normal. This procedure, sometimes referred to as telomerase
therapy, may indeed one day provide a means of transferring
some of the powers of immortal renewal into at least some of
the cells of the body. But it has proven difficult to efficiently
introduce this, or indeed any gene, into most tissues in the
And so, in the meantime, my mind turned to other ways
to mine the rich vein of gold of the immortal germ-line.
One fall day several years earlier, I took a break from work-
ing on telomerase and walked along the San Francisco Bay
waterfront. I began thinking about what are called stem cells.
A stem cell is a cell that can branch like the stems of a tree,
either making another stem cell or changing to become a more
specialized cell. There are all kinds of stem cells in the body,
some more potent than others (that is, some have the poten-
tial to become more similar in cell type than others do).
I wondered that day whether it would be possible to grow a
human totipotent stem cell in the laboratory. A human toti-
potent stem cell, though entirely theoretical at the time, could
potentially branch into any cell in the body. If we imagine
the branching of the fertilized egg cell into all the cells in the
body, these totipotent stem cells would be analogous to the
trunk of the tree of cellular life, the mother of all stem cells.
I was well aware of Weismanns work from my years work-
ing on cellular aging, and it occurred to me that if we could
isolate and culture such cells from the human germ-line, they
might be naturally immortal and telomerase positive, at least
until they are directed to become a specific mortal cell type.
And, most important of all, all the cells that come from them
would be young, just as babies are born young.