How can you harvest living cells from
someone who is dead? Doesn't the fact that they are dead mean
that their cells are dead?
It is quite clear that clinically defined death, which in
most states is simply the cessation of cardiac and respiratory
activity, does not mean that all of the cells of the body
have died. It simply means that the cells required to maintain/sustain
life, namely, cardiac muscle cells and diaphragmatic muscle
cells no longer function adequately. Those cells may still
be alive, just not functioning.
In addition, the point you raised about brain death illustrates
the dichotomy from yet another angle. Brain death, cessation
of brain function as defined by flat-line EEG and lack of
cerebral blood flow, is a legal definition (which varies slightly
state to state), not a clinical one.
In brain death cases, there may be adequate cardiorespiratory
activity but a complete lack of cerebral activity. Under these
circumstances, removal of organs for transplant is legally
allowed, including heart and lungs. Clearly in this case the
organs responsible for maintaining life (as clinically defined)
are adequately functioning, otherwise they would be of no
use to a transplant recipient. In addition, clearly, once
the heart and lungs have been removed the patient would be
considered clinically, as well as legally, dead.
Even after the criteria for legal and clinical definitions
of death have been satisfied, many, if not most, cells of
the body remain viable (alive) for quite some time. For example,
it is well known that skin fibroblasts may be harvested and
grown from a cadaver as long as three days after death.
However, other cells do not last nearly as long. The capacity
of cells to remain viable after death of an organism is relatively
linearly related to their lack of functional and/or respiratory
complexity as a cell. That is, cells that have elaborated,
through expression of specific genes, a biochemical repertoire
that relies heavily on energy production tend to be the cells
that lose viability the earliest.
For example, in the brain neurons, with their much higher
metabolic rates, die sooner than glia. From a whole body point-of-view,
organs with a higher metabolic rate are much more susceptible
to a loss of energetic potential -- brain and muscle "die"
sooner that skin or bone.
To extend this whole scenario to stem cells, therefore, is
quite simple. Stem cells, by definition, have not elaborated
complex biochemical machinery; they are awaiting instructions
to do so. These cells should logically be more resistant to
a cessation of nutrient input. They should remain viable much
longer than highly metabolic cells after the delivery of glucose
and oxygen has ceased. And this is, in fact, what we have
shown in our Nature paper.