Our brains have more in common with testicles than you ever wanted to know

That delightful saying about men thinking with their nether regions has gained a new meaning. A new study has found an unnerving lot of similarities between men’s brains and the innards of their scrotums.

“Brain and testis have the highest number of common proteins, compared with other human body tissues,” a team led by biomedical scientist Bárbara Matos from the University of Aveiro in Portugal writes in their new paper.

While the brain has a highly complex role – controlling our bodies, receiving and interpreting signals from sensory organs, not to mention doing all our thinking and feeling, human testes have just two main functions – the production of sperm and hormones. (Although, many of us should be forgiven for attributing these gonads with their own thoughts and feelings too.)

Previous studies have suggested there are links between sexual dysfunction and brain disorders, and even between intelligence and semen quality. Of course, such links do not mean much by themselves, but now the team of researchers from Portugal and the UK has found an explanation for why they might exist.

They compared proteins across 33 tissue types, including the heart, intestine, cervix, ovaries and placenta, and found that testes and brains share 13,442 proteins in common. This is corroborated by gene expression studies showing these two distantly positioned organs share the highest number of genes among all the organs in the body.

Taking a closer look at the shared proteins most highly expressed in these tissues, Matos and colleagues found they’re mostly involved in tissue development and cell communication. These shared proteins make sense when you consider how unexpectedly similar the two tissues are in many ways, the team explains.

The brain and testes are both greedy for energy to fuel highly demanding processes like thinking and the production of several million little sperms per day. So both organs have specialized cells to support the hard-working neurons in the brain and germ cells in the testes – to keep them well fed and physically comfortable.

Also, despite being very differently purposed cells, neurons function similarly to sperm in several ways. Both cells have important tasks involving moving stuff from within themselves to their outside environment – a process called exocytosis.

This is how brain cells pass neurotransmitters between each other. In sperm, the same process is used to release important fertilization factors.

In neurons, exocytosis is also involved in the growth of their reaching little branching arms collectively called neurites (dendrites and axons), while in sperm this process allows its innards to fuse with an egg.

“This is an underexplored topic, and the connection between these tissues needs to be clarified, which could help to understand the dysfunctions affecting brain and testis,” the team wrote.

These findings raise a lot of questions, the obvious being how did two such disparate organs end up sharing so much in common? The researchers suspect it’s because they’re both strongly influenced by the speciation process.

Just like animals separated by millions of years of evolution and evolved half a world away from each other can develop the same traits, so too can different tissue groups within the human body.

For example, unlike most other animals, koalas have fingerprints confusingly similar to ours – thanks to the obvious selection pressure exerted by our (well, our primate ancestors’) need to grip trees – despite 70 million years of evolution between us. This process is called convergent evolution

In this case, the researchers propose the same selection pressures involved in keeping species distinct from each other may be imposed on both organs, causing them to evolve convergently. They point to 60 protein-coding genes, unique to humans, many of which are found within the brain and testis.

“The highest expression levels in cerebral cortex and testis suggested that these genes may contribute to phenotypic features that are exclusive of humans, such as the improved cognitive ability,” the team wrote.

While owners of testes may not be so thrilled by these biological revelations, the rest of us might be inclined to think it makes an awful lot of sense. But before we get too ahead of ourselves, this finding means female brains share these similarities with balls, too.

Their research was published in Royal Society Open Biology.

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