•The supply chain
mechanisms appears to be operable throughout life in a manner controlled by several feedback mechanisms.
E.g. In the absence of
P21, hematopoietic stem cells would not remain quiescent in their niches but would instead prematurely
differentiate when stress occurs exhausting the pools of those cells
leading to premature death.
•The balance of cell
Types is highly dependent on the stage of development of the organism, favoring gradual shift to
the more-differentiated cell
types
• Adult stem cells live in niches - stem cell
microenvironments and the health
of the stem cells and their ability to reproduce or differentiate both depend upon and condition the states of their
niches.
•Health for people
of all ages requires continuing operation of the supply chain throughout life. But it is a once-through process that runs out of adult stem cells with old age.
•