GPLindsey

Let's say I'm 50 years old today (I'm not really, but bear with me)

How old is the oldest living cell in my body? I don't know the answer to this, but I'm hoping some biology majors out there do. I do know that many individual cells go through life cycles of their own, some lasting only a few weeks. If the oldest cell in my body is only 6 months old say, then in what sense am I 50 years old? Are there any cells in my body that have been with me unchanged since birth (except for eggs--see below)?

Let's take this in the other direction. I have often heard that females are born with all the egg cells their bodies will ever produce. Since I am the product of an egg and a sperm, then that means that 50% of me, in a sense, is 50 years old plus the number of years my mother was alive carrying my egg prior to my birth. Half of me could then be considered 83 years old!

We also gain weight throughout our lives (at least I do sadly). There is at least 10lbs of me that wasn't around a year ago. Instead of saying I'm 50 years old, should I break it down into percentages (Well, about 20% of me is a year old, 60% of me is between 49 and 18 years old, etc.).

Maybe this all comes down to a much more basic question--when I say that "I am 50 years old today," what is the "I" that is being referred to? <img src="confused.gif" border="0">

One of the last sane

Heart muscle cells and cerebral neurones have long been considered to be postmitotic by the time you are born, although I think some doubt has been shed on this recently.

Some other cells only undergo mitosis if they absolutely have to, as in response to serious injury, like skeletal muscle cells for example.

Your intestinal epithelial cells and bladder epithelial cells, on the other hand, are mere infants.

How do you figure that? The 50% of your nuclear DNA that came from your mother, (plus the 100% of your mitochondrial DNAthat came from your mother) has been replicated over and over using newly synthesized nucleotides, it isn't the original stuff.

Abacus

The protons, neutrons, and electrons that make up the atoms in your body came to be billions and billions of years ago.

tronvillain

When you say "I am fifty years old today" the "I" is the label given to each of relatively identical organisms that have existed from moment to moment from your conception or birth until now. The "you" who existed when you were ten is not the "you" who exists today, but we call both organisms "you" because they are connected by a continuous line of virtually identical selves occupying the intervening years.

GPLindsey

I think the "I" is a consistent pattern of electrical signals that is self aware and that my body carries around with it. The cells that carry this pattern may have changed or been replaced many times, but my memories, behavior patterns, sense of self, etc., continue. When I die, this pattern will cease to be.

I concede two wrinkles to this argument, however:

1) People who have had head injuries and suffered amnesia can have almost wholesale personality changes. Does this mean that the old "I" is gone, and a new one has emerged?

2) What of instinctive, unlearned behavior, such as territoriality, fear of fire and heights, etc.? Should the definition of "I" be restricted to only those things that are unique to me as an individual, and not to things that everyone shares?

NumberTenOx

I took the OP differently. You might be able to break "aging" down into three categories:

- "Material aging": where the original material and structure has reduced capability due to wear and tear. Metal fatigue, for example. It's the original atoms in the (mostly) original structure, just beat up.

- "Pattern aging": where the material is new (renewed) but the structure has suffered degradation, due to transcription errors or impurities in the new materials.

- "Programmed aging": where the original design includes an element that measures time and changes the structure accordingly.

This is interesting from a purely practical point of view: I want to live longer. You might assume that medical science will eventually deal with the second two types of aging, but the first one poses a problem. How much of our bodies, and specifically our brains, are the original material (the original atoms)? Inquiring minds want to know...

{Added}However, on the subject of "what is me", it's the ongoing sense of self created by the workings of the brain. This includes instinctual thoughts (it's still a part of your sense of self even if it's not unique).

[ October 09, 2002: Message edited by: NumberTenOx ]</p>