Every sip of this morning’s coffee was tasted by cells only about a week old. By the end of next week, plenty of them will have cleared their desks, and a fresh intake will be reading the flat whites instead.

Taste buds are the small clusters of cells, arranged like the segments of an orange, tucked into the bumps and grooves of the tongue. An adult carries between 2,000 and 4,000 of them, per the InformedHealth.org guide to taste from Germany’s Institute for Quality and Efficiency in Health Care. Each bud holds 10 to 50 sensory cells, and those are renewed about once a week. Extra taste cells turn up at the back of the throat and even in the upper part of the food pipe.

None of this comes with a memo.

How the ten-day figure was first measured

In 1965, two biologists at Florida State University set out to time the shifts. They injected rats with thymidine, one of the raw ingredients a cell uses to copy its DNA before dividing, tagged with a faint radioactive label. Cells that split shortly afterwards soaked up the tag and passed it to their daughters, and it showed up as dark specks once thin slices of tongue were coated in photographic emulsion and left to develop.

Writing in the Journal of Cell Biology, the pair reported that new cells were born in the lining around each bud, then drifted inward toward its centre. An average cell lasted about 250 hours, give or take 50, which rounds to roughly ten days. Some burned out much sooner and others stuck around far longer. All of it was measured in rats.

Some taste cells outlast others

Does every cell in a bud clock off on the same schedule? A University of Miami team put that to the test in mice, in a paper published in PLOS One in 2013, sorting the cells by type before starting the clock.

Type II cells, which carry the receptors for sweet, bitter and savoury (umami) flavours, had a half-life of eight days. Half-life means the time it takes for half of a batch to disappear, so within just over a week, half the receptor cells from any given intake were gone. Type III cells, the most nerve-like members of the bud, were in no hurry, with a half-life of 22 days.

That is one study, looking at one patch at the rear of the tongue, so the exact figures belong to that setup. It does fit the spread the Florida State pair spotted decades earlier, and it recasts ten days as an average smoothed across very different careers.

Where the replacement cells come from

“Taste tissue regenerates continuously throughout the life span in mammals.” That line opens the plain-language summary of a 2014 paper in PNAS from the Monell Chemical Senses Center, which went looking for the source of all those new recruits.

Its authors isolated single stem cells from mouse taste tissue, identifying them by a surface protein called Lgr5 (or its close relative Lgr6), which marks out adult stem cells in several tissues. Grown in a dish, a single one of these cells could multiply into a small three-dimensional clump known as an organoid. Those clumps contained all the mature taste cell types, the cells reacted to taste chemicals, and they managed all of it with no nerve attached.

That knack for renewal is rare among the body’s sensing cells. As a taste biology lab at the University of Colorado Anschutz Medical Campus points out, the light-sensing rods and cones of the eye and the hair cells of the inner ear lack it. Wear those out and no replacement order is coming.

What happens when the renewal stops

Basal cell carcinoma, a common skin cancer, can be treated with drugs that block a growth signal called Hedgehog, a chemical message that helps developing cells decide what to become. Many patients on these drugs report severe taste disturbances, which prompted University of Michigan researchers to give one of them, sonidegib, to mice and track the fallout, as documented in PNAS in 2017.

The precursor cells that feed each bud were thrown off their usual pattern of dividing and maturing, and the buds disappeared. Nerve responses to taste were wiped out, while responses to touch and cold carried on. Once the drug was withdrawn, the buds grew back and the taste signals returned.

It is a mouse result, though it lines up neatly with reports of patients’ taste recovering after they finish treatment.

Taste can feel like one of the most settled things about a person, as fixed as an accent. A decades-long grudge against coriander has been maintained by taste cells replaced hundreds of times over since the feud began.