Most people probably think of tastebuds as existing only on their tongues, but did you know there are taste buds in testes? It’s true. Sort of. They aren’t exactly like the taste buds in your mouth. Male germ cells–the cells that are destined to become sperm–have molecules on them that can detect bitter tastes.
These bitter taste receptors, called T2rs, were recently discovered on male germ cells by researchers at the Monell Chemical Senses Center in Philadelphia, PA. Obviously, the first (and from my perspective, only) question that came to mind was: why? Why do pre-sperm cells need to be able to “taste” bitter compounds?
The researchers first looked at what happens when these cells come into contact with bitter-tasting stuff. But it still remains a mystery why testes need taste receptors in the first place.
Bitter taste receptors (T2rs) were first discovered in taste bud cells. Later, these receptors (proteins that sit on the cell surface and sense the environment) were also found in the gastrointestinal tract. Apparently, your body can track bitter compounds–from hops to quinine–all the way from your mouth to your intestines. Other places T2rs are found in the nose, lungs….and now the testes.
Many poisonous or toxic compounds taste bitter. The researchers at Monell wanted to know if male germ cells–that is, the sperm or cells that are destined to become sperm–could use bitter taste receptors to sense toxic chemicals. They used cells from mouse testes for their studies.
First, they looked only for T2rs that are known to sense poisonous compounds (there are 9 of these in the mouse). All 9 were expressed in developing sperm cells called spermatids. They then said ‘what the hay,’ and looked for all known mouse T2rs. All 35 of the mouse T2r genes were expressed in spermatids.
But do they work?
The researchers tested several bitter-tasting compounds on the spermatids and mature sperm of mice. Some compounds were naturally occurring, like caffeine. Others were synthetic. Throwing some bitter compounds on these tasteticular cells (oh, yes I did) caused the amount of calcium in the cells to increase. When these kind of receptors are activated, they cause calcium to increase. So, it looks like the receptors work. The results were tested further by using a chemical that blocks T2rs. No more calcium response.
Okay, so your sperm can taste. But why? The paper doesn’t get this far, but you might imagine that swimming sperm could use it to avoid certain toxic chemicals in the female tract. Alternatively, there might be some unknown bitter compound, or compounds, that signal to the sperm or spermatids to express specific genes.
Why do you think sperm need taste buds?
Xu J, Cao J, Iguchi N, Riethmacher D, & Huang L (2013). Functional characterization of bitter-taste receptors expressed in mammalian testis. Molecular human reproduction, 19 (1), 17-28 PMID: 22983952