Epithelial cells of the intestine colored by fluorescent proteins. Image courtesy of Hans Clevers, Hubrecht Institute
Showing some guts
On an organ-izational chart, your intestines aren’t likely to draw the same attention as, say, your heart or brain, but they too are marvels of nature and activity. For one thing, much of what constitutes you lives in your gut.
The average human body contains roughly 100 trillion cells, but only one in 10 of those cells is, well, human. The rest are bacteria, viruses and other microorganisms.
A majority of these non-human cells reside in the gut, representing 500 to 1,000 species. For the most part, your gut flora perform valuable services to their host: extracting vitamins, converting foods into energy, generally aiding digestion and keeping nastier bugs at bay. It’s a thankless job, and temporary at that: up to 60 percent of dry fecal matter is composed of excreted bacteria.
Non-human microbes, though, aren’t the only intestinal occupants stuck on the workplace treadmill. While many of the tissues in your body are constantly renewing – you shed an estimated 30,000 to 40,000 skin cells every hour; getting an entirely new skin roughly once a month – no place turns over its employees more rapidly than the lining of your gut.
The epithelial cell lining of the human intestine lasts about a week. These cells begin their brief work lives in deep pits called crypts. As they get older, they are pushed up the sides of tiny projections called villi, which protrude into the intestinal space and multiply exponentially the active surface area of the intestines. Once an epithelial cell has climbed to the top, it dies, detaches and joins the progression of dead bacteria and other biological flotsam on the road to excretion. It’s not much of a career. Neurons, by comparison, can live as long as you do.
Scientists know much about how this process works through clever technologies like the one that produced the image above. Researchers induced stem cells in the intestinal crypts to randomly switch on one of four fluorescent proteins (red, yellow, green or blue) in each newly minted intestinal epithelial cell.
After that, it’s just a matter of tracking and tracing these cells back to their creators.