Somewhere a human egg cell is hooking up with a sperm cell.
Whether it be in the Fallopian tube of a woman, or in a glass Petri dish, there is now the full complement of the 46 chromosomes that make up the human genome, all in a single cell. But then again, just about every cell in your body has this full set genes bundled as chromosomes. How is this cell different?
Here again, it depends. It could continue to move along the Fallopian tube while it divides into 8 cells and then implant in the uterus. Or, it could be a thawed embryo from a IVF clinic that is used to generate an embryonic stem cell line. The 8-celled blastocyst in the uterus, as we know eventually grows into an organism that lives for on average 70 years, while the embryonic stem cell line is essentially immortal, but even though that's the term used, it's a little overdramatic. But they can be cultured and grown in labs for what seems endlessly. The other immortal cells that can be grown in labs are cancer cells.
Lets look at the adult for now. (By "adult" I mean "not a fetus." Really it's about differentiated versus undifferentiated tissue,) There has never been any good evidence that embryonic stem cells exist in the adult human. It's assumed that cells a little bit along the way of differentiation do exist and are ready to produce the tissue that needs replacing. The liver is good at doing this, the nervous system, no. And there are times when this repair system doesn't always work out the best way for the human.
For instance, during the typical heart attack, a coronary artery is blocked, decreases blood flow to cardiac muscle which needs the oxygen without fail. The result is the muscle cells die, but the problem is they're not replaced with new muscle tissue but with non-functioning scar tissue. If enough of this scar tissue is present, the heart can't work as a pump as well as it used to.
more to follow...