Greg asks:

> I am puzzled by the failure to distinguish between stem cells, and embryonic
> stem cells. Bone marrow transplants are more or less a stem cell therapy.

Technically, they're both stem cells, but they are also different. A decade
or more ago, cells were divided into only two basic categories: totipotent
(Latin for "all powerful") and "fated".

Fated cells have been known for more than a century to be capable of
differentiating into only a small subset of the tissue types that characterize an
animal or plant soma (body), although we still don't fully understand this fating
mechanism. Once such fating occurs, it becomes impossible to convert a
developing cell lineage that has been fated to become a muscle cell back into a nerve
or gut cell.

However in the last decade, we've been able to get a more clear sense of the
differentiating mechanisms, and thus three words are now used to describe the
various levels of fating: totipotent, pluripotent ("many potentialities"), and
multipotent ("still many, but less, potentialities").

Totipotent cells exist in mammals only for the cell that results from the
first union of sperm and egg (technically called a "zygote") and its first few
divisions. After that, the cells are already, although only very thinly, fated
to become one type of cell or another. Quite quickly after that, no totipotent
(cells capable of developing into any form of tissue) remain in the developing
mammalian embryo.

Stem cells do exist in bone marrow, but they have been biochemically stamped
in some manner not yet completely understood so that they are now capable of
developing into just a subset of all tissue types, thus these stem cells are
termed multipotent.

If I'm going to answer these questions technically, I'm also going to assign
homework. There is no reason for me to simply retype what is already available
on the web. For more information on these levels of fatedness, please see:

   http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Stem_Cells.html

How then is totipotency restored? We're not at all sure, but what we can say
with definitiveness, while being of comfort to the hedonists among us, that
the secret to eternal life comes through sex, not prayer. But that statement
shouldn't come as a surprise to anyone who has ever had a child. The most salient
attribute of that child lies in its perfection, and that its age has been
reset.

Nor does life begin at conception. Life began four billion years ago, and the
sperm and egg that form the zygote were just as alive as the embryo-to-be
will be. "Life" is a very complex set of biochemical machinery that has been
passed on, generation after generation, for billions of years now. But what is
true at the moment of conception in sexual reproduction is that the aging clock
and cell fating are somehow reset.

It's for these two reasons, accumulated errors in the adult and a significant
reduction of cellular potency, that embryonic stem cells are so highly prized
over adult stem cells.

Wirt Atmar

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