Page 278 - 20dynamics of cancer

STEM CELLS: TISSUE RENEWAL 263

H H
E E
E(s) E(s) E(s)
SG
SG
B(s) Hair B(s)
FG(s?) cycle
DP

Resting (telogen) phase
DP
FG(s)

Growing (anagen) phase

E Epidermis B Bulge
H Hair SG Sebaceous gland
DP Dermal papilla (s) Stem cells
FG Follicle germ

Figure 12.9 Life cycle of a mammalian hair follicle. As the follicle moves from
the rest phase to the growth phase, the follicular germ region moves downward
and becomes an active site of cell division. Transit cells from the follicular germ
move upward to form the growing hair. After a growth phase, the follicular germ
region regresses to reform the rest phase morphology. From Potten and Booth
(2002).

So far, I have discussed the keratinocyte lineages that produce the hair
and the epidermal surface. In those tissues, melanocyte cell lineages
provide pigmentation. Recent studies suggest that, in the hair follicles,
the bulge region contains melanocyte stem cells (Nishimura et al. 2002;
Lang et al. 2005; Sommer 2005). In each hair cycle, the melanocyte
stem cells produce some daughters cells that migrate to the base of the
follicle where the active keratinocyte transit lineages will be generated.
Melanocytes in each new hair cycle seem to derive from the melanocyte
stem cells in the bulge region.
Cancer risk concentrates in long-lived cell lineages—the stem lineages.
Morris (2004) recently summarized evidence that various skin cancers
derive from keratinocyte stem lineages. Similarly, melanomas probably
descend from transformed melanocyte stem cells. Alternatively, trans-
formed transit cells may de-differentiate into cancer cells with stem-like
properties of renewal.