular-subventricular zone, an area of the brain in which stem cells can form new nerve cells that migrate to the olfactory bulb. However, outside of pregnancy, many of these stem cells are in a resting state.
“Our studies show that several regionally different neural stem cells, including those that are normally dormant, are recruited on different days during pregnancy,” the team reports. The nerve cells that mature from these stem cells migrate to the olfactory bulb. “The timing is very precise,” explains Chaker’s colleague Fiona Doetsch. “The new neurons are ready just in time for birth.”
Recognition of one’s own offspring
As soon as the young are born, the mother mouse can reliably recognize them by smell and distinguish them from foreign young animals thanks to the additional olfactory neurons. In behavioral experiments, however, female mice without their own offspring were unable to distinguish the smell of the offspring of different mothers. “However, the neurons created during pregnancy are only needed temporarily and are eliminated when the offspring is older and independent,” says Doetsch. Between the 20th and 30th day after birth, when the young are weaned, the specific olfactory cells also regress.
To find out whether the olfactory cells are influenced by the presence of the young animals, the researchers took their babies away from some mouse mothers just one day after birth. In fact, in these mice the nerve cells specifically adapted to the young animals had disappeared from the olfactory bulb after just six days. On the other hand, if the researchers repeatedly placed new newborn mouse babies with a mother mouse, she retained the specific nerve cells in the olfactory bulb for significantly longer and also recognized her adopted children by smell.
Also in humans?
Even though smell plays a much smaller role in recognizing offspring in humans, the research team suspects that similar mechanisms are at work in expectant mothers. “Some women report changes in their sense of smell during pregnancy,” says Chaker. “It could be similar in humans.” In the corresponding brain area in humans, there are also stem cells that can develop nerve cells for the olfactory bulb – at least until early infancy. According to current knowledge, the stem cells are then inactive.
“As observed in the mice, it would also be conceivable that pregnancy awakens the stem cells from their dormant state,” says Chaker. For other areas of the brain, it is already known that the formation of new nerve cells from neural stem cells is involved in learning processes and has an influence on our memory. In an accompanying commentary on the study, which also appeared in the journal Science, Gerd Kempermann from the German Center for Neurodegenerative Diseases in Dresden, who was not involved in the study, writes: “The new data convincingly prove that the formation of new nerve cells in the olfactory bulb contributes to important brain function in adult individuals that goes beyond learning and memory.”
Source: Zayna Chaker (University of Basel, Switzerland) et al., Science, doi: 10.1126/science.abo5199