We all know the cells in our body are constantly dying and being replaced by new ones. If I started now, how long would it be until every cell currently living in my body will have been replaced?
It is going to differ a lot for different types of cells, and I think your premise is a bit questionable. Until fairly recently, it was thought that nerve cells (including neurons in the brain) were never replaced in adult humans. When they die, you have lost them. In recent years it has become apparent that this is not always true - under some circumstances, new neurons can grow - but I think it still is true for the most part. I do not know if neurons are the only cells which are generally not replaced, but I doubt it. On the other hand, epidermal cells, for example, turn over pretty quickly. I don’t think there is any general answer to your question.
You have cells in your pancreas that aren’t replaced either. If you gain weight and cause yourself type II diabetes (i.e. by overstressing these cells until they start to die off), you aren’t cured by simply losing weight. The damage is permanent.
Here’s a previous thread with a good bit of info in it:
Yes, it varies tremendously. To further complicate issues, when you “replace” a cell, what you really mean is that a stem cell has divided. One of the daughter cells differentiates into whatever cell type, and the other daughter remains a stem cell*. So has the stem cell been replaced, or is it still there? It’s not like you can point to the daughters and say this one’s new, and this one’s old.
*Deliberately oversimplified.
I know it’s an old thread but here’s a recent podcast on the topic:
Ovaries already have all the eggs they will ever have when the person is born. So I would say that eggs are not replaced.