Page 269 - 20dynamics of cancer

254 CHAPTER 12

Figure 12.1 Pattern of cell division in the epithelial layer of the skin. At the
deepest layer, each basal stem cell divides and produces one cell that remains
at the base to continue as a stem cell and one cell that moves up to form the
transit lineage. The transit lineage divides a few times, and the cells progress
through various developmental stages as they migrate to the surface. Eventu-
ally, the cells lose their nucleus and synthesize the insoluble proteins of the
skin (keratin). As the basal stem cells continue to divide, the flow of cells from
the basal layer pushes the cells above toward the surface. The surface layer
continually sheds dead cells, which are replaced by new cells from below. From
Figure 4.1 of Cairns (1997).

many years. The short-lived transit cells derive from stem cells, divide
several times to provide a temporary population of surface cells, and
then die. Cairns (1975) wrote:
The turnover that occurs in the self-renewing epithelia is the re-
sult of continual shedding of superficial cells balanced by contin-
ual multiplication of the deeper cells. In the simplest examples,
like the skin, cell division is restricted to the deepest (basal) layer
of cells [Figure 12.1]. To keep the number of basal cells constant,
one of the two daughter cells resulting from each cell division must
on average remain in the basal layer and the other must escape and
be discarded.

Cairns contrasted two alternative patterns by which tissues may re-
new themselves. In Figure 12.2a, the lower left cell is the single stem
cell that will renew the local area of tissue. Each stem cell division pro-
duces one new daughter stem cell to the right and one new transit cell
to the top. The transit cell migrates up through the tissue and dies on
the surface. The new stem cell repeats the process. Through 16 cell di-
visions, the original stem cell produces 16 new transit cells that renew