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STEM CELLS: TISSUE RENEWAL 261

Section view
Cornified layer
Granular layer

Spinous layer
Basal layer
S
S
Surface view

Figure 12.8 Architecture of skin renewal in the mouse based on Potten’s model
of epidermal proliferative units. The top cross-sectional view shows the epider-
mal layers. About one in ten basal cells are stem cells (S). The neighboring basal
cells and all cells in the layers directly above derive from the stem cell in a typical
stem-transit architecture. The surface view shows that each unit derived from
a single stem cell forms a roughly hexagonal shape that encompasses about
ten basal cells. Each black cell denotes the single stem cell in each unit. From
Potten and Booth (2002).

would divide about 5,000 times. However, the actual history of stem
lineages and the number of divisions over time remains unknown.

EPIDERMAL RENEWAL

The epidermal layer of the skin turns over about every 7 days in mice
(Potten 1981; Ghazizadeh and Taichman 2001) and approximately every
60 days in humans (Hunter et al. 1995); however, those numbers must
be taken only as rough estimates.
Several lines of indirect evidence suggest that the skin renews by a
stem-transit architecture (Watt 1998; Janes et al. 2002). For example,
about 60% of basal epidermal cells are progressing through the cell cycle,
but in mice only about 10% of those cells can continue through several
rounds of cell division after irradiation. Human epidermal cells plated
in cell culture also show a distinction between rare cells that have a high