Why men and women have different brain sizes

(ORDO NEWS) — Thousands of brain scans have shown differences in brain volumes in men and women. It was found that this difference is dictated by sex chromosomes, which can influence the anatomical structure of the brain during growth and development, writes Science and Technology Daily.

Scientific analysis of over 2,000 brain scans has shown that there are strong sex differences in the volume of certain areas of the human brain. This pattern of gender differences in brain volume is consistent with the patterns of sex chromosome gene expression observed in cerebral autopsy samples, indicating that sex chromosomes may affect the anatomical structure of the brain during growth and development. The study was led by American scientists from the National Institute of Mental Health (NIMH) at the US National Institutes of Health (NIH), and the results were published in the journal Proceedings of the National Academy of Sciences.

“A clearer understanding of the gender differences in human brain tissue is important as it helps us to understand the established gender differences in perception, behavior and risk of mental illness. Inspired by the sex differences found in experimental animal models, we wanted to try to bridge the gap between this animal data and our patterns of sex differences in the human brain, ”said Armin Raznahan, head of the NIMH’s Evolving Neurogenomics Section, who took active participation in the study.

For a long time, researchers have observed gender differences in the subcortical structures of the brains of mice. Several studies suggest that these anatomical differences are largely due to the effects of sex hormones, and have advanced a “gonadocentric” theory to explain sex differences in brain development. However, later studies in mice also revealed persistent sex differences in cortical structures. At the same time, gene expression data suggest that sex chromosomes may play a role in the formation of these anatomical sex differences. While there are many similarities between the mouse brain and the human brain, it is not yet clear if the key findings apply to mice and humans.

To explore the neurobiological underpinnings of gender differences in the human brain, Raznahan, lead study author Dr.Liu Xiyuan, and his colleagues first analyzed neuroimaging data collected through the Human Connectome Project (HCP). These data were collected from 976 healthy adults, aged 22 to 35 years, and the results revealed consistent gender differences in the volume of certain cortical structures. On average, women had relatively larger cortical volumes in the medial and lateral prefrontal cortex, orbitofrontal cortex, superior temporal sulcus and lateral parietal sulcus, while men on average had relatively larger cortical volumes in the ventral temporal and occipital regions, including the temporal pole, fusiform gyrus and primary visual cortex.

Liu Xiyuan and his colleagues then used two complementary approaches to determine the reproducibility of these results. First, the researchers randomly split the HCP dataset into two 1000-unit pieces, and then compared the results of both halves, finding that the pattern of sex differences in cerebral cortex volume was very stable. The researchers then obtained consistent results with the findings of the HCP in an unrelated neuroimaging dataset from the British Biobank. Although the datasets had marked demographic and methodological differences, the researchers found that the overall picture of gender differences in cerebral volume was very consistent.

After that, Liu Xiyuan and his co-authors linked their anatomical data to public maps of gene expression in the brain, which are based on 1,317 postmortem tissue samples from six human donors. The results showed that the spatial pattern of sex differences in cortical volume was similar to the spatial pattern of sex chromosome gene expression in the cerebral cortex. In particular, areas of the cortex with relatively high expression of sex chromosome genes tend to have a larger cortical volume in males than in females.

This alignment of cortical gene expression on the sex chromosomes is also consistent with previous studies in mice. Research has shown that gender differences in brain anatomy can be at least partially explained by genetic mechanisms that have persisted throughout mammalian evolution. These data indicate that genes located on the X and Y sex chromosomes may affect sex differences in cerebral volume.

“Men and women differ in many genetic and environmental factors, and these factors can potentially influence brain development. Because human experimentation is complex, we often rely on observational data to infer potential genetic or environmental factors that influence sex differences in the brain, Raznahan said. – We observed a very high level of reproducibility of anatomical sex differences in different groups of men and women, as well as the relationship between these differences and the expression of genes of sex chromosomes. All of the above facts show that these differences are probably not just the result of environmental influences. ”

The researchers also compared their anatomical data with data from over 11,000 functional neuroimaging studies. The results indicate that there is a spatial overlap between brain regions that showed sex differences in cortical volume in the HCP dataset, and brain regions associated with facial recognition in functional neuroimaging studies.

Using these results as a roadmap, future research can more effectively investigate the causes and consequences of sex differences in the human brain.

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