Imaging taste cells in action

An international team of scientists has used a specially designed microscope to capture the first live images of taste taking place on a tongue.

The human tongue has over 2000 tastebuds which can distinguish at least five tastes: salty, sweet, sour, bitter and umami. Yet the relationship between the many taste cells within a tastebud and our perception of taste has been a long-standing mystery, according to Professor Seok Hyun Yun from Harvard Medical School.

Writing in the journal Scientific Reports, Professor Yun and his co-authors stated that intravital multiphoton microscopy - an otherwise powerful tool in neuroscience - has not been adapted to the tongue due to anatomical constraint. Together, the team “developed an imaging window to facilitate microscopic access to the murine tongue in vivo”.

The tongue, illuminated by two-photon excitation. Tastebuds are blue, amongst the yellow papillae. Image credit: S Lee, S Yun, M Choi.

The scientists shone a bright infrared laser onto a mouse’s tongue, which caused different parts of the tongue and the flavour molecules to fluoresce. This fluorescence was captured through intravital multiphoton microscopy. The scientists were then able to pick out the individual taste cells within each tastebud, as well as blood vessels up to 240 µm below the surface of the tongue.

“With this new imaging tool, we have shown that each tastebud contains taste cells for different tastes,” said Professor Yun. The team also discovered that taste cells responded not only to molecules contacting the surface of the tongue, but also to molecules in the blood circulation.

“We were surprised by the close association between taste cells and blood vessels around them,” said co-author Assistant Professor Myunghwan Choi from Sungkyunkwan University.

“We think that tasting might be more complex than we expected, and involve an interaction between the food taken orally and blood composition.”

A tastebud showing receptor cells (green), blood cells (red) and collagen surrounding the bud (blue). Image credit: S Lee, S Yun, M Choi.

The breakthrough complements recent studies by other research groups that identify the areas in the brain associated with taste. The team now hopes to develop an experiment to monitor the brain while imaging the tongue to track the full process of taste sensation.

However, co-author Dr Steve Lee from The Australian National University said it could take years to fully understand the complex interactions that form our basic sense of taste. He explained, “Until we can simultaneously capture both the neurological and physiological events, we can’t fully unravel the logic behind taste.”