Speaking of cool science about the human eye and other things, I loved this story about the age of the body's cells. It's got cell biology, plant biology, and nuclear physics all rolled up into one neat result. Apparently, while most of the body's cells regenerate on a fairly regular time period, the same is not true of the visual cortex. Here was how this was studied:
It was a dispute over whether the cortex ever makes any new cells that got Dr. Frisen looking for a new way of figuring out how old human cells really are. Existing techniques depend on tagging DNA with chemicals but are far from perfect. Wondering if some natural tag might already be in place, Dr. Frisen recalled that the nuclear weapons tested above ground until 1963 had injected a pulse of radioactive carbon 14 into the atmosphere. Breathed in by plants worldwide and eaten by animals and people, the carbon 14 gets incorporated into the DNA of cells each time the cell divides and the DNA is duplicated. Most molecules in a cell are constantly being replaced but the DNA is not. All the carbon 14 in a cell's DNA is acquired on the cell's birth date, the day its parent cell divided. Hence the extent of carbon 14 enrichment could be used to figure out the cell's age, Dr. Frisen surmised. In practice, the method has to be performed on tissues, not individual cells, because not enough carbon 14 gets into any single cell to signal its age. Dr. Frisen then worked out a scale for converting carbon 14 enrichment into calendar dates by measuring the carbon 14 incorporated into individual tree rings in Swedish pine trees.
The average age of most of the body's cells is apparently around 10 years, though there seems to be a ton of variation: the epithelial cells that line the surface of the gut last five days, and the liver totally replaces itself in 300-500 days. And the study found the cells of the visual cortex are exactly the same age as the individual -- apparently the only other cells outside of the cortex with this property are the inner lens cells of the eye and perhaps the muscle cells of the heart. Cool method, cool result. And it's good to know that at least something interesting came out of these above ground nuclear tests.
