There are on average approximately 2,500 stem cells per mL
of blood, broadly ranging from 100 up to 10, 000 cells per mL.
Although little information is available on the circadian cycle of
stem cells in the blood, Micklem’’ reported a two-fold diurnal
variation in the number ofcirculating stem cells in mice. On the
other hand, Méndez-Ferrer et al.2! reported that the number
of stem cells in the blood peaks 5 hours after the initiation of
light and reaches a nadir 5 hours after darkness. In this study
the circadian oscillations were markedly altered when mice
were subjected to continuous light or to a ‘jet lag’, defined as a
shift of 12 hours in the light cycle. In humans, Abrahamsen?!
reported a consistent 6-fold greater yield of stem cells when
bone marrow cells were aspirated during daytime and late
70 + CHAPTER FOUR
afternoon, as compared to nighttime, suggesting that the bone
marrow might produce more SC during the day. However, this
does not provide much information about circulating stem cells.
Kreutzmann and Fliedner’* studied three normal volunteers
three times a week for 70 days, and concluded that the number
of circulating stem cells could vary by a factor of 3 over a number of days, with cycles of about 21 days. So it seems that the
number of circulating SC may vary throughout the day, being
more numerous during daytime, and also over longer periods
of time, though there is truly not enough information to reach
any reliable conclusion.
Studies aimed at determining parameters that would predict the extent to which an individual can release stem cells
prior to cancer treatment have established that one of the
main predictors for the ability to release stem cells is the very
number of circulating stem cells. In other words, the greater
the number of circulating stem cells, the greater one’s ability
to mobilize additional stem cells. Gender was also another
parameter, as women on average release more stem cells than
men, as well as age with younger individuals being on average
better mobilizers than older individuals.