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I'm Malcolm Moore.
And I'm a professor of
cell biology at Memorial
Sloan-Kettering Cancer
Center in New York.
And I would like to talk to you
today about hematopoietic stem
cells and progenitor cells and their
role in normal blood formation.
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What are stem cells?
Well, the simple definition is cells
that replicate and self-regenerate.
This is called self-renewal.
And they can do so extensively, only
limited by a progressive shortening
of the ends of the chromosomes.
This is called telomeric shorting.
And when they lose a
sufficient amount of DNA
at the ends of these
chromosomes, they
undergo death by a
DNA damage response.
So this is called
the Hayflick limit.
The shortening of
telomeres can be prevented
by telomerase enzyme expression.
And many stem cells, there are different types,
actually produce telomerase.
Embryonic stem cells, those that are
identified as capable of generating
embryos, undergo
symmetric self-renewal.
And they give rise to all the
specialized tissues of the body.
Symmetric self-renewal is
basically where the stem cell
gives rise to two
daughter stem cells.
Somatic stem cells, that is to say stem cells of the adult,
include hematopoietic stem
cells, a rare sub-population
of tissue-specific, relatively
undifferentiated cells
that undergo asymmetric division.
They're capable of extensive self-renewal.
And they can give rise not only to
the different types of blood cell,
the red cells, platelets, and white blood cells,
but also the cells of the immune
system and other sub-populations,
such as dendritic cells.
This definition was established
for a hematopoietic stem cells
45 years ago and is
still valid today.
