We answered this question on the show...We posed this question to Kirsty Spalding, regeneration expert at the Karolinska Institute in Sweden...
Kirsty - For starters in the brain, most of your nerve cells that you're born
with will be the ones that you have when you're older. Although we do continue to lose nerve cells as we age, there's very little areas in the brain that actually make new nerve cells in adulthood.
Also the lens capsule in the eye, this is also not turning over at all so the lens capsule will be as old as you are.
And with teeth, once we have our mature teeth, the enamel that's laid down in these teeth, this is not turned over at all.
Other organs in the body have much more dynamic turnover. An example of this is fat. We replace 50% of our fat cells per year. Other tissues, such as muscle, by the end of a relatively healthy lifespan of about 75 years, less than half of the muscle cells in the heart will have turned over.
Bone is a rather complex structure and in the middle, it has the bone marrow which is making the white and the red blood cells. Here, this is a highly proliferative area, such that millions of cells are being born per second.
So as you can see, there's quite a range in contrast of turnover rates of all the different cell types in the body. And with regards to your question on what happens to the shed tissue, sometimes our cells are broken down into their components and recycled or if they're toxic, they can be packaged up for removal of a further processing, or they can be used up in the form of energy.